[larcenasb]

Bottlehead Crack Upgrading Guide

EDITED 2/4/2021 4:26 PM: grammatical edits, restructured some parts, added bits of info, and added links for my Bottlehead headphone amps comparison review & 6SN7GT comparison review

Introduction

It's been a little while for me reading or posting in this thread, but I’ve gotten a couple of messages last week about what mods most improve the sound quality of the Crack. So, I’ll post my take on the subject here in case anyone else is wondering. The mods will be ranked in tiers 4-1: tier 4 being the top and with the most obvious audible improvement, and tier 1 being no noticeable audible improvement, or merely audio gear jewelry. Within each tier, the mods are further listed in order of most audible improvement...except tiers 2 and 1 because no audible improvement was discerned by me. Included are only the more popular--and reasonably affordable--mods that you’ll see on many, many builds (for example, my suggestions will favor things like a $30 stepped volume attenuator, not an exotic $200 attenuator). This is by no means comprehensive; look at this as more of a table of contents to guide you on your exploration through this and Bottlehead's forums. If you're interested in a particular upgrade listed, Google it, along with keywords: "bottlehead" and "crack" to direct you to detailed forum posts and guides. And of course, feel free to reply/ask here if you can't find the info you need and I, or many of the other bottlehead tinkerers, will surely try to help.

Here for reference are some photos of my most recent build at various stages:

These rankings are of course just my humble opinion and with my gear (see my sig below)! However, I have built 3 Cracks since 2010, installed upgrades one at a time over the course of months, and have done a sit-down listening comparison with the Crack, Crackatwoa, SEX, and Mainline at Bottleheadquarters [review can be found here: https://www.head-fi.org/threads/bot...ck-sex-mainline.683012/page-131#post-15398582]. So, I think I have a decent level of experience to reference when it comes to differences in sound with Bottlehead gear.

Lastly, since the list will be ranked in order of most audible improvement, price isn't factored in. So, I will use asterisks in front of the upgrades that I think represent amazing value for what you get. Let's go!

Tier 4: Obvious audible improvements

• Add Speedball upgrade (dramatically increased clarity, separation, excitement, dynamics, bass slam...) [$115]
• *Replace stock 6080 w/ 6AS7G (in general... wider soundstage, a fuller sound, more natural tone, deeper bass...) [$20-ish]
  • Changing the output tube (6080/6AS7) has a bigger effect on the overall sound compared to the input tube (12AU7/6SN7), though the input tube has many more options for tuning the signature to your precise liking.
• Replace stock 12AU7 w/ 6SN7GT, using a 6SN7-to-12AU7 6.3V adapter (in general... full-bodied vocals, more balanced across frequency range, more natural tone...and just better overall for what I consider to be lifelike sound) [a good-testing old-stock 6SN7GT can be had for around $20-30 on auction sites, $20-ish for the adapter]
  • Some history: "The main competitor of the 6SN7GT after the war was the 12AU7 and its variants. A dual version of the 6C4 triode, the 12AU7 was developed by RCA in late 1946. It used half as much heater current and had a smaller footprint than the 6SN7GT. RCA and GE (having bought Ken-Rad in 1945) heavily pushed miniature tubes after the war, and many miniatures were designed-in where octals or loctals would have been the natural choice. The higher cathode emission and plate dissipation of the 6SN7GT kept it popular, though...In 1954, RCA came out with the 6CG7, which was pitched as a direct equivalent of the 6SN7GT. Although audiophiles found that the 6SN7GT typically sounded better, this was the beginning of the end for widespread usage of the 6SN7GT." [source: https://www.effectrode.com/knowledge-base/the-6sn7gt-the-best-general-purpose-dual-triode/]
  • You could also use a 6F8G w/ a 6F8G-to-12AU7 6.3V adapter. It's the older brother of the 6SN7GT and sounds just as good, if not minimally better. But availability and price make the 6SN7GT a more viable option for most people. Also the 6SN7GT tends to be less noisy and microphonic IME...but I still love my 6F8G tubes. [My comparison review of some of the more popular NOS choices can be found here: https://www.head-fi.org/threads/6sn7gt-6f8g-an-exploration-of-wwii-era-octal-tubes.890205/]
• Volume pot upgrade (pick one):
  • *Pad stock pot w/ 75K & 33K Ohm resistors for -12dB of attenuation (no more channel imbalance at low volumes, more usable volume range and control) [$5-ish]
    • Only pad the volume pot if the volume gets too loud too quickly w/ your headphones. I find it does even w/ my power-hungry AKG K240 Sextetts 600 Ohm...
  • *Replace stock pot w/ 100K stereo stepped attenuator (perfect channel balance at all levels, and, as a bonus, subtly increased clarity and separation) [$20-30 on auction sites]
    • No need to pad a stepped attenuator...is already just a series of resistors and provides good usable volume range adjustment.
    • I use a Valab 23-step but it doesn't seem to be available on auction sites anymore. Dale 23-step is another option around $20-30 that other Crack owners have used w/ success.
    • You may be wondering why I'm not suggesting an upgraded potentiometer like the popular Alps 100KAX2... I've used it in the Crack but, IMO, a stepped attenuator is better for the money: channel imbalance isn't a concern, you get more usable volume range (not restricted to the 7-9 o'clock positions). With the Alps you really should get a PCB and not solder wires directly to those fragile pins, you may have to cut/dremel the shaft if it's too high, and you will likely still need to pad it with resistors...I'd just rather get a stepped attenuator and enjoy the easier install and better sound. But if you like the smooth feel of a volume pot over the clicky turns of an attenuator, or if you like that the pot allows you to really fine-tune the volume instead of being limited to 23 set volume levels, then go for it.
    • Also, I've used an Audio Note 100K stereo volume pot [$35-ish] in one of my Crack builds and I was very impressed with the build quality, smooth feel of the turns, good tracking, and that it had solder lugs for easy installation. Though it would still need to be padded to get more usable volume range. It's my pick if you prefer a pot over an attenuator. And, again, there's nothing wrong with an Alps if you don't mind how it's high maintenance...You are a tinkerer after all, right!?

Tier 3: Subtle but clearly noticeable

• Replace electrolytic output caps w/ film caps (smoother treble and midrange) [anywhere from $50-100 and beyond]
  • Don't stress so much on which brand to get or how much to spend...the important thing is to have film caps, in general, handling the audio signal. Any respectable brand will do just fine. Having said that... Dayton 250V, Audyn Q4 400V, and Panasonic EZPE 500V are good value options around $50 for 2.
  • Here's a chart from the Bottlehead forums that I saved years ago. It shows bass roll-off of different output cap capacitance values depending on your headphones' impedance. So, with my AKG K240 Sextetts 600 Ohms, I could use 33uF output caps and notice no bass roll-off vs the stock 100uF caps...however, it's nice to have a little breathing room in case I want to plug in different headphones.
    
    
  • With the above chart in mind... I actually use 68uF output caps (Audyn Q4 68uF 400V). There are multiple reasons for this: 1) Due to the Crack's 120 Ohm output impedance, it's not recommended to use headphones lower than 200 Ohms regardless of what output capacitance is used for the output caps [search "Crack damping factor" to learn more]...it basically just means the Crack doesn't control the drivers of low-impedance cans well because it was designed for high-impedance cans, 2) The lower-value cap is also used as my last power supply cap, so the lower capacitance means more savings for those nicer bypass caps, 3) Higher voltage caps, like my 400V Audyns, are physically bigger and the 100uF version wouldn't fit with the way I'm mounting them.
• *Replace first 270 Ohm resistor (the one on the left terminal strip of the power supply) w/ Triad C-7X choke (better dynamics, separation, bass definition, quieter background...) [$12-ish]
• Add a higher-quality 1/100 film cap as a bypass on the last power supply film cap (replacing last power supply electrolytic cap first w/ one identical to the output film caps is advisable since lower-capacitance bypass caps are cheaper, see tier 2) (adjusts the sound signature to your liking w/ an influence from the qualities of the nicer cap) [from $20 on upwards]
  • Bottlehead designer, Paul Joppa has said, "There are no reliable 'rules' about bypassing power supply caps. Sometimes they help, sometimes they hinder, and sometimes you can't tell the difference. The '1% rule' is really more like 'something between 0.1% and 10%' and even then exceptions are not at all uncommon...The only reliably useful answer is 'try it and see!'" [source: Bottlehead forum, Re: Power supply bypass cap values, Reply #15, May 22, 2013, 08:20:51 PM]
  • So, for my 68uF last PS cap, I would prefer a 0.68uF bypass, but wouldn't fret at all if the cap I wanted to try was only available in 0.47uF or 1uF. Even .068uF or 6.8uF, as Mr. Joppa explained, are worth a try.
  • I've tried the following 0.68uF caps: Jantzen Superior-Z [$15], Audyn Tri-Reference [$25], and Audyn True Copper [$50]. The Jantzen sounded detailed and natural, but it was subtle. The Audyn Tri-Reference had a bold sound in comparison, with natural tone and thrilling dynamics. Switching to the Audyn True Copper didn't live up to what I imagined it would be after months of dreaming of it (yeah, I'm a nerd lol)...the True Copper sounded very refined but lacked the boldness and dynamics of the Tri, sounding very polite...and my tastes just couldn't live with that. More expensive doesn't always mean better for what you prefer. And though I love my Audyn Tri-Reference, its thick copper leads were a pain to deal with...
  • Many swear by bypassing PS caps, but loathe the idea of bypassing or "contaminating" output/signal caps. Try if you like and discover what you think is best. I personally haven't bypassed the output caps, but with Mr. Joppa's comment below (tier 2, 1st bullet point) about the last power supply cap being in the audio current loop, and being "as important as the output coupling cap," then theoretically my bypassing the last PS cap isn't much different--from a purist's perspective--than bypassing the output caps! I love the sound from the Audyn Tri-Reference bypass though, so I don't care about being a purist in this sense.
• Add sound deadening material (Dynamat, Kilmat, Noico, MAT66, etc.) to underside of top-plate (reduces microphonics when turning on amp, adjusting volume, tapping top plate if you ever do that lol, etc. ...just improves the user experience and adds a dense, quality feel to your amp) [<$20]
  • You don't need full coverage... Like with sound-deadening car doors, follow the 25% rule to avoid diminishing returns. I apply larger pieces where I can and focus on putting smaller pieces all around the tube sockets, volume pot/attenuator, and the headphone socket. See second-to-last photo at top of post.

Tier 2: I’d like to think I hear a difference, but it’s more placebo than anything

• Replace last power supply cap (C3, the sideways one) w/ film cap (ideally, double the capacitance of the output caps [100uF output caps, 220uF last PS cap]...but the same value [100uF output caps, 100uF last PS cap] is commonly used due to space constraints) [$20-30 for Dayton, Audyn, or Panasonic]
  • Paul Joppa has said, "The output audio current flows through the power supply, mostly the last capacitor, as well as through the output coupling cap and the output triode. So the final power supply cap should, in theory, be as important as the output coupling cap with regard to the sound of the amp." [source: Bottlehead forum, Re: Upgrading the power supply capacitors in the Crack, Reply #4, April 18, 2011, 08:08:44 AM]
  • It was hard for me to notice, but then bypassing it w/ a higher-quality cap was clearly noticeable. So, this is an important upgrade to me as a gateway to bypassing it. On its own though, I can't say I can tell the difference for sure.
  • If on a budget, you could install the last PS film cap before installing the output film caps...maybe then the difference would be more noticeable...and it would be half the cost. I know it seems like common sense to take care of the output/signal caps first, but here's a situation where an audio engineer, Mr. Joppa, is telling us the theory of the circuit design...an engineer's theory should take precedence over a layman's common sense. So, if you install this instead of the film output caps, it could potentially be a tier 3 upgrade...it's just not my experience, so I can't place it there.
• Replace the 220uF 250V first power supply cap (C1, beside RCA jacks) w/ 470uF 250V electrolytic cap (more power supply filtration) [$5-ish for Nichicon KX, which are "great for power supply upgrades," according to hificollective...I went with Nippon Chemi-Con KMH]
  • Bottlehead designer, Paul Birkeland, has said, "...there's nothing wrong with increasing the capacitance of cap 1 or 2 in the power supply (I put a 500uF cap for C1 in one Crack I built)." [source: Bottlehead forum, Re: Choking the Crack, Reply #12, November 10, 2012, 04:15:11 PM]
  • This is something I do since I'm not using the ideal doubled-capacitance last power supply film cap...I use the same capacitance for the PS film cap as the output caps. So, with the larger capacitance PS electrolytic, it's nice to know I'm not lacking in PS filtration.
  • Actually, as you can see in my above photos, I replaced both PS electrolytics w/ 470uF caps, but it's clearly overkill... for my next build, I would just upgrade the first PS cap and return to the land of reason.
• Add 1/100 film bypass cap to each electrolytic power supply cap (C1 & C2) [<$5 for 2.2uF or 4.7uF 250V film caps]
  • For power supply electrolytics, basic film caps are all you need (orange drop, Dayton, Audyn Q4, Wima, etc.). Caps w/ beeswax and, as Audyn calls it, "fairy-like oils" don't make much sense in this application, or arguably in any audio application for that matter.
  • I went with ClarityCap PX 4.7uF 250V caps. They fit very nicely as you can see, but the thicker leads were less than ideal to work with. I'd go with orange drop types next time.
• Replace power supply rectifiers (18, 19, 20, & 21) w/ Cree Schottky diodes [$5-ish for 4 "CSD01060A" diodes & "Discrete Bridge Rectifier PCB - TYPE 2 (TO-220 Radial Style)" PCB]
  • Many say there's increased detail, dynamics, and clarity, but I didn't notice...maybe if I didn't already install the choke, I might've noticed?...
  • For only $5, perhaps try this before the choke and report back here if you notice any audible improvements. I'll do this for my next build. This is another upgrade that has the potential to be a tier 3 upgrade if installed instead of the choke or other PS upgrades.
• Replace input wires (from RCA jacks to volume pot) w/ Mogami W2549 cable, or other OFC cable/wires [$0.80/ft. for Mogami W2549, $0.50/ft. for Canare L-2T2S, and you only need about 1 ft.]
  • Some say the sound in the Crack is smoother w/ OFC...I would love to see them identify it in a blind test.
  • But this is something I will always do anyway since I have so much OFC wire/cable lying around...it's one of those "nice to haves" that don't cost much at all. For less than $1, it's comforting to know your audio signal is comfortably traveling in style!

Tier 1: Audio Gear Jewelry

• Replace plastic volume knob w/ solid aluminum knob [$5-ish]
  • Of course, has no effect on sound quality, but the weighty feel turning the knob is really nice/premium.
  • Especially if using a pot, a larger diameter knob allows you to really fine-tune the volume--similar to what joystick extenders do on game controllers. Up to 50mm diameter knobs will fit, even if using the wider 6SN7GT tube.
• Replace RCA jacks w/ exotic noble-metal-plated jacks [up to $20 or more]
  • Bottlehead upgraded their RCA jacks since 2010, the gold-plated jacks they supply now are already of great quality...I feel no need to "upgrade" them.
• Replace tube sockets w/ gold-plated ceramic tube sockets [$5 or more]
  • This is another thing I always do anyway because I like to show my NOS tubes that I appreciate them lol...another "nice to have".
• Replace rubber feet w/ metal spikes [$15 or more]
  • Taller variants of course can supply more ventilation...but are mostly just pretty.

My Recommendations

• If you have a basic Crack built, and objective sound quality improvement is what you're after, just work down tiers 4 & 3.
• If you have a basic Crack built, and objective sound quality improvement is what you're after...but money is a concern, I would buy upgrades in stages every few months or so (but perhaps still install each part one at a time to try and hear the differences):
  • STAGE 1 $25: 6AS7G, and pad stock volume pot
  • STAGE 2 $50: 6SN7GT + adapter, and choke
  • STAGE 3 $100: Speedball kit
  • STAGE 4 $100: stepped attenuator, film output caps, and sound deadening material
• If you have a basic Crack built, and you're a starving student who gives up Cup Noodles in order to get good audio, buy one upgrade at a time! I'd focus first on padding the volume pot, then trying to get a good deal on tubes on auction sites, and then trying some of the low-cost power supply upgrades like the 470uF first PS cap, Cree Schottky diodes, or bypassing the PS electrolytics. And ask for the Speedball kit for your birthday...
• If I were starting a new Crack build, here's how I'd go about it: basic Crack kit, gold-plated ceramic tube sockets, Mogami W2549 input cable, stepped attenuator, sound deadening material, and solid aluminum volume knob.
  • This would run you about $360. The reasoning for these upgrades is that they don't cost too terribly much and some are easier to install right from the get-go (good luck if you decide to replace the stock tube sockets later on...). Also, these initial upgrades make the basic kit feel much more premium and make for a good foundation to install the more expensive upgrades down the road--think of it like prioritizing a good-quality motherboard for a new PC. For me, sound quality is most important...but it's also nice to have a really premium looking and feeling product.
  • For the sound deadening material, be sure to plan the layout: leave open spaces for future upgrades such as the zip-tie mounts for the output caps, and standoffs for the choke.
• If cost is no object and you want the best sound this circuit has to offer ASAP, then consider getting the Crackatwoa kit instead. In stock form, it's better than the Crack w/ all the above upgrades...and would actually be cheaper overall (don't forget separate tax and shipping for all the Crack upgrades). [$269 premium for the Crackatwoa over the Crack + Speedball]
  • For upgrading the Crackatwoa, Bottlehead owner, Doc B., explained to me that all the PS upgrades for the Crack--including replacing the last PS cap with a film cap--would be pointless, as the more sophisticated, shunt voltage regulated power supply is a far better solution than all the Crack PS upgrades combined...I'm paraphrasing but I think that's accurate to what he intended. He can correct me here if I'm wrong.
  • For the Crackatwoa, the only upgrades I would do are: 6AS7G, 6SN7GT + adapter, stepped attenuator, film output caps, gold-plated ceramic tube sockets, sound deadening material, and solid aluminum volume knobs. [around $150]

Conclusion

This turned out to be longer than I expected haha, but I wanted to add details and notes that I think are important to clarify for anyone starting their Crack journey. This amp is certainly a mainstay--being available for over a decade--and, as you can see, there are many upgrade paths you could go down, but hopefully my post here is a good primer and starting guide if you're planning your first build or first upgrades to an existing build. It should help you to not go overboard and to get the most for your money. Like I said in the intro though, this isn't comprehensive and you should always seek other forum members' inputs, so as to not take one person's word as the end-all-be-all. Good luck with your build!

 

[evonimos]

Regarding tubes and mods that go well with them, I'd ike to add my two cents.
I know this is probably a contentious issue, but my compulsive A/B tests seem to indicate that tubes do very well with carbon composite resistors.

The difference (or improvement in my opinion) is not that huge, but it is there.
They seem to add further to the 'tubiness' of the sound.
By 'tubiness', I mean the subjective effect that comes along with tubes, especially in zero-feedback, single ended topologies.
It certainly doesn't hurt trying them out, despite the 'bad press' they've been getting from people compared to other types like metal-film resistors etc. The only issue is that they are getting harder to come by (and cost accordingly) and the current production is limited to 250mW and 500mW power ratings only.

Larger wattage carbon films are also a good match, especially when you need something above the 500mW mark.

If I had to make a guess of what's going on, I'd say they might add slightly more second order harmonics which seems to be the reason tubes can sound so unique.
But that is just me speculating so it might be something else entirely.

 

[HiFiHawaii808]

Bottlehead Crack Upgrading Guide

Introduction

It's been a little while for me reading or posting in this thread, but I’ve gotten a couple of messages last week about what mods most improve the sound quality of the Crack. So, I’ll post my take on the subject here in case anyone else is wondering. The mods will be ranked in tiers 4-1: tier 4 being the top and with the most obvious audible improvement, and tier 1 being no noticeable audible improvement, or merely decorative. Within each tier, the mods are further listed in order of most audible improvement...except tiers 2 and 1 because no audible improvement was discerned by me. Included are only the more popular--and reasonably affordable--mods that you’ll see on many, many builds (for example, my suggestions will favor things like a $30 Valab attenuator, not a $165 Goldpoint attenuator). This is by no means comprehensive; look at this as more of a table of contents to guide you on your exploration through this and Bottlehead's forums. If you're interested in a particular upgrade listed, Google it, along with keywords: "bottlehead" and "crack" to direct you to detailed forum posts and guides. And of course, feel free to reply/ask here if you can't find the info you need and I, or many of the other bottlehead tinkerers, will surely try to help.

Here for reference are some photos of my most recent build at various stages:







These rankings are of course just my humble opinion and with my gear (see my sig below)! However, I have built 3 Cracks since 2010, installed upgrades one at a time over the course of months, and have done a sit-down listening comparison with the Crack, Crackatwoa, SEX, and Mainline at Bottleheadquarters. So, I think I have a decent level of experience to reference when it comes to differences in sound with Bottlehead gear.

Lastly, since the list will be ranked in order of most audible improvement, price isn't factored in. So, I will use asterisks in front of the upgrades that I think represent amazing value for what you get. Let's go!

Tier 4: Obvious audible improvements

• Add Speedball upgrade (dramatically increased clarity, separation, excitement, dynamics, bass slam...) [$115]
• *Replace stock 6080 w/ 6AS7G (in general... wider soundstage, a fuller sound, more natural tone, deeper bass...) [$20-ish]
  • Changing the output tube (6080/6AS7) has a bigger effect on the overall sound compared to the input tube (12AU7/6SN7), though the input tube has many more options for tuning the signature to your precise liking.
• Replace stock 12AU7 w/ 6SN7GT, using a 6SN7-to-12AU7 6.3V adapter (in general... full-bodied vocals, more balanced across frequency range, more natural tone...and just better overall for what I consider to be lifelike sound) [a good-testing old-stock 6SN7GT can be had for around $20-30 on auction sites, $20-ish for the adapter]
  • Some history: "The main competitor of the 6SN7GT after the war was the 12AU7 and its variants. A dual version of the 6C4 triode, the 12AU7 was developed by RCA in late 1946. It used half as much heater current and had a smaller footprint than the 6SN7GT. RCA and GE (having bought Ken-Rad in 1945) heavily pushed miniature tubes after the war, and many miniatures were designed-in where octals or loctals would have been the natural choice. The higher cathode emission and plate dissipation of the 6SN7GT kept it popular, though. The combined maximum plate dissipation was upped to 7.5 watts in 1950 in the 6SN7GTA. In 1954, the 6SN7GTB added controlled heater warm-up time to make it more reliable in series heater-string TV sets. In 1954, RCA came out with the 6CG7, which was pitched as a direct equivalent of the 6SN7GT. Although audiophiles found that the 6SN7GT typically sounded better, this was the beginning of the end for widespread usage of the 6SN7GT." [source: https://www.effectrode.com/knowledge-base/the-6sn7gt-the-best-general-purpose-dual-triode/]
  • You could also use a 6F8G w/ a 6F8G-to-12AU7 6.3V adapter, but availability and price make the 6SN7GT a more viable option for most people. Also the 6SN7GT tends to be less noisy and microphonic IME...but I still love my 6F8G tubes.
• Volume pot upgrade (pick one):
  • *Pad stock pot w/ 75K & 33K Ohm resistors for -12dB of attenuation (no more channel imbalance at low volumes, more usable volume range and control) [$5-ish]
    • Only pad the volume pot if the volume gets too loud too quickly w/ your headphones. I find it does even w/ my power-hungry AKG K240 Sextetts 600 Ohm...
  • *Replace stock pot w/ 100K stereo stepped attenuator (Valab 23-step is a common choice and is what I currently use) (perfect channel balance at all levels, and, as a bonus, subtly increased clarity and separation) [$20-30 on auction sites]
    • No need to pad an attenuator...is already just a series of resistors and provides good usable volume range adjustment.
    • You may be wondering why I'm not suggesting an upgraded potentiometer like the popular Alps 100KAX2... Well, IMO, a stepped attenuator is better for the money: channel imbalance isn't a concern, you get more usable volume range (not restricted to the 7-9 o'clock positions). With the Alps you really should get a PCB and not solder wires directly to those fragile pins, you may have to cut/dremel the shaft if it's too high, and you will likely still need to pad it with resistors...I'd just rather get a stepped attenuator and enjoy the easier install and better sound. But if you like the smooth feel of a volume pot over the clicky turns of an attenuator, or if you like that the pot allows you to really fine-tune the volume instead of being limited to 23 set volume levels, then go for it.
    • Also, I've used an Audio Note 100K stereo volume pot [$35-ish] in one of my Crack builds and I was very impressed with the build quality, smooth feel of the turns, good tracking, and that it had solder lugs for easy installation. Though it would still need to be padded to get more usable volume range. It's my pick if you prefer a pot over an attenuator. And, again, there's nothing wrong with an Alps if you don't mind how it's high maintenance...You are a tinkerer after all, right!?

Tier 3: Subtle but clearly noticeable

• Replace electrolytic output caps w/ film caps (smoother treble and midrange) [anywhere from $50-100 and beyond]
  • Don't stress so much on which brand to get or how much to spend...the important thing is to have film caps, in general, handling the audio signal. Any respectable brand will do just fine. Having said that... Dayton 250V, Audyn Q4 400V, and Panasonic EZPE 500V are good value options around $50 for 2.
  • Here's a chart from the Bottlehead forums that I saved years ago. It shows bass roll-off of different output cap capacitance values depending on your headphones' impedance. So, with my AKG K240 Sextetts 600 Ohms, I could use 33uF output caps and notice no bass roll-off vs the stock 100uF caps...however, it's nice to have a little breathing room in case I want to plug in different headphones.
    
  • With the above chart in mind... I actually use 68uF output caps (Audyn Q4 68uF 400V). There are multiple reasons for this: 1) Due to the Crack's 120 Ohm output impedance, it's not recommended to use headphones lower than 300 Ohms regardless of what output capacitance is used for the output caps [search "Crack damping factor" to learn more], 2) The lower-value cap is also used as my last power supply cap, so the lower capacitance means more savings for those nicer bypass caps, 3) Higher voltage caps, like my 400V Audyns, are physically bigger and the 100uF version wouldn't fit with the way I'm mounting them.
• *Replace first 270 Ohm resistor (the one on the left terminal strip of the power supply) w/ Triad C-7X choke (better dynamics, separation, bass definition, quieter background...) [$12-ish]
• Add a higher-quality 1/100 film cap as a bypass on the last power supply film cap (requires replacing last power supply electrolytic cap w/ film cap, see tier 3) (adjusts the sound signature to your liking) [from $20 on upwards]
  • Bottlehead designer, Paul Joppa has said, "There are no reliable 'rules' about bypassing power supply caps. Sometimes they help, sometimes they hinder, and sometimes you can't tell the difference. The '1% rule' is really more like 'something between 0.1% and 10%' and even then exceptions are not at all uncommon...The only reliably useful answer is 'try it and see!'" [source: Bottlehead forum, Re: Power supply bypass cap values, Reply #15, May 22, 2013, 08:20:51 PM]
  • So, for my 68uF last PS cap, I would prefer a 0.68uF bypass, but wouldn't fret at all if the cap I wanted to try was only available in 0.47uF or 1uF. Even .068uF or 6.8uF, as Mr. Joppa explained, are worth a try.
• Add sound deadening material (Dynamat, Kilmat, Noico, MAT66, etc.) to underside of top-plate (reduces microphonics when turning on amp, adjusting volume, tapping top plate if you ever do that lol, etc. ...just improves the user experience and adds a dense, quality feel to your amp) [<$20]
  • You don't need full coverage... Like with sound-deadening car doors, follow the 25% rule to avoid diminishing returns. I apply larger pieces where I can and focus on putting smaller pieces all around the tube sockets, volume pot/attenuator, and the headphone socket. See middle-bottom photo at top of post.

Tier 2: I’d like to think I hear a difference, but it’s more placebo than anything

• Replace last power supply cap (C3, the sideways one) w/ film cap (ideally, double the capacitance of the output caps--100uF output caps, 220uF last PS cap--but the same value--100uF for outputs and 100uF for last PS--are commonly used due to space constraints) [$20-30 for Dayton, Audyn, or Panasonic]
  • Paul Joppa has said, "The output audio current flows through the power supply, mostly the last capacitor, as well as through the output coupling cap and the output triode. So the final power supply cap should, in theory, be as important as the output coupling cap with regard to the sound of the amp." [source: Bottlehead forum, Re: Upgrading the power supply capacitors in the Crack, Reply #4, April 18, 2011, 08:08:44 AM]
  • It was hard for me to notice, but then bypassing it w/ a higher-quality cap was clearly noticeable. So, this is an important upgrade to me as a gateway to bypassing it. On its own though, I can't say I can tell the difference for sure.
• Replace the 220uF 250V first power supply cap (C1, beside RCA jacks) w/ 470uF 250V electrolytic cap (more power supply filtration) [$5-ish for Nichicon KX, which are "great for power supply upgrades," according to hificollective]
  • Bottlehead designer, Paul Birkeland, has said, "...there's nothing wrong with increasing the capacitance of cap 1 or 2 in the power supply (I put a 500uF cap for C1 in one Crack I built)." [source: Bottlehead forum, Re: Choking the Crack, Reply #12, November 10, 2012, 04:15:11 PM]
  • This is something I do since I'm not using the ideal doubled-capacitance last power supply film cap...I use the same capacitance for the PS film cap as the output caps. So, with the larger capacitance PS electrolytic, it's nice to know I'm not lacking in PS filtration.
  • Actually, as you can see in my above photos, I replaced both PS electrolytics w/ 470uF caps, but it's clearly overkill... for my next build, I would just upgrade the first PS cap and return to the land of reason.
• Add 1/100 film bypass cap to each electrolytic power supply cap (C1 & C2) [<$5 for 2.2uF or 4.7uF 250V film caps]
  • For power supply electrolytics, basic film caps are all you need (orange drop, Dayton, Audyn Q4, Wima, etc.). Caps w/ beeswax and, as Audyn calls it, "fairy-like oils" don't make much sense in this application, or arguably in any audio application for that matter.
  • Many swear by bypassing PS caps, but loathe the idea of bypassing or "contaminating" output/signal caps. Try if you like and discover what you think is best. I personally haven't bypassed the output caps.
• Replace power supply rectifiers (18, 19, 20, & 21) w/ Cree Schottky diodes [$5-ish for 4 diodes & PCB]
  • Many say there's increased dynamics and clarity, but I didn't notice...maybe if I didn't already install the choke, I might've noticed?...
  • For only $5, perhaps try this before the choke and report back here if you notice any audible improvements. I'll do this for my next build.
• Replace input wires (from RCA jacks to volume pot) w/ Mogami W2549 cable, or other OFC cable/wires [$0.80/ft. for Mogami W2549, $0.50/ft. for Canare L-2T2S, and you only need about 1 ft.]
  • Some say the sound in the Crack is smoother w/ OFC...I would love to see them identify it in a blind test.
  • But this is something I will always do anyway since I have so much OFC wire/cable lying around...it's one of those "nice to haves" that don't cost much at all. For less than $1, it's comforting to know your audio signal is comfortably traveling in style!

Tier 1: Merely decorative

• Replace plastic volume knob w/ solid aluminum knob (of course has no effect on sound quality, but the weighty feel turning the knob is really nice/premium) [$5-ish]
• Replace RCA jacks w/ exotic noble-metal-plated jacks (Bottlehead upgraded their RCA jacks since 2010, the gold-plated jacks they supply now are already of great quality...I feel no need to "upgrade" them) [up to $20 or more]
• Replace tube sockets w/ gold-plated ceramic tube sockets (this is another thing I always do anyway because I like to show my NOS tubes that I appreciate them lol...another "nice to have") [$5 or more]
• Replace rubber feet w/ metal spikes (taller variants of course can supply more ventilation...but are mostly just pretty) [$15 or more]

My Recommendations

• If you have a basic Crack built and sound quality is your only concern, just work down tiers 4 & 3, and then maybe 2 if you're curious.
• If you have a basic Crack built and money is a concern, I would upgrade in stages every few months or so:
  • STAGE 1 $25: 6AS7G, pad stock volume pot, and 470uF first PS cap
  • STAGE 2 $50: 6SN7GT + adapter, and choke
  • STAGE 3 $100: Speedball kit
  • STAGE 4 $100: Valab attenuator, Mogami W2549 input cable, film output caps, Cree Schottky diodes, and film bypasses for PS electrolytics
• If cost is no object and you want the best sound this circuit has to offer ASAP, then consider getting the Crackatwoa kit instead. In stock form, it's better than the Crack w/ all the above upgrades...and would actually be cheaper overall (don't forget separate tax and shipping for all the Crack upgrades). [$269 premium for the Crackatwoa over the Crack + Speedball]
  • For upgrading the Crackatwoa, Bottlehead owner, Doc B., explained to me that all the PS upgrades for the Crack--including replacing the last PS cap with a film cap--would be pointless, as the more sophisticated, shunt voltage regulated power supply is a far better solution than all the Crack PS upgrades combined...I'm paraphrasing but I think that's accurate to what he intended. He can correct me here if I'm wrong.
  • For the Crackatwoa, the only upgrades I would do are: 6AS7G, 6SN7GT + adapter, Valab attenuator, film output caps, and gold-plated ceramic tube sockets. [around $150]
• If I were starting a new Crack build, here's how I'd go about it: basic Crack kit, gold-and-ceramic tube sockets, Mogami W2549 input cable, Valab attenuator, 470uF first PS cap, Cree Schottky diodes, Noico sound deadening material, and solid aluminum volume knob.
  • This would run you about $375. The reasoning for these upgrades is that they don't cost too much and some are easier to install right from the get-go (good luck if you decide to replace the stock tube sockets later on...). Also, these initial upgrades make the basic kit feel much more premium and make for a good foundation to install the more expensive upgrades down the road--think of it like prioritizing a good-quality motherboard for a new PC.
  • Then from here, I'd just go down the tiers as my budget allows.

Conclusion

This turned out to be longer than I expected haha, but I wanted to add details and notes that I think are important for anyone starting their Crack journey. The Crack is certainly a mainstay--being available for over a decade--and, as you can see, there are many upgrade paths you could go down, but hopefully my post here is a good primer and starting guide if you're planning your first build or first upgrades to an existing build. It should help you to not go overboard and to get the most for your money. Like I said in the intro though, this isn't comprehensive and you should always seek other forum members' inputs, so as to not take one person's word as the end-all-be-all. Good luck with your build!

Very nice. One of the best overviews and upgrade how-to guides I've every seen for the Crack.

All you need is the same type of analysis for Tubes and you've got one of the best how-to guides in the industry.

 

[DeweyCH]

Bottlehead Crack Upgrading Guide

Introduction

It's been a little while for me reading or posting in this thread, but I’ve gotten a couple of messages last week about what mods most improve the sound quality of the Crack. So, I’ll post my take on the subject here in case anyone else is wondering. The mods will be ranked in tiers 4-1: tier 4 being the top and with the most obvious audible improvement, and tier 1 being no noticeable audible improvement, or merely decorative. Within each tier, the mods are further listed in order of most audible improvement...except tiers 2 and 1 because no audible improvement was discerned by me. Included are only the more popular--and reasonably affordable--mods that you’ll see on many, many builds (for example, my suggestions will favor things like a $30 Valab attenuator, not a $165 Goldpoint attenuator). This is by no means comprehensive; look at this as more of a table of contents to guide you on your exploration through this and Bottlehead's forums. If you're interested in a particular upgrade listed, Google it, along with keywords: "bottlehead" and "crack" to direct you to detailed forum posts and guides. And of course, feel free to reply/ask here if you can't find the info you need and I, or many of the other bottlehead tinkerers, will surely try to help.

Here for reference are some photos of my most recent build at various stages:







These rankings are of course just my humble opinion and with my gear (see my sig below)! However, I have built 3 Cracks since 2010, installed upgrades one at a time over the course of months, and have done a sit-down listening comparison with the Crack, Crackatwoa, SEX, and Mainline at Bottleheadquarters. So, I think I have a decent level of experience to reference when it comes to differences in sound with Bottlehead gear.

Lastly, since the list will be ranked in order of most audible improvement, price isn't factored in. So, I will use asterisks in front of the upgrades that I think represent amazing value for what you get. Let's go!

Tier 4: Obvious audible improvements

• Add Speedball upgrade (dramatically increased clarity, separation, excitement, dynamics, bass slam...) [$115]
• *Replace stock 6080 w/ 6AS7G (in general... wider soundstage, a fuller sound, more natural tone, deeper bass...) [$20-ish]
  • Changing the output tube (6080/6AS7) has a bigger effect on the overall sound compared to the input tube (12AU7/6SN7), though the input tube has many more options for tuning the signature to your precise liking.
• Replace stock 12AU7 w/ 6SN7GT, using a 6SN7-to-12AU7 6.3V adapter (in general... full-bodied vocals, more balanced across frequency range, more natural tone...and just better overall for what I consider to be lifelike sound) [a good-testing old-stock 6SN7GT can be had for around $20-30 on auction sites, $20-ish for the adapter]
  • Some history: "The main competitor of the 6SN7GT after the war was the 12AU7 and its variants. A dual version of the 6C4 triode, the 12AU7 was developed by RCA in late 1946. It used half as much heater current and had a smaller footprint than the 6SN7GT. RCA and GE (having bought Ken-Rad in 1945) heavily pushed miniature tubes after the war, and many miniatures were designed-in where octals or loctals would have been the natural choice. The higher cathode emission and plate dissipation of the 6SN7GT kept it popular, though. The combined maximum plate dissipation was upped to 7.5 watts in 1950 in the 6SN7GTA. In 1954, the 6SN7GTB added controlled heater warm-up time to make it more reliable in series heater-string TV sets. In 1954, RCA came out with the 6CG7, which was pitched as a direct equivalent of the 6SN7GT. Although audiophiles found that the 6SN7GT typically sounded better, this was the beginning of the end for widespread usage of the 6SN7GT." [source: https://www.effectrode.com/knowledge-base/the-6sn7gt-the-best-general-purpose-dual-triode/]
  • You could also use a 6F8G w/ a 6F8G-to-12AU7 6.3V adapter, but availability and price make the 6SN7GT a more viable option for most people. Also the 6SN7GT tends to be less noisy and microphonic IME...but I still love my 6F8G tubes.
• Volume pot upgrade (pick one):
  • *Pad stock pot w/ 75K & 33K Ohm resistors for -12dB of attenuation (no more channel imbalance at low volumes, more usable volume range and control) [$5-ish]
    • Only pad the volume pot if the volume gets too loud too quickly w/ your headphones. I find it does even w/ my power-hungry AKG K240 Sextetts 600 Ohm...
  • *Replace stock pot w/ 100K stereo stepped attenuator (Valab 23-step is a common choice and is what I currently use) (perfect channel balance at all levels, and, as a bonus, subtly increased clarity and separation) [$20-30 on auction sites]
    • No need to pad an attenuator...is already just a series of resistors and provides good usable volume range adjustment.
    • You may be wondering why I'm not suggesting an upgraded potentiometer like the popular Alps 100KAX2... Well, IMO, a stepped attenuator is better for the money: channel imbalance isn't a concern, you get more usable volume range (not restricted to the 7-9 o'clock positions). With the Alps you really should get a PCB and not solder wires directly to those fragile pins, you may have to cut/dremel the shaft if it's too high, and you will likely still need to pad it with resistors...I'd just rather get a stepped attenuator and enjoy the easier install and better sound. But if you like the smooth feel of a volume pot over the clicky turns of an attenuator, or if you like that the pot allows you to really fine-tune the volume instead of being limited to 23 set volume levels, then go for it.
    • Also, I've used an Audio Note 100K stereo volume pot [$35-ish] in one of my Crack builds and I was very impressed with the build quality, smooth feel of the turns, good tracking, and that it had solder lugs for easy installation. Though it would still need to be padded to get more usable volume range. It's my pick if you prefer a pot over an attenuator. And, again, there's nothing wrong with an Alps if you don't mind how it's high maintenance...You are a tinkerer after all, right!?

Tier 3: Subtle but clearly noticeable

• Replace electrolytic output caps w/ film caps (smoother treble and midrange) [anywhere from $50-100 and beyond]
  • Don't stress so much on which brand to get or how much to spend...the important thing is to have film caps, in general, handling the audio signal. Any respectable brand will do just fine. Having said that... Dayton 250V, Audyn Q4 400V, and Panasonic EZPE 500V are good value options around $50 for 2.
  • Here's a chart from the Bottlehead forums that I saved years ago. It shows bass roll-off of different output cap capacitance values depending on your headphones' impedance. So, with my AKG K240 Sextetts 600 Ohms, I could use 33uF output caps and notice no bass roll-off vs the stock 100uF caps...however, it's nice to have a little breathing room in case I want to plug in different headphones.
    
  • With the above chart in mind... I actually use 68uF output caps (Audyn Q4 68uF 400V). There are multiple reasons for this: 1) Due to the Crack's 120 Ohm output impedance, it's not recommended to use headphones lower than 300 Ohms regardless of what output capacitance is used for the output caps [search "Crack damping factor" to learn more], 2) The lower-value cap is also used as my last power supply cap, so the lower capacitance means more savings for those nicer bypass caps, 3) Higher voltage caps, like my 400V Audyns, are physically bigger and the 100uF version wouldn't fit with the way I'm mounting them.
• *Replace first 270 Ohm resistor (the one on the left terminal strip of the power supply) w/ Triad C-7X choke (better dynamics, separation, bass definition, quieter background...) [$12-ish]
• Add a higher-quality 1/100 film cap as a bypass on the last power supply film cap (requires replacing last power supply electrolytic cap w/ film cap, see tier 3) (adjusts the sound signature to your liking) [from $20 on upwards]
  • Bottlehead designer, Paul Joppa has said, "There are no reliable 'rules' about bypassing power supply caps. Sometimes they help, sometimes they hinder, and sometimes you can't tell the difference. The '1% rule' is really more like 'something between 0.1% and 10%' and even then exceptions are not at all uncommon...The only reliably useful answer is 'try it and see!'" [source: Bottlehead forum, Re: Power supply bypass cap values, Reply #15, May 22, 2013, 08:20:51 PM]
  • So, for my 68uF last PS cap, I would prefer a 0.68uF bypass, but wouldn't fret at all if the cap I wanted to try was only available in 0.47uF or 1uF. Even .068uF or 6.8uF, as Mr. Joppa explained, are worth a try.
• Add sound deadening material (Dynamat, Kilmat, Noico, MAT66, etc.) to underside of top-plate (reduces microphonics when turning on amp, adjusting volume, tapping top plate if you ever do that lol, etc. ...just improves the user experience and adds a dense, quality feel to your amp) [<$20]
  • You don't need full coverage... Like with sound-deadening car doors, follow the 25% rule to avoid diminishing returns. I apply larger pieces where I can and focus on putting smaller pieces all around the tube sockets, volume pot/attenuator, and the headphone socket. See middle-bottom photo at top of post.

Tier 2: I’d like to think I hear a difference, but it’s more placebo than anything

• Replace last power supply cap (C3, the sideways one) w/ film cap (ideally, double the capacitance of the output caps--100uF output caps, 220uF last PS cap--but the same value--100uF for outputs and 100uF for last PS--are commonly used due to space constraints) [$20-30 for Dayton, Audyn, or Panasonic]
  • Paul Joppa has said, "The output audio current flows through the power supply, mostly the last capacitor, as well as through the output coupling cap and the output triode. So the final power supply cap should, in theory, be as important as the output coupling cap with regard to the sound of the amp." [source: Bottlehead forum, Re: Upgrading the power supply capacitors in the Crack, Reply #4, April 18, 2011, 08:08:44 AM]
  • It was hard for me to notice, but then bypassing it w/ a higher-quality cap was clearly noticeable. So, this is an important upgrade to me as a gateway to bypassing it. On its own though, I can't say I can tell the difference for sure.
• Replace the 220uF 250V first power supply cap (C1, beside RCA jacks) w/ 470uF 250V electrolytic cap (more power supply filtration) [$5-ish for Nichicon KX, which are "great for power supply upgrades," according to hificollective]
  • Bottlehead designer, Paul Birkeland, has said, "...there's nothing wrong with increasing the capacitance of cap 1 or 2 in the power supply (I put a 500uF cap for C1 in one Crack I built)." [source: Bottlehead forum, Re: Choking the Crack, Reply #12, November 10, 2012, 04:15:11 PM]
  • This is something I do since I'm not using the ideal doubled-capacitance last power supply film cap...I use the same capacitance for the PS film cap as the output caps. So, with the larger capacitance PS electrolytic, it's nice to know I'm not lacking in PS filtration.
  • Actually, as you can see in my above photos, I replaced both PS electrolytics w/ 470uF caps, but it's clearly overkill... for my next build, I would just upgrade the first PS cap and return to the land of reason.
• Add 1/100 film bypass cap to each electrolytic power supply cap (C1 & C2) [<$5 for 2.2uF or 4.7uF 250V film caps]
  • For power supply electrolytics, basic film caps are all you need (orange drop, Dayton, Audyn Q4, Wima, etc.). Caps w/ beeswax and, as Audyn calls it, "fairy-like oils" don't make much sense in this application, or arguably in any audio application for that matter.
  • Many swear by bypassing PS caps, but loathe the idea of bypassing or "contaminating" output/signal caps. Try if you like and discover what you think is best. I personally haven't bypassed the output caps.
• Replace power supply rectifiers (18, 19, 20, & 21) w/ Cree Schottky diodes [$5-ish for 4 diodes & PCB]
  • Many say there's increased dynamics and clarity, but I didn't notice...maybe if I didn't already install the choke, I might've noticed?...
  • For only $5, perhaps try this before the choke and report back here if you notice any audible improvements. I'll do this for my next build.
• Replace input wires (from RCA jacks to volume pot) w/ Mogami W2549 cable, or other OFC cable/wires [$0.80/ft. for Mogami W2549, $0.50/ft. for Canare L-2T2S, and you only need about 1 ft.]
  • Some say the sound in the Crack is smoother w/ OFC...I would love to see them identify it in a blind test.
  • But this is something I will always do anyway since I have so much OFC wire/cable lying around...it's one of those "nice to haves" that don't cost much at all. For less than $1, it's comforting to know your audio signal is comfortably traveling in style!

Tier 1: Merely decorative

• Replace plastic volume knob w/ solid aluminum knob (of course has no effect on sound quality, but the weighty feel turning the knob is really nice/premium) [$5-ish]
• Replace RCA jacks w/ exotic noble-metal-plated jacks (Bottlehead upgraded their RCA jacks since 2010, the gold-plated jacks they supply now are already of great quality...I feel no need to "upgrade" them) [up to $20 or more]
• Replace tube sockets w/ gold-plated ceramic tube sockets (this is another thing I always do anyway because I like to show my NOS tubes that I appreciate them lol...another "nice to have") [$5 or more]
• Replace rubber feet w/ metal spikes (taller variants of course can supply more ventilation...but are mostly just pretty) [$15 or more]

My Recommendations

• If you have a basic Crack built and sound quality is your only concern, just work down tiers 4 & 3, and then maybe 2 if you're curious.
• If you have a basic Crack built and money is a concern, I would upgrade in stages every few months or so:
  • STAGE 1 $25: 6AS7G, pad stock volume pot, and 470uF first PS cap
  • STAGE 2 $50: 6SN7GT + adapter, and choke
  • STAGE 3 $100: Speedball kit
  • STAGE 4 $100: Valab attenuator, Mogami W2549 input cable, film output caps, Cree Schottky diodes, and film bypasses for PS electrolytics
• If cost is no object and you want the best sound this circuit has to offer ASAP, then consider getting the Crackatwoa kit instead. In stock form, it's better than the Crack w/ all the above upgrades...and would actually be cheaper overall (don't forget separate tax and shipping for all the Crack upgrades). [$269 premium for the Crackatwoa over the Crack + Speedball]
  • For upgrading the Crackatwoa, Bottlehead owner, Doc B., explained to me that all the PS upgrades for the Crack--including replacing the last PS cap with a film cap--would be pointless, as the more sophisticated, shunt voltage regulated power supply is a far better solution than all the Crack PS upgrades combined...I'm paraphrasing but I think that's accurate to what he intended. He can correct me here if I'm wrong.
  • For the Crackatwoa, the only upgrades I would do are: 6AS7G, 6SN7GT + adapter, Valab attenuator, film output caps, and gold-plated ceramic tube sockets. [around $150]
• If I were starting a new Crack build, here's how I'd go about it: basic Crack kit, gold-and-ceramic tube sockets, Mogami W2549 input cable, Valab attenuator, 470uF first PS cap, Cree Schottky diodes, Noico sound deadening material, and solid aluminum volume knob.
  • This would run you about $375. The reasoning for these upgrades is that they don't cost too much and some are easier to install right from the get-go (good luck if you decide to replace the stock tube sockets later on...). Also, these initial upgrades make the basic kit feel much more premium and make for a good foundation to install the more expensive upgrades down the road--think of it like prioritizing a good-quality motherboard for a new PC.
  • Then from here, I'd just go down the tiers as my budget allows.

Conclusion

This turned out to be longer than I expected haha, but I wanted to add details and notes that I think are important for anyone starting their Crack journey. The Crack is certainly a mainstay--being available for over a decade--and, as you can see, there are many upgrade paths you could go down, but hopefully my post here is a good primer and starting guide if you're planning your first build or first upgrades to an existing build. It should help you to not go overboard and to get the most for your money. Like I said in the intro though, this isn't comprehensive and you should always seek other forum members' inputs, so as to not take one person's word as the end-all-be-all. Good luck with your build!

Wow, this is super helpful. I'm planning my first build and will follow as much of this as I can - I already have a stepped attenuator I was considering putting in my Darkvoice, but that's got an Alps and a shunt mod so I'm pretty happy with it... and I'll probably use the fancier tube sockets I ordered for the DV here, since I am not really looking forward to trying to replace the sockets in an already-built box.

Thanks!

 

[larcenasb]

Very nice. One of the best overviews and upgrade how-to guides I've every seen for the Crack.

All you need is the same type of analysis for Tubes and you've got one of the best how-to guides in the industry.

Thanks so much for your kind words. And I actually do have a write-up for input tubes(!), you can read it here:

https://www.head-fi.org/threads/6sn7gt-6f8g-an-exploration-of-wwii-era-octal-tubes.890205/

I'll also add the link to my post above under the 6SN7GT entry.

Wow, this is super helpful. I'm planning my first build and will follow as much of this as I can - I already have a stepped attenuator I was considering putting in my Darkvoice, but that's got an Alps and a shunt mod so I'm pretty happy with it... and I'll probably use the fancier tube sockets I ordered for the DV here, since I am not really looking forward to trying to replace the sockets in an already-built box.

Thanks!

You're welcome! I'm glad I wrote it at just the right time for you. Yeah, replacing tube sockets is not a quick-and-easy task...best to do it at the start instead. I'd love to see photos of your build here once you're done! Cheers!

 

[cddc]

Bottlehead Crack Upgrading Guide

Introduction

It's been a little while for me reading or posting in this thread, but I’ve gotten a couple of messages last week about what mods most improve the sound quality of the Crack. So, I’ll post my take on the subject here in case anyone else is wondering. The mods will be ranked in tiers 4-1: tier 4 being the top and with the most obvious audible improvement, and tier 1 being no noticeable audible improvement, or merely decorative. Within each tier, the mods are further listed in order of most audible improvement...except tiers 2 and 1 because no audible improvement was discerned by me. Included are only the more popular--and reasonably affordable--mods that you’ll see on many, many builds (for example, my suggestions will favor things like a $30 Valab attenuator, not a $165 Goldpoint attenuator). This is by no means comprehensive; look at this as more of a table of contents to guide you on your exploration through this and Bottlehead's forums. If you're interested in a particular upgrade listed, Google it, along with keywords: "bottlehead" and "crack" to direct you to detailed forum posts and guides. And of course, feel free to reply/ask here if you can't find the info you need and I, or many of the other bottlehead tinkerers, will surely try to help.

Here for reference are some photos of my most recent build at various stages:







These rankings are of course just my humble opinion and with my gear (see my sig below)! However, I have built 3 Cracks since 2010, installed upgrades one at a time over the course of months, and have done a sit-down listening comparison with the Crack, Crackatwoa, SEX, and Mainline at Bottleheadquarters. So, I think I have a decent level of experience to reference when it comes to differences in sound with Bottlehead gear.

Lastly, since the list will be ranked in order of most audible improvement, price isn't factored in. So, I will use asterisks in front of the upgrades that I think represent amazing value for what you get. Let's go!

Tier 4: Obvious audible improvements

• Add Speedball upgrade (dramatically increased clarity, separation, excitement, dynamics, bass slam...) [$115]
• *Replace stock 6080 w/ 6AS7G (in general... wider soundstage, a fuller sound, more natural tone, deeper bass...) [$20-ish]
  • Changing the output tube (6080/6AS7) has a bigger effect on the overall sound compared to the input tube (12AU7/6SN7), though the input tube has many more options for tuning the signature to your precise liking.
• Replace stock 12AU7 w/ 6SN7GT, using a 6SN7-to-12AU7 6.3V adapter (in general... full-bodied vocals, more balanced across frequency range, more natural tone...and just better overall for what I consider to be lifelike sound) [a good-testing old-stock 6SN7GT can be had for around $20-30 on auction sites, $20-ish for the adapter]
  • Some history: "The main competitor of the 6SN7GT after the war was the 12AU7 and its variants. A dual version of the 6C4 triode, the 12AU7 was developed by RCA in late 1946. It used half as much heater current and had a smaller footprint than the 6SN7GT. RCA and GE (having bought Ken-Rad in 1945) heavily pushed miniature tubes after the war, and many miniatures were designed-in where octals or loctals would have been the natural choice. The higher cathode emission and plate dissipation of the 6SN7GT kept it popular, though. The combined maximum plate dissipation was upped to 7.5 watts in 1950 in the 6SN7GTA. In 1954, the 6SN7GTB added controlled heater warm-up time to make it more reliable in series heater-string TV sets. In 1954, RCA came out with the 6CG7, which was pitched as a direct equivalent of the 6SN7GT. Although audiophiles found that the 6SN7GT typically sounded better, this was the beginning of the end for widespread usage of the 6SN7GT." [source: https://www.effectrode.com/knowledge-base/the-6sn7gt-the-best-general-purpose-dual-triode/]
  • You could also use a 6F8G w/ a 6F8G-to-12AU7 6.3V adapter, but availability and price make the 6SN7GT a more viable option for most people. Also the 6SN7GT tends to be less noisy and microphonic IME...but I still love my 6F8G tubes.
• Volume pot upgrade (pick one):
  • *Pad stock pot w/ 75K & 33K Ohm resistors for -12dB of attenuation (no more channel imbalance at low volumes, more usable volume range and control) [$5-ish]
    • Only pad the volume pot if the volume gets too loud too quickly w/ your headphones. I find it does even w/ my power-hungry AKG K240 Sextetts 600 Ohm...
  • *Replace stock pot w/ 100K stereo stepped attenuator (Valab 23-step is a common choice and is what I currently use) (perfect channel balance at all levels, and, as a bonus, subtly increased clarity and separation) [$20-30 on auction sites]
    • No need to pad an attenuator...is already just a series of resistors and provides good usable volume range adjustment.
    • You may be wondering why I'm not suggesting an upgraded potentiometer like the popular Alps 100KAX2... Well, IMO, a stepped attenuator is better for the money: channel imbalance isn't a concern, you get more usable volume range (not restricted to the 7-9 o'clock positions). With the Alps you really should get a PCB and not solder wires directly to those fragile pins, you may have to cut/dremel the shaft if it's too high, and you will likely still need to pad it with resistors...I'd just rather get a stepped attenuator and enjoy the easier install and better sound. But if you like the smooth feel of a volume pot over the clicky turns of an attenuator, or if you like that the pot allows you to really fine-tune the volume instead of being limited to 23 set volume levels, then go for it.
    • Also, I've used an Audio Note 100K stereo volume pot [$35-ish] in one of my Crack builds and I was very impressed with the build quality, smooth feel of the turns, good tracking, and that it had solder lugs for easy installation. Though it would still need to be padded to get more usable volume range. It's my pick if you prefer a pot over an attenuator. And, again, there's nothing wrong with an Alps if you don't mind how it's high maintenance...You are a tinkerer after all, right!?

Tier 3: Subtle but clearly noticeable

• Replace electrolytic output caps w/ film caps (smoother treble and midrange) [anywhere from $50-100 and beyond]
  • Don't stress so much on which brand to get or how much to spend...the important thing is to have film caps, in general, handling the audio signal. Any respectable brand will do just fine. Having said that... Dayton 250V, Audyn Q4 400V, and Panasonic EZPE 500V are good value options around $50 for 2.
  • Here's a chart from the Bottlehead forums that I saved years ago. It shows bass roll-off of different output cap capacitance values depending on your headphones' impedance. So, with my AKG K240 Sextetts 600 Ohms, I could use 33uF output caps and notice no bass roll-off vs the stock 100uF caps...however, it's nice to have a little breathing room in case I want to plug in different headphones.
    
  • With the above chart in mind... I actually use 68uF output caps (Audyn Q4 68uF 400V). There are multiple reasons for this: 1) Due to the Crack's 120 Ohm output impedance, it's not recommended to use headphones lower than 300 Ohms regardless of what output capacitance is used for the output caps [search "Crack damping factor" to learn more], 2) The lower-value cap is also used as my last power supply cap, so the lower capacitance means more savings for those nicer bypass caps, 3) Higher voltage caps, like my 400V Audyns, are physically bigger and the 100uF version wouldn't fit with the way I'm mounting them.
• *Replace first 270 Ohm resistor (the one on the left terminal strip of the power supply) w/ Triad C-7X choke (better dynamics, separation, bass definition, quieter background...) [$12-ish]
• Add a higher-quality 1/100 film cap as a bypass on the last power supply film cap (requires replacing last power supply electrolytic cap w/ film cap, see tier 3) (adjusts the sound signature to your liking) [from $20 on upwards]
  • Bottlehead designer, Paul Joppa has said, "There are no reliable 'rules' about bypassing power supply caps. Sometimes they help, sometimes they hinder, and sometimes you can't tell the difference. The '1% rule' is really more like 'something between 0.1% and 10%' and even then exceptions are not at all uncommon...The only reliably useful answer is 'try it and see!'" [source: Bottlehead forum, Re: Power supply bypass cap values, Reply #15, May 22, 2013, 08:20:51 PM]
  • So, for my 68uF last PS cap, I would prefer a 0.68uF bypass, but wouldn't fret at all if the cap I wanted to try was only available in 0.47uF or 1uF. Even .068uF or 6.8uF, as Mr. Joppa explained, are worth a try.
• Add sound deadening material (Dynamat, Kilmat, Noico, MAT66, etc.) to underside of top-plate (reduces microphonics when turning on amp, adjusting volume, tapping top plate if you ever do that lol, etc. ...just improves the user experience and adds a dense, quality feel to your amp) [<$20]
  • You don't need full coverage... Like with sound-deadening car doors, follow the 25% rule to avoid diminishing returns. I apply larger pieces where I can and focus on putting smaller pieces all around the tube sockets, volume pot/attenuator, and the headphone socket. See middle-bottom photo at top of post.

Tier 2: I’d like to think I hear a difference, but it’s more placebo than anything

• Replace last power supply cap (C3, the sideways one) w/ film cap (ideally, double the capacitance of the output caps--100uF output caps, 220uF last PS cap--but the same value--100uF for outputs and 100uF for last PS--are commonly used due to space constraints) [$20-30 for Dayton, Audyn, or Panasonic]
  • Paul Joppa has said, "The output audio current flows through the power supply, mostly the last capacitor, as well as through the output coupling cap and the output triode. So the final power supply cap should, in theory, be as important as the output coupling cap with regard to the sound of the amp." [source: Bottlehead forum, Re: Upgrading the power supply capacitors in the Crack, Reply #4, April 18, 2011, 08:08:44 AM]
  • It was hard for me to notice, but then bypassing it w/ a higher-quality cap was clearly noticeable. So, this is an important upgrade to me as a gateway to bypassing it. On its own though, I can't say I can tell the difference for sure.
• Replace the 220uF 250V first power supply cap (C1, beside RCA jacks) w/ 470uF 250V electrolytic cap (more power supply filtration) [$5-ish for Nichicon KX, which are "great for power supply upgrades," according to hificollective]
  • Bottlehead designer, Paul Birkeland, has said, "...there's nothing wrong with increasing the capacitance of cap 1 or 2 in the power supply (I put a 500uF cap for C1 in one Crack I built)." [source: Bottlehead forum, Re: Choking the Crack, Reply #12, November 10, 2012, 04:15:11 PM]
  • This is something I do since I'm not using the ideal doubled-capacitance last power supply film cap...I use the same capacitance for the PS film cap as the output caps. So, with the larger capacitance PS electrolytic, it's nice to know I'm not lacking in PS filtration.
  • Actually, as you can see in my above photos, I replaced both PS electrolytics w/ 470uF caps, but it's clearly overkill... for my next build, I would just upgrade the first PS cap and return to the land of reason.
• Add 1/100 film bypass cap to each electrolytic power supply cap (C1 & C2) [<$5 for 2.2uF or 4.7uF 250V film caps]
  • For power supply electrolytics, basic film caps are all you need (orange drop, Dayton, Audyn Q4, Wima, etc.). Caps w/ beeswax and, as Audyn calls it, "fairy-like oils" don't make much sense in this application, or arguably in any audio application for that matter.
  • Many swear by bypassing PS caps, but loathe the idea of bypassing or "contaminating" output/signal caps. Try if you like and discover what you think is best. I personally haven't bypassed the output caps.
• Replace power supply rectifiers (18, 19, 20, & 21) w/ Cree Schottky diodes [$5-ish for 4 diodes & PCB]
  • Many say there's increased dynamics and clarity, but I didn't notice...maybe if I didn't already install the choke, I might've noticed?...
  • For only $5, perhaps try this before the choke and report back here if you notice any audible improvements. I'll do this for my next build.
• Replace input wires (from RCA jacks to volume pot) w/ Mogami W2549 cable, or other OFC cable/wires [$0.80/ft. for Mogami W2549, $0.50/ft. for Canare L-2T2S, and you only need about 1 ft.]
  • Some say the sound in the Crack is smoother w/ OFC...I would love to see them identify it in a blind test.
  • But this is something I will always do anyway since I have so much OFC wire/cable lying around...it's one of those "nice to haves" that don't cost much at all. For less than $1, it's comforting to know your audio signal is comfortably traveling in style!

Tier 1: Merely decorative

• Replace plastic volume knob w/ solid aluminum knob (of course has no effect on sound quality, but the weighty feel turning the knob is really nice/premium) [$5-ish]
• Replace RCA jacks w/ exotic noble-metal-plated jacks (Bottlehead upgraded their RCA jacks since 2010, the gold-plated jacks they supply now are already of great quality...I feel no need to "upgrade" them) [up to $20 or more]
• Replace tube sockets w/ gold-plated ceramic tube sockets (this is another thing I always do anyway because I like to show my NOS tubes that I appreciate them lol...another "nice to have") [$5 or more]
• Replace rubber feet w/ metal spikes (taller variants of course can supply more ventilation...but are mostly just pretty) [$15 or more]

My Recommendations

• If you have a basic Crack built and sound quality is your only concern, just work down tiers 4 & 3, and then maybe 2 if you're curious.
• If you have a basic Crack built and money is a concern, I would upgrade in stages every few months or so:
  • STAGE 1 $25: 6AS7G, pad stock volume pot, and 470uF first PS cap
  • STAGE 2 $50: 6SN7GT + adapter, and choke
  • STAGE 3 $100: Speedball kit
  • STAGE 4 $100: Valab attenuator, Mogami W2549 input cable, film output caps, Cree Schottky diodes, and film bypasses for PS electrolytics
• If cost is no object and you want the best sound this circuit has to offer ASAP, then consider getting the Crackatwoa kit instead. In stock form, it's better than the Crack w/ all the above upgrades...and would actually be cheaper overall (don't forget separate tax and shipping for all the Crack upgrades). [$269 premium for the Crackatwoa over the Crack + Speedball]
  • For upgrading the Crackatwoa, Bottlehead owner, Doc B., explained to me that all the PS upgrades for the Crack--including replacing the last PS cap with a film cap--would be pointless, as the more sophisticated, shunt voltage regulated power supply is a far better solution than all the Crack PS upgrades combined...I'm paraphrasing but I think that's accurate to what he intended. He can correct me here if I'm wrong.
  • For the Crackatwoa, the only upgrades I would do are: 6AS7G, 6SN7GT + adapter, Valab attenuator, film output caps, and gold-plated ceramic tube sockets. [around $150]
• If I were starting a new Crack build, here's how I'd go about it: basic Crack kit, gold-and-ceramic tube sockets, Mogami W2549 input cable, Valab attenuator, 470uF first PS cap, Cree Schottky diodes, Noico sound deadening material, and solid aluminum volume knob.
  • This would run you about $375. The reasoning for these upgrades is that they don't cost too much and some are easier to install right from the get-go (good luck if you decide to replace the stock tube sockets later on...). Also, these initial upgrades make the basic kit feel much more premium and make for a good foundation to install the more expensive upgrades down the road--think of it like prioritizing a good-quality motherboard for a new PC.
  • Then from here, I'd just go down the tiers as my budget allows.

Conclusion

This turned out to be longer than I expected haha, but I wanted to add details and notes that I think are important for anyone starting their Crack journey. The Crack is certainly a mainstay--being available for over a decade--and, as you can see, there are many upgrade paths you could go down, but hopefully my post here is a good primer and starting guide if you're planning your first build or first upgrades to an existing build. It should help you to not go overboard and to get the most for your money. Like I said in the intro though, this isn't comprehensive and you should always seek other forum members' inputs, so as to not take one person's word as the end-all-be-all. Good luck with your build!

Excellent write-up, easily one of the best in the Crack thread!

I was thinking why you need to pad the stock 100K pot with some resistor, while skipping it on the same 100K stepped attenuator? Is it because the 100K stepped attenuator is on logarithmic scale and the stock pot is on linear scale?

 

[cddc]

Regarding tubes and mods that go well with them, I'd ike to add my two cents.
I know this is probably a contentious issue, but my compulsive A/B tests seem to indicate that tubes do very well with carbon composite resistors.

The difference (or improvement in my opinion) is not that huge, but it is there.
They seem to add further to the 'tubiness' of the sound.
By 'tubiness', I mean the subjective effect that comes along with tubes, especially in zero-feedback, single ended topologies.
It certainly doesn't hurt trying them out, despite the 'bad press' they've been getting from people compared to other types like metal-film resistors etc. The only issue is that they are getting harder to come by (and cost accordingly) and the current production is limited to 250mW and 500mW power ratings only.

Larger wattage carbon films are also a good match, especially when you need something above the 500mW mark.

If I had to make a guess of what's going on, I'd say they might add slightly more second order harmonics which seems to be the reason tubes can sound so unique.
But that is just me speculating so it might be something else entirely.

Interesting finding!

I guess you'll have to find some new production carbon composite resistors, as carbon composite resistors are notorious for drifting. The ones recycled from antique radios normally drifted miles away.

 

[larcenasb]

Excellent write-up, easily one of the best in the Crack thread!

I was thinking why you need to pad the stock 100K pot with some resistor, while skipping it on the same 100K stepped attenuator? Is it because the 100K stepped attenuator is on logarithmic scale and the stock pot is on linear scale?

Thanks! And about not padding the attenuator... The Alps pot I had (100KAX2) was logarithmic but still needed to be padded. So, I don't know but I'd love to see a reply on this from some of the more technical head-fiers here. Anyone?

 

[evonimos]

I guess you'll have to find some new production carbon composite resistors, as carbon composite resistors are notorious for drifting. The ones recycled from antique radios normally drifted miles away.

I only use current production.
Usually by Ohmite (USA) or Kamaya (Japan), depending on availability.
They are both excellent, but cost 2-3 times more than film types.

Vintage stuff like Allen Bradley are way too much money and trouble for my projects at the moment.

However, the current production ones are getting increasingly harder to come by.
Also, all current production is limited to 250mW or 500mW power ratings and 10% or 5% accuracy.
Strangely this seems to be shared among all the manufacturers left making them.

 

[Jimmyblues1959]

I finally got around to uploading some photos of my Crack......

External:


Upskirt, mostly stock:


Plus Speedball and film output caps:


Highlights:
TKD 2CP2511 50kohm pot
50kohm 1W PRP resistors in series with the pot for approx -6dB pre-attenuation
Speedball with a couple of resistor upgrades
Axon 91µF/250V film output coupling caps
Cardas RCA jacks (rhodium plating is a BITCH to solder)
Teflon tube sockets plus PCBs (Woo Audio branded)
2.2µF Axon film bypass cap on the last electrolytic filter cap
Better PSU filter resistors
PRP resistors for bleeding DC offset
Neutrik jack with chrome ferrule

The wood was finished with Watco Danish Oil in cherry, while the top plate and transformer cover were finished with Krylon hammered metal finish. All the screws were replaced with stainless steel socket cap jobbies. It is currently running a JJ ECC802S and a Cetron branded 5998. With the HD650, it gives a very different presentation to what I am used to from the AD900 or SR-Lambda, but it is definitely growing on me - particularly for anything with brass or woodwind. All in all a great intro to PTP and tubes!

Nice clean build. One of the nicest finished Bottlehead Cracks I have seen.

 

[Bruc3]

Hi all,
After many Years of wanting one of these and never getting around to actually buying one, a friend may be selling his to me with stock tubes.

I plan to get the TS5998 as power tube, but what is a good/recommended driver tube?

I like nice vocals/mids.

 

[DenverW]

So, I am going to entitle this post: The Frustration with the Crackatwoa.

Having built and modded a crack and speedball. I've really been excited to get my hands on a crackatwoa kit. Black friday sale was just the ticket, and I've been taking my time (cause I have no time) to get to the build. Worked the base...stained...lacquer...looks pretty good for my usually screw ups.

First issue comes from mounting the sockets into the chassis. One of the sockets is too large. Since I'm a manly man, I try to force it. It crumbles in my hand. Well, that was a mistake. I order a new socket.

Meanwhile I finish mounting the other components, and wire the power supply. First power test! Zippo. No power. Is my volt-o-meter broken? Nope. After troubleshooting, resoldering all the connections, and getting annoyed, I guess that its a faulty fuse. Amazon to the rescue. I order more fuses.

Fuses arrive two days later. I was right, the fuse I was sent was bad. Everything tests within the parameters. I continue wiring.

Socket arrives! I determine that the hole in the chassis was cut too small. I spend a significant amount of time shaving and sanding out the chassis to get the socket to fit. Finally it does. I don't break this one (yet).

Ok, time to finish mounting the stand offs around the last socket. Well guess what? I'm short two screws. Before you ask, YES, i'm positive I'm short, as I put all the components in an ice cube tray with a plastic cover to keep them separate and secure. All screws gone. Two screws needed. Tomorrow will be Home Depot.

I love Bottlehead. This build is cursed though. If you're thinking of building the crackatwoa be aware that its not a step up in complication from the crack, its a LEAP.

Editing this post: The rectifiers that I thought were missing were packed in with the c4s board, so it was my fault for assuming the packaging for the c4s didn't have additional parts. Maybe not the place I would have packed them, but its a good learning experience to look in every plastic bag, even if one is clearly for a specific part of the build.

 

[markkr]

My crackatwoa build was difficult to say the least, first the skill level is about 10 notches higher than a regular Crack... I was missing 4 screws... and ran out of wire due to my attempt at “clean wiring”. Also the 7 pin sockets are very loose, the tubes are essentially “rattling” in the looseness of the socket. I had a bad wire which required disgnosis as well as a mistake that I made in the regular potentiometer.

Tonight I wired up the two-quiet... that was quite a chore but I cant test it yet since I’m awaiting 2 new 7 pin tube sockets.

None of this is BH’s fault, 4 missing screws is no big deal... DIY always has challenges. Prior to the two-quiet, the amp sounded fantastic so I expect even better things with the upgrade. I also installed a pair of Mundorf caps for good measure.

 

[DenverW]

My crackatwoa build was difficult to say the least, first the skill level is about 10 notches higher than a regular Crack... I was missing 4 screws... and ran out of wire due to my attempt at “clean wiring”. Also the 7 pin sockets are very loose, the tubes are essentially “rattling” in the looseness of the socket. I had a bad wire which required disgnosis as well as a mistake that I made in the regular potentiometer.

Tonight I wired up the two-quiet... that was quite a chore but I cant test it yet since I’m awaiting 2 new 7 pin tube sockets.

None of this is BH’s fault, 4 missing screws is no big deal... DIY always has challenges. Prior to the two-quiet, the amp sounded fantastic so I expect even better things with the upgrade. I also installed a pair of Mundorf caps for good measure.

Let me know what you think of the mundorf caps; I assume you're replacing the 100uf? I'm considering those, but I've heard they're a bit warm, so I'm also looking at some clarity cap CSA that are on sale.

I love BH, and nothing that's happened will stop my love for them, but I do get frustrated when I want to work on the amp with limited time and end up having to stop for delays from missing or faulty parts. The screws werent a big deal for me either, I got some the next day at Home Depot, but waiting on a fuse took a couple days, and now I'm waiting on the missing rectifiers. I'm super slow, so these delays just make things take longer and longer.

 

[cddc]

Hi all,
After many Years of wanting one of these and never getting around to actually buying one, a friend may be selling his to me with stock tubes.

I plan to get the TS5998 as power tube, but what is a good/recommended driver tube?

I like nice vocals/mids.

Get the Mullard and Amperex, but don't go for Telefunken, Telefunken is clean & clear sounding, but kind of lean to my taste.