[DDDamian]

^^ this

 

[Mich4lle]

   

 
I have so much penis envy for your crack, that is the most beautiful wiring I have ever seen, well done.
I am just curious how much of an effect that volume pot has on the sound over the stock one, it does look a lot better, but does it sound better?
The same question goes for the tube sockets.
But good job on the crack, it looks great.

 

[FunyunBreath]

   
No, it doesn't work like that. A volume pot is a divider - so at 'half' the volume knob, approximately half of the source voltage gets through, regardless of the value of the pot. Using a different value pot doesn't just mean you can turn the volume down more or less.
 
The only difference would arise if the pots have a different attenuation curve. But unless something is borked, or one was audio and the other linear, there will be no difference.

 
Good to know, thanks for posting What would the different affects of a 25k, 50k, or 100k pot be on the system then?
 

   
I have so much penis envy for your crack, that is the most beautiful wiring I have ever seen, well done.
I am just curious how much of an effect that volume pot has on the sound over the stock one, it does look a lot better, but does it sound better?
The same question goes for the tube sockets.
But good job on the crack, it looks great.

 
Thanks! I can't do a direct a/b test with the Alps pot vs. stock but my other Crack has a Valab 100k stepped attenuator and I don't notice much difference in clarity. I much prefer the buttery smooth tracking of the Alps pot over the hard clicks on the attenuator.
 
The biggest reason I went with the Alps pot over the 100k stock pot is the Alps tracks perfectly at lower volumes. Whereas the stock pot had a channel imbalance until the volume is around a medium listening level. The stock pot also tends to collect dust easily and over time starts to get that scratchy sound when turned.

 

[krikor]

   
No, it doesn't work like that. A volume pot is a divider - so at 'half' the volume knob, approximately half of the source voltage gets through, regardless of the value of the pot. Using a different value pot doesn't just mean you can turn the volume down more or less.
 
The only difference would arise if the pots have a different attenuation curve. But unless something is borked, or one was audio and the other linear, there will be no difference.

 
Thanks for this tidbit... helps me better understand how these things work as well. 
 

  What would the different affects of a 25k, 50k, or 100k pot be on the system then?
 

 
I'll venture a guess that it changes the input impedance of the amp to 25k, 50k and 100k respectively. I'd also expect it to have some effect on the input tube since the pot acts as both a grid stopper resistor and grid-to-ground resistor.
 
Caveat - I know just enough to convincingly sound like I know what I'm talking about (which really pisses my wife off), so take this with a grain of salt. I did find some good explanations of voltage dividers and grid resistors at the Aiken guitar amplifier website:
 

1. White papers: http://www.aikenamps.com/index.php/white-papers#
2. The Voltage Divider Rule: http://www.aikenamps.com/index.php/the-voltage-divider-rule
3. Grid Resistors - Why Are They Used: http://www.aikenamps.com/index.php/grid-resistors-why-are-they-used

 
 
So why does this interest me?
 
Well, I've been contemplating ditching the Crack's volume pot and using an external passive attenuator (Bottlehead Submissive or the Tortuga LDR). As a trial, I've been doing this with another preamp I have on hand by simply cranking the volume pot to max to effectively bypass the pot (zero resistance in the signal path). However, I've noticed some rising static/noise at the very end of the pots travel.
 
I live about a mile from 13,500 watt radio tower transmitting 6 different stations' signals. This has caused problems for me with other  headphones/amps (most notably the Schiit Fulla/AKG K7XX combo) in which I've been able to actually "tune in" a station well enough to hear it depending on how I arrange the cords and tilt my head.
 
My theory... based on the white papers linked above, cranking the volume all the way up reduces what is the "grid stopper" resistance (the pot resistance that is in series with the 12AU7) to zero. Since grid stoppers "act as a very high frequency low-pass filter," I'm basically opening up my Crack up to RFI (wait, that sounds bad).
 
Do I have these concepts correct?
 
The long and short of it... are there ideal resistor values for the input voltage divider when using the Crack amp sans volume control? I've seen it recommended at the Bottlhead forum that a simple 100k grid-to-ground resistor is all that's needed (no series grid stopper), but is this the ideal situation? Or is mine a unique situation that requires a voltage divider?
 
Thanks.
 
I should probably just cut to the chase post this over at the Bottlhead forum 

 

 

[Beefy]

  I'll venture a guess that it changes the input impedance of the amp to 25k, 50k and 100k respectively. I'd also expect it to have some effect on the input tube since the pot acts as both a grid stopper resistor and grid-to-ground resistor. 

 
Yep, this. The thing that sees the biggest difference is really the source though. Some tube sources like a high impedance because they can't run a lot of current. Solid state amps are less picky.
 
I am less sure about how the pot alters the input stage of the amp, but unless you go really crazy low it shouldn't make any audible difference. But if you want to swap out for something more boutique, definitely ask at Bottlehead......

 

[hdtv00]

Did I just score a Western Electric 421a for $175. It tested results at bottom does it look great test wise?

 

 

 


 

 

[JamieMcC]

  Did I just score a Western Electric 421a for $175. It tested results at bottom does it look great test wise?

 

 

 


 

 
If it is not a WE421a it will be Tung sol 5998 clear top from the early 50's I think? I have a couple and they look identical to my WE421a I actually prefer the clear tops and find them a bit more  musical and noticeably different to latter silver topped 5998's .

 

[Maxhawk]

I posted this on the Bottlehead forum, but there seems to be more activity here so I thought I'd repeat it.
 
This thread (http://bottlehead.com/smf/index.php?topic=5989.131) on the Bottlehead forum describes methods to mod the Crack to optimize bias for various tube types. I had added a switch and resistors to allow use of the 12BH7 as well as E80CC/5687 but wanted something neater and consequently more reliable long-term. Here's the before shot:

 
 
I decided to make my own PCB, integrating a DPDT switch and resistors, and condensing the dual boards into a single PCB.
Here's the bare PCB:

 

And stuffed with parts:

 

Installed in place of the 2 separate boards:

 

The first try wasn't perfect, as the mounting holes needed some dremeling to allow the PCB to clear the standoff from the other PCB. Also some of the pads are on the small side and the holes for the LEDs are too large, but it did work first time.
 
After spending more time switching between the 12BH7, and 5687, I find that the 5687 excels in imaging and placement, but is down on bass compared to the 12BH7.
 
I had to buy 10 pcs minimum and I've only got one Crack, so I've got extras if anyone is interested.

 

[DDDamian]

   
If it is not a WE421a it will be Tung sol 5998 clear top from the early 50's I think? I have a couple and they look identical to my WE421a I actually prefer the clear tops and find them a bit more  musical and noticeably different to latter silver topped 5998's .

Interesting on the difference but maybe not surprising - tubes of the same batches may sound slightly different let alone with design differences. Here's my Tung Sol 5998 silver-topped. Love it.
 

​

 

[FunyunBreath]

5998 is a great tube, I'm really liking the Tung Sol 7236 as well. Similar, but a bit cleaner in the bass/low mids and with better imaging I think.
 

 
On another note, as you can see I currently have 2 Cracks sitting on my desk, one speedball'd and the other not
 
I've been comparing each with different tube combos and surprised to find that I'm actually liking the sound of the stock crack a bit more so far. The speedball makes for a tighter sound, especially with drums and cymbals, but there's a natural reverb to the stock crack that the speedball doesn't have.
 
Going to keep doing some tube combo shoot-outs to see whether I'm going to install the speedball in my new build or not.

 

[DDDamian]

Going to keep doing some tube combo shoot-outs to see whether I'm going to install the speedball in my new build or not.

 
Keep us posted

 

[grausch]

   
I had to buy 10 pcs minimum and I've only got one Crack, so I've got extras if anyone is interested.

 
I saw this on the Bottlehead forums, but since I really had no clue about the mods I did not want to comment. However, this is a great idea and definitely the cleanest way of implementing the switches that I have seen.
 
How much wider is the pcb vs the standard Speedball pcb? Reason I am asking is I have the VALabs stepped attentuator and the Speedball will barely fit with that in there. If it needs any more space, then it will need to either to be such a pcb or the attentuator.

 

[Maxhawk]

It spans the same width at 2.75". Let me know if yours measures differently.

 

[i luvmusic 2]

  I posted this on the Bottlehead forum, but there seems to be more activity here so I thought I'd repeat it.
 
This thread (http://bottlehead.com/smf/index.php?topic=5989.131) on the Bottlehead forum describes methods to mod the Crack to optimize bias for various tube types. I had added a switch and resistors to allow use of the 12BH7 as well as E80CC/5687 but wanted something neater and consequently more reliable long-term. Here's the before shot:

 
 
I decided to make my own PCB, integrating a DPDT switch and resistors, and condensing the dual boards into a single PCB.
Here's the bare PCB:

 

And stuffed with parts:

 

Installed in place of the 2 separate boards:

 

The first try wasn't perfect, as the mounting holes needed some dremeling to allow the PCB to clear the standoff from the other PCB. Also some of the pads are on the small side and the holes for the LEDs are too large, but it did work first time.
 
After spending more time switching between the 12BH7, and 5687, I find that the 5687 excels in imaging and placement, but is down on bass compared to the 12BH7.
 
I had to buy 10 pcs minimum and I've only got one Crack, so I've got extras if anyone is interested.

NICE!

 

[bigfatpaulie]

  I posted this on the Bottlehead forum, but there seems to be more activity here so I thought I'd repeat it.
 
This thread (http://bottlehead.com/smf/index.php?topic=5989.131) on the Bottlehead forum describes methods to mod the Crack to optimize bias for various tube types. I had added a switch and resistors to allow use of the 12BH7 as well as E80CC/5687 but wanted something neater and consequently more reliable long-term. Here's the before shot:
 
 
 
I decided to make my own PCB, integrating a DPDT switch and resistors, and condensing the dual boards into a single PCB.
Here's the bare PCB:
 
 

And stuffed with parts:
 
 

Installed in place of the 2 separate boards:
 
 

The first try wasn't perfect, as the mounting holes needed some dremeling to allow the PCB to clear the standoff from the other PCB. Also some of the pads are on the small side and the holes for the LEDs are too large, but it did work first time.
 
After spending more time switching between the 12BH7, and 5687, I find that the 5687 excels in imaging and placement, but is down on bass compared to the 12BH7.
 
I had to buy 10 pcs minimum and I've only got one Crack, so I've got extras if anyone is interested.

 
 
May I be the first to extend a very warm welcome to Head-fi 

 
I very much look forward to your future contributions.
 
-Paul