[cddc]

I have mixed feelings about the negative feedback (NFB)

I do read some folks do not like NFB. Though NFB lowers distortion and improves various measurements, some folks think NFB reduces the speed of the circuit, hence transient is more sluggish. Imagining a boxer is trying to hit the punchbag with his fist, but suddenly some heavy gel (like NFB) fills the space between his fist and the punchbag, now the more force the boxer applies to his fist the more pushback from the gel his fist will receive, so he'll never hit the punchbag as fast as if there's no such drag from the gel.

But a cathode follower OTL that operates at 100% local internal NFB does sound very good, just like the BHC. So I think I have no problem with some local NFB, besides, the cathode resistor always comes with some NFB if not bypassed. The global NFB with some flying leads is somewhat more difficult for me to swallow though, but in the end if the implementation does sound very good, I think I'll still be fine with it.

I have never spent too much time on SS components, but I remember I read that BJTs and Mosfets have worse linearity than tubes, so there is always lots of NFB used in SS amplification circuits to improve distortion.

 

[L0rdGwyn]

@cddc it is all about implementation! My main system amplifier uses some small amount of local NFB to lower the output impedance, it is very dynamic! I think the bad reputation comes from poor circuits that use very high degrees of global NFB. Yes, BJT and MOSFETs have characteristics similar to pentodes and often use negative feedback. Used as a source follower, a MOSFET also operates at 100% NFB, so very similar characteristics to a cathode follower - low distortion, low output impedance, no gain. I use a SiC MOSFET source follower in another amplifier, a hybrid design with a tube gain stage. They can sound quite good if done well! There is one transistor that is very interesting, the VFET (aka SIT), which actually has triode-like curves, making it very much usable in class A designs that do not use any NFB, much like class A tube amplifiers. These are popular right now in the DIY community, popularized by Nelson Pass in his First Watt line, but they are all NOS and numbers are dwindling. I bought some so I have them around when I am ready to build something with them



And the curves! Like from a triode datasheet.

 

[Wes S]

Just in case any BHC owners are looking for some amazing and rare driver tubes, I have a few listed in the classifieds. I have several really nice early Hamburg Valvo Long Plates listed, that were one of my favorite tubes in the BHC.

Cheers!

 

[wabibito]

Does anyone know how to replace the PT-3 from the crack with a PT-7? I heard this is possible but PT7 is missing the terminal 9.

 

[CaptainFantastic]

In some of the more recent posts above there have been queries on whether it is too risky to buy an already assembled BHC. The risks would generally be build quality and safety of the unit during shipment. I would like to share my very positive experience with a BHC project I pursued with Tom-s, a fellow member of these forums. As we are both based in Benelux, this is more directed toward the European crowd.

For those short on time the summary in one paragraph is this:

It was a very fun project where we collaborated in picking the mods and options available at all points along the way. Tom did the build, the painting of the steel plate and a great super-safe shipping job. I could not be happier with the result. If you are in the EU and interested in getting a Crack built, send Tom a message, depending on his availability and what you’re looking for it might lead to an interesting and engaging project like mine was. And you will have a great sounding tube amp that outperforms what else I’ve heard in the sub-1k range.

The details:

I have zero experience with soldering and do-it-yourself projects in electronics. So despite being very interested in what the BHC has to offer, I always kept away knowing that I would not entertain the idea of building one myself. I was interested in the BHC because my stable of headphones is perfectly suited for it – mainly 300Ω Sennheisers and a ZMF, soon two.

My goal has been for a while to have a high performing solid state amp and a very enjoyable tube amp. For the tube amp I had the Woo Audio WA3, an amp I enjoyed a lot and acquired some very good tubes for. In fact, I enjoyed it so much that I owned two of them. And it was in the context of tubes for the WA3 that I started speaking with Tom. What this particular BHC-build was promising over the stock WA3 was hand-picked upgraded components and therefore the hope to take the performance up a notch or even two in the same compact format.

We discussed the build in detail and chose the mods together. One particular feature I wanted to have was the ability to use my E88CC tubes. Tom therefore installed a switch that allows three different current settings for the driver tube bias - to optimize it for 6SN7 (12AU7, 6J5, 76) first and E88CC (12BH7) second. We also went with a 6SN7 socket as standard, a nice mod since most of the time I run 6SN7s not E88CCs or 12AU7s.

The other main mods were:

• Audio Grade 21 steps DACT Type Stepped Attenuator
• Mundorf MCap Classic 250V 100 uF ±5%
• Hammond Choke 158M
• Schottky diodes
• PRP Audio Metal Film Resistors
• and upgraded internal wiring to silver plated solid core copper

Tom completed the build over the course of something like two-three weeks, which included the painting of the plate to the color/finish I chose and also did a few days of testing that everything works well (note – I did the wood treatment myself, hence I am responsible for the imperfections on that front as seen in the pictures, but it was still enjoyable/personal and I now know what I would do slightly differently next time). This project was never about getting it done quickly. We knew from ordering the parts from the U.S., to choosing and sourcing the mods, to the build, that this would be about taking it easy and advancing with the right decisions and quality rather than speed.

The shipment from Tom to me required particular care to protect all the internal components. He used a cloth material wrapped/molded almost on the unit which ensured zero movement inside. It arrived in top shape and (out of an abundance of caution) we even did a one hour video session together where I ran through a number of tests of the unit with a multimeter at Tom’s direction to ensure nothing moved somehow during shipping. Everything checked out.

So what are the results:

• Almost 3x the tube rolling possibilities with 12AU7s and 6SN7s now added to my E88CCs.
• A sense of ownership of a custom-built amp that is much greater than toward the off-the-shelf units.
• Satisfying, rich sound with deeper impact while maintaining good clarity. It’s not the transparency and detail of the TT2, of course, but I listen to the TT2 50% of the time and to this great tube sound the other 50% of the time. As someone else said in the TT2 forum recently “audiophile terms are squishy” and “getting into the weeds with actual adjectives” sometimes promotes misunderstanding and blowback. So I won’t go deeper into this, but to say that if you are interested in the BHC you have surely researched it and know what it promises to deliver. Well, imagine that stock BHC very nicely upgraded as detailed above… even better.
• Tube hum and microphonics depend on the tube, but I understand that sometimes the amp can accentuate the issues as well. I am very pleased that as we built it, this BHC sounds clean, even when running an E88CC tube.

Hope this “review” is helpful to others and a big thanks to Tom for the project.

 

[HTSkywalker]

In some of the more recent posts above there have been queries on whether it is too risky to buy an already assembled BHC. The risks would generally be build quality and safety of the unit during shipment. I would like to share my very positive experience with a BHC project I pursued with Tom-s, a fellow member of these forums. As we are both based in Benelux, this is more directed toward the European crowd.

For those short on time the summary in one paragraph is this:

It was a very fun project where we collaborated in picking the mods and options available at all points along the way. Tom did the build, the painting of the steel plate and a great super-safe shipping job. I could not be happier with the result. If you are in the EU and interested in getting a Crack built, send Tom a message, depending on his availability and what you’re looking for it might lead to an interesting and engaging project like mine was. And you will have a great sounding tube amp that outperforms what else I’ve heard in the sub-1k range.

The details:

I have zero experience with soldering and do-it-yourself projects in electronics. So despite being very interested in what the BHC has to offer, I always kept away knowing that I would not entertain the idea of building one myself. I was interested in the BHC because my stable of headphones is perfectly suited for it – mainly 300Ω Sennheisers and a ZMF, soon two.

My goal has been for a while to have a high performing solid state amp and a very enjoyable tube amp. For the tube amp I had the Woo Audio WA3, an amp I enjoyed a lot and acquired some very good tubes for. In fact, I enjoyed it so much that I owned two of them. And it was in the context of tubes for the WA3 that I started speaking with Tom. What this particular BHC-build was promising over the stock WA3 was hand-picked upgraded components and therefore the hope to take the performance up a notch or even two in the same compact format.

We discussed the build in detail and chose the mods together. One particular feature I wanted to have was the ability to use my E88CC tubes. Tom therefore installed a switch that allows three different current settings for the driver tube bias - to optimize it for 6SN7 (12AU7, 6J5, 76) first and E88CC (12BH7) second. We also went with a 6SN7 socket as standard, a nice mod since most of the time I run 6SN7s not E88CCs or 12AU7s.

The other main mods were:

• Audio Grade 21 steps DACT Type Stepped Attenuator
• Mundorf MCap Classic 250V 100 uF ±5%
• Hammond Choke 158M
• Schottky diodes
• PRP Audio Metal Film Resistors
• and upgraded internal wiring to silver plated solid core copper

Tom completed the build over the course of something like two-three weeks, which included the painting of the plate to the color/finish I chose and also did a few days of testing that everything works well (note – I did the wood treatment myself, hence I am responsible for the imperfections on that front as seen in the pictures, but it was still enjoyable/personal and I now know what I would do slightly differently next time). This project was never about getting it done quickly. We knew from ordering the parts from the U.S., to choosing and sourcing the mods, to the build, that this would be about taking it easy and advancing with the right decisions and quality rather than speed.

The shipment from Tom to me required particular care to protect all the internal components. He used a cloth material wrapped/molded almost on the unit which ensured zero movement inside. It arrived in top shape and (out of an abundance of caution) we even did a one hour video session together where I ran through a number of tests of the unit with a multimeter at Tom’s direction to ensure nothing moved somehow during shipping. Everything checked out.

So what are the results:

• Almost 3x the tube rolling possibilities with 12AU7s and 6SN7s now added to my E88CCs.
• A sense of ownership of a custom-built amp that is much greater than toward the off-the-shelf units.
• Satisfying, rich sound with deeper impact while maintaining good clarity. It’s not the transparency and detail of the TT2, of course, but I listen to the TT2 50% of the time and to this great tube sound the other 50% of the time. As someone else said in the TT2 forum recently “audiophile terms are squishy” and “getting into the weeds with actual adjectives” sometimes promotes misunderstanding and blowback. So I won’t go deeper into this, but to say that if you are interested in the BHC you have surely researched it and know what it promises to deliver. Well, imagine that stock BHC very nicely upgraded as detailed above… even better.
• Tube hum and microphonics depend on the tube, but I understand that sometimes the amp can accentuate the issues as well. I am very pleased that as we built it, this BHC sounds clean, even when running an E88CC tube.

Hope this “review” is helpful to others and a big thanks to Tom for the project.

Good job there

 

[cddc]

In some of the more recent posts above there have been queries on whether it is too risky to buy an already assembled BHC.

I thought you'd have talked a little bit about how your $200+ GEC 6AS7G got toasted on the already assembled BHC, as you started your article with the above statement. To my surprise, there is none.

Before - clear glass:

After - smoked glass:

 

[CaptainFantastic]

@cddc I didn't talk about it because this was about the amp, not about that particular tube. I bought two of those 6AS7Gs from Billington (so a trusted source - but they certainly didn't test them in amps), One did this, the other did not. None of my other power tubes had any issues and all testing showed it was the tube, not the amp. By the way, the tube works great still and I use it. It just looks "different".

 

[CaptainFantastic]

I thought you'd have talked a little bit about how your $200+ GEC 6AS7G got toasted on the already assembled BHC, as you started your article with the above statement. To my surprise, there is none.

Before - clear glass:

After - smoked glass:

By the way, am I getting old, or the top picture is not mine. Did you own this particular tube before me?

 

[cddc]

By the way, am I getting old, or the top picture is not mine. Did you own this particular tube before me?

No, the top picture is from the web just for comparison, I have several GEC 6080's and A2293's, but I prefer 7236/5998/421A, so I don't want to go this route.

 

[cddc]

@cddc I didn't talk about it because this was about the amp, not about that particular tube. I bought two of those 6AS7Gs from Billington (so a trusted source - but they certainly didn't test them in amps), One did this, the other did not. None of my other power tubes had any issues and all testing showed it was the tube, not the amp. By the way, the tube works great still and I use it. It just looks "different".

So you're questioning a trusted source Billington not testing his tubes and hence sending you a faulty GEC 6AS7G? Have you thought about other possibilities?

 

[CaptainFantastic]

So you're questioning a trusted source Billington not testing his tubes and hence sending you a faulty GEC 6AS7G? Have you thought about other possibilities?

No, I am not questioning Billington nor did I intend to suggest that above, the tube works as I said. I don't think any tube seller has time to test tubes in amps for hours.

 

[cddc]

Then, good luck...LOL

 

[DenverW]

@CaptainFantastic switch looks great. I’ve been talking about putting one in for a while, and never manned up.

 

[wabibito]

Bought a cheap damaged basic crack that has broken based and bent top plate recently. In the process of fixing and restoring it. Deciding to build it a little different then the last one I build for my friend. I also using the BT-7 instead of the BT-3 so the wiring is a little different.


So far it sounds good. Time to order the mods.