Because the 6SN7 isn't actually a very good driver for a 300B despite its popularity in that application. The 6DN7 will offer better gain structure and do a better job of overcoming the 300B's miller capacitance.
It sounds like Thomas is open to customers requesting modifications, so you could always ask for a C3g or D3a driver implementation which is what I would probably go for.
This is what Glenn, a custom amp builder on here, had to say in regards to 6SN7 with 300B a few years ago. Edits are mine to not specifically name any manufacturer at the time. Remember, this is just one mans opinion and there are plenty of very good amps out there using 6SN7 as drivers for a 300B amp. Also some notes on OTL amps.
"Every thing is a compromise. These company's are in business to make money not have the best sounding amplifier. It just has to sound good enough so you will buy it. These company's have to give the people what they want and everyone wants a 6SN7 so they can tube roll.
The 6SN7 is not a proper driver for a 300B. The xxx only uses 1/2 of the 6SN7 to do the driving so this is essentially the same as the one 6SN7. After 10 or more years of dealing with the 300B trying the 6SN7 then going to the 6DN7 that is really 1/2 a 6SN7 and 1/2 a 6BL7 and also trying the 6BM8/ECL82. I found that
#1) the 6SN7 cannot drive the 300B properly
#2) even though the 6DN7 and 6BM8 can drive the 300B neither of these tubes sound that great in the first place.
So I went to the #10/210 this sounded fantastic but I had to go through the DC on the filament thing to use this tube. Especially as the driver It had to run it on DC. The #10 can drive the 300B with 1.5 watts. Recently I have experimented with the C3g and this tube can also drive the 300B with 1.5 watts of drive and not have to run the filament on DC.
And these C3g tubes sound almost as good as the #10 and have more gain. I have sold a couple of amps that use this tube as the driver for a 300B to xxx you can ask him about the SQ.
When I made the OTL I used a Brimmer 13D1 / 25SN7 so I can easily run on DC and filter 150 ma. But everyone wanted the 6SN7 so I switched to the 6SN7 and ran it on AC. every one happy. But the OTL can pick up a slight hum in some tubes. I can't be convinced that there are 6SN7s that sound that much better then a Brimmer 13D1 maybe
I don't know what I am hearing.
I didn't understand why some company's used large Input capacitors after a 5U4 causing some of them to blow up. Now I know. People want to hear what they call a "black background" - no hum or noise with the volume turned all the way up and no input. What does this matter? This causes the rectifier to produce switching noise that you can't hear but it makes the amp sound harsh.
Running the filament's on high frequency AC is a cheaper way of getting rid of the hum then doing DC. To go DC It has to have filtering and on high current it has to use large capacitors and Choke or resistors maybe a voltage regulator. These things take up real estate under the chassis. And resistors or voltage regulator give off heat that has to be gotten rid of. So a high frequency power supply with a small transformer can supply several DHT filaments. Even though this can also make the amp sound harsh.
It's also noteworthy that Thomas's high-end builds tend to use the 10/801A as a 300B driver. I have heard the implementation is tricky but the results are excellent.
There are ways to make the 6SN7 work pretty well of course and many people have, but it's not ideal for the job and if you want to stick to a 2-stage design (and you do) then honestly there aren't very many small signal tubes that truly are well suited to the job of driving the 300B. It's a tug of war between having enough gain and having low enough plate resistance.
edit: I should also mention for people not very familiar with Thomas that his #10 driven 300B amp designs are intended to be paired with a pre-amp. It is very difficult to implement the 10 as a 300B driver in an integrated amplifier, you just don't have enough gain. At a minimum you'd probably be resorting to using a 4:1 step-up input transformer which comes with its own downsides. Ultimately you can make many tube combinations work but there reaches a point where you are bending over backward to do so and the trade-offs don't make sense.
The GZ33 and 37 haven't been reproduced to my knowledge so you would have to look for NOS Mullards there. There are many 5AR4 options both new and old. JJ makes one that's very inexpensive that would be an easy low-risk way of finding out if you like what a 5AR4 does to the sound of your system.
This is a massive generalization but a rough rule of thumb is the more voltage drop you have in your rectifier the more "romantic" and slow of a sound you'll get, and vise-versa less Vdrop will tighten up the sound. This is all very circuit dependent of course...... like I said a massive generalization, but if you're experimenting with rectifiers and don't want to buy them at random this might help you with choosing a direction.
The 5R4GY will have the most drop of all the common rectifiers, and the 5AR4 the least. 5U4G has quite a bit of drop too. If you google search "rectifier voltage drop chart" you'll get plenty of useful resources on this.
The 5R4GY will have the most drop of all the common rectifiers, and the 5AR4 the least. 5U4G has quite a bit of drop too. If you google search "rectifier voltage drop chart" you'll get plenty of useful resources on this.
I just dropped a 5R4GY into my EC Studio B and it bent it slightly away from its default dry voicing to a nice balance point. Very pleased with the outcome.
This is what Glenn, a custom amp builder on here, had to say in regards to 6SN7 with 300B a few years ago. Edits are mine to not specifically name any manufacturer at the time. Remember, this is just one mans opinion and there are plenty of very good amps out there using 6SN7 as drivers for a 300B amp. Also some notes on OTL amps.
"Every thing is a compromise. These company's are in business to make money not have the best sounding amplifier. It just has to sound good enough so you will buy it. These company's have to give the people what they want and everyone wants a 6SN7 so they can tube roll.
The 6SN7 is not a proper driver for a 300B. The xxx only uses 1/2 of the 6SN7 to do the driving so this is essentially the same as the one 6SN7. After 10 or more years of dealing with the 300B trying the 6SN7 then going to the 6DN7 that is really 1/2 a 6SN7 and 1/2 a 6BL7 and also trying the 6BM8/ECL82. I found that
#1) the 6SN7 cannot drive the 300B properly
#2) even though the 6DN7 and 6BM8 can drive the 300B neither of these tubes sound that great in the first place.
So I went to the #10/210 this sounded fantastic but I had to go through the DC on the filament thing to use this tube. Especially as the driver It had to run it on DC. The #10 can drive the 300B with 1.5 watts. Recently I have experimented with the C3g and this tube can also drive the 300B with 1.5 watts of drive and not have to run the filament on DC.
And these C3g tubes sound almost as good as the #10 and have more gain. I have sold a couple of amps that use this tube as the driver for a 300B to xxx you can ask him about the SQ.
When I made the OTL I used a Brimmer 13D1 / 25SN7 so I can easily run on DC and filter 150 ma. But everyone wanted the 6SN7 so I switched to the 6SN7 and ran it on AC. every one happy. But the OTL can pick up a slight hum in some tubes. I can't be convinced that there are 6SN7s that sound that much better then a Brimmer 13D1 maybe
I don't know what I am hearing.
I didn't understand why some company's used large Input capacitors after a 5U4 causing some of them to blow up. Now I know. People want to hear what they call a "black background" - no hum or noise with the volume turned all the way up and no input. What does this matter? This causes the rectifier to produce switching noise that you can't hear but it makes the amp sound harsh.
Running the filament's on high frequency AC is a cheaper way of getting rid of the hum then doing DC. To go DC It has to have filtering and on high current it has to use large capacitors and Choke or resistors maybe a voltage regulator. These things take up real estate under the chassis. And resistors or voltage regulator give off heat that has to be gotten rid of. So a high frequency power supply with a small transformer can supply several DHT filaments. Even though this can also make the amp sound harsh.
Glenn is right. I believe his 10Y to 300B monos were direct coupled as well.
Probably what Thomas is doing is using the 6SN7 section of the 6DN7 for gain, then wiring the other section as a cathode follower buffer. It's a simple solution to overcome the wimpy driving capabilities of a 6SN7. Looks like that amp is capacitor coupled...
I might be building Zach a 300B amp at some point in the next year. I'll post some pics here if that happens. Still under discussion, but probably similar topology wise to the 45 amp I built him, meaning all voltage regulated power supplies, fixed bias, and transformer coupled. Thinking Monolith Magnetics iron since they can also wind me a custom mains and filament transformers. How far up the ladder as far as amorphous core materials for the interstage and output transformers would be up to him. Would be a fun one to build.
Glenn is right. I believe his 10Y to 300B monos were direct coupled as well.
Probably what Thomas is doing is using the 6SN7 section of the 6DN7 for gain, then wiring the other section as a cathode follower buffer. It's a simple solution to overcome the wimpy driving capabilities of a 6SN7. Looks like that amp is capacitor coupled...
I might be building Zach a 300B amp at some point in the next year. I'll post some pics here if that happens. Still under discussion, but probably similar topology wise to the 45 amp I built him, meaning all voltage regulated power supplies, fixed bias, and transformer coupled. Thinking Monolith Magnetics iron since they can also wind me a custom mains and filament transformers. How far up the ladder as far as amorphous core materials for the interstage and output transformers would be up to him. Would be a fun one to build.
I am absolutely loving what I am hearing with my 300B amp that uses 6SN7's ( I am actually using a quad of GEC CV1135 with adapters at the moment). However, after reading this I would love to hear what a proper run 300B amp sounds like, and if the 6SN7 is wimpy I can't imagine what a better tube choice would sound like. . .
This is what Glenn, a custom amp builder on here, had to say in regards to 6SN7 with 300B a few years ago. Edits are mine to not specifically name any manufacturer at the time. Remember, this is just one mans opinion and there are plenty of very good amps out there using 6SN7 as drivers for a 300B amp. Also some notes on OTL amps.
"Every thing is a compromise. These company's are in business to make money not have the best sounding amplifier. It just has to sound good enough so you will buy it. These company's have to give the people what they want and everyone wants a 6SN7 so they can tube roll.
The 6SN7 is not a proper driver for a 300B. The xxx only uses 1/2 of the 6SN7 to do the driving so this is essentially the same as the one 6SN7. After 10 or more years of dealing with the 300B trying the 6SN7 then going to the 6DN7 that is really 1/2 a 6SN7 and 1/2 a 6BL7 and also trying the 6BM8/ECL82. I found that
#1) the 6SN7 cannot drive the 300B properly
#2) even though the 6DN7 and 6BM8 can drive the 300B neither of these tubes sound that great in the first place.
So I went to the #10/210 this sounded fantastic but I had to go through the DC on the filament thing to use this tube. Especially as the driver It had to run it on DC. The #10 can drive the 300B with 1.5 watts. Recently I have experimented with the C3g and this tube can also drive the 300B with 1.5 watts of drive and not have to run the filament on DC.
And these C3g tubes sound almost as good as the #10 and have more gain. I have sold a couple of amps that use this tube as the driver for a 300B to xxx you can ask him about the SQ.
When I made the OTL I used a Brimmer 13D1 / 25SN7 so I can easily run on DC and filter 150 ma. But everyone wanted the 6SN7 so I switched to the 6SN7 and ran it on AC. every one happy. But the OTL can pick up a slight hum in some tubes. I can't be convinced that there are 6SN7s that sound that much better then a Brimmer 13D1 maybe
I don't know what I am hearing.
I didn't understand why some company's used large Input capacitors after a 5U4 causing some of them to blow up. Now I know. People want to hear what they call a "black background" - no hum or noise with the volume turned all the way up and no input. What does this matter? This causes the rectifier to produce switching noise that you can't hear but it makes the amp sound harsh.
Running the filament's on high frequency AC is a cheaper way of getting rid of the hum then doing DC. To go DC It has to have filtering and on high current it has to use large capacitors and Choke or resistors maybe a voltage regulator. These things take up real estate under the chassis. And resistors or voltage regulator give off heat that has to be gotten rid of. So a high frequency power supply with a small transformer can supply several DHT filaments. Even though this can also make the amp sound harsh.
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