I've built a few amplifiers which are all-DHT. I like the sound so much I wouldn't use anything other than DHTs. So I have a stock of all the common DHTs - 26, 30, 31, 71A, 01A, 112A, 10Y, 46 and so on.
I think for the application you describe an effective solution is three stages. I'd start with the 26 and direct couple to the second stage, which is interstage coupled to the 45. My present amp is very similar - 30sp direct coupled to 31 interstage coupled (Hammond 126C) to 300b.
The 26 could use battery bias. A 9 volt battery would suggest 130v plate at 5mA or 135v at 6mA. Simple solution. Or filament bias - another excellent sounding option. Use a 10 ohm cathode resistor, 145v plate for 6mA. Filament supply is connected between the opposite side of the 26 filament from the resistor, and ground. The 10 ohm cathode resistor then has 1 amp going through it plus the 6mA bias.
I'm fond of using the Hammond 126C interstage - good sound. This is 1:1 at 10K, with 106H inductance. So max plate resistance of about 5k for a tube using it. So choose from 31, 46, 71A, 10Y or even a 45, 6B4G or 2a3. My choice is a 10Y which I use in my preamp with filament bias. The 46 sounded very clean. 31 is cheap and a nice musical sounding tube, easy on the filament supply.
So that's my solution - built and working nicely into a 300b. Check out the "26 pre amp" thread for all the options for the 26, including Rod Coleman's fine filament supply for which he can now supply PCBs.
Andy
I think for the application you describe an effective solution is three stages. I'd start with the 26 and direct couple to the second stage, which is interstage coupled to the 45. My present amp is very similar - 30sp direct coupled to 31 interstage coupled (Hammond 126C) to 300b.
The 26 could use battery bias. A 9 volt battery would suggest 130v plate at 5mA or 135v at 6mA. Simple solution. Or filament bias - another excellent sounding option. Use a 10 ohm cathode resistor, 145v plate for 6mA. Filament supply is connected between the opposite side of the 26 filament from the resistor, and ground. The 10 ohm cathode resistor then has 1 amp going through it plus the 6mA bias.
I'm fond of using the Hammond 126C interstage - good sound. This is 1:1 at 10K, with 106H inductance. So max plate resistance of about 5k for a tube using it. So choose from 31, 46, 71A, 10Y or even a 45, 6B4G or 2a3. My choice is a 10Y which I use in my preamp with filament bias. The 46 sounded very clean. 31 is cheap and a nice musical sounding tube, easy on the filament supply.
So that's my solution - built and working nicely into a 300b. Check out the "26 pre amp" thread for all the options for the 26, including Rod Coleman's fine filament supply for which he can now supply PCBs.
Andy
Sorry for resurrecting an old thread, but enjoyed reading it and an all-DHT power amp is definitely my next project.
It's interesting to read an old post of mine and see what I was doing in 2010.
These days it's 01A with Ale Moglia's Gyrator into PSE 4P1L into an amorphous core 3.5K OPT. Just 2 stages, all filament bias. Mark Audio Alpair 10M single unit speaker columns. This is the most transparent system I've ever had - crystal clear. Very happy with it! I've only used DHTs since 2010 and in fact before that.
These days it's 01A with Ale Moglia's Gyrator into PSE 4P1L into an amorphous core 3.5K OPT. Just 2 stages, all filament bias. Mark Audio Alpair 10M single unit speaker columns. This is the most transparent system I've ever had - crystal clear. Very happy with it! I've only used DHTs since 2010 and in fact before that.
I'm 100% with your thinking and that's exactly what I'll embark upon on my next project. Have lots of Cunningham 01A and 4P1L. Yes, it'll be DHT for me from now on!
Thanks, Andy!
It's a question of how much gain you need. Depending on speakers and room size, a 3 stage system from line source (phono pre or DAC) may be required, in which case I'd still use a 01A front end and a 4P1L driver and then a final tube which could be anything from a 45 or 2a3 to a 300b to a big transmitting tube like 845 or 211, all directly heated. Single ended for me, but fine to parallel up the output tubes.
But if you don't need more than 2 stages, a PSE 4P1L output stage is the solution for me, and also for Ale Moglia and others. We have the same basic system. It's all in filament bias so no cathode bypass caps. That's something you can do with the 4P1L that you can't do with tubes like 45, 2a3, 300b without very big and hot cathode resistors and a high voltage/current filament supply. Not practical.
And as said many times, whereas the 45, 2a3 and 300b are great tubes they are low gain. The only way to implement a 2 stage design is with a high mu indirectly heated input tube so all-DHT goes out the window, unless you add a solid state stage or have very high efficiency speakers like horns. Not a solution for me. The 4P1L, which is also a great tube, has a gain of around 11 which makes a 2-stage all-DHT setup possible for many of us, with all the gains in clarity it brings. The best DHT input tubes like 26, 01A, 10Y have a mu of around 8. You can squeeze a bit more gain out with a 4P1L input - that's not impossible though I like mixing the tubes up. So for 2 stages that passes on the issue of gain to the output tube.
There are some esoteric higher gain DHTs that can be used as input tubes but I leave those to the guys with deep pockets. Same with the 211 as output tube - mu of 12 so great solution if you can stomach the high voltages - I can't. But NOS is very high dollar, though modern Chinese can work. And you lose the filament bias so back to cathode bypass caps or fixed bias. The 4P1L is just such a perfect solution - it transformed how I build amplifiers.
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3A5 is a nice linear tube, mu 15, fine for filament bias, and cheap, but no warm glow.
It is indeed a nice little tube. Probably at least as good as a 6SN7. I have plenty, but haven't used them for ages. I also used 1J6 at one point - bigger and warmer sound. Hard to choose between them. The other double triode DHT is the 3B7 but I never liked that much. The 30 tube was also pretty so-so and also microphonic. The tall bottle ones were better sounding - have a few of all these. The 30SP were disappointing - less microphonic but a bit boring.
The 1G4GT is a nice tube, especially in the tall bottle form. Pretty microphonic, though. Must try it again sometime - I remember liking it. The 1E4G should be useful - mu of 14. For some reason I never got round to that one. The 71A and 31 are low mu so no reason to use them and no better than a 4P1L.
Overall I think the 01A, 26 and 10Y stand out, and of course the 4P1L. I prefer them to any of the above, though most of these DHTs will probably sound pretty nice with Ale Moglia's Gyrator. Must pull some out and try them just for completeness' sake.
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