Direct coupled PP 2a3 amp - maths needed!!

[Eusebius]

I wonder if I could ask the theorists on the group here to take me through the drive requirements for a 2a3 in push pull. Let's say 350v B+, cathode bias, 40mA (or maybe 50mA), 6.6K output transformer.

My specific question is about the preceding stages - I am planning a three stage all balanced amp, and wondering if I can direct couple the first and second stages.

The problem area is that I want to use DHTs all through, and they have lower max anode voltage - for the first stage this won't matter, but can I do anything with the 1J6g in the second stage - it's max anode voltage is 135v? Will I have enough drive for the 2a3? If yes, then great.

If no, then the next step up is 180v anode to cathode for the second stage, since this is the max anode voltage of the 12A, 31 and 26 - all possible contenders. Can this be achieved direct coupled from the first stage?

I'm being a bit of a cheapskate here - my present amp is 1G4 into 12a into 2a3 but it's all transformer coupled - sounds great too.

If I direct couple the first two stages, I could use either caps or a transformer to couple to the 2a3.

So is this likely to work - maths needed! Thanks for your help. Andy

[Tweeker]

Its doable. But, your either going to need a negative rail, or a B+ of at least 675V.

Edit:misread. 675V for direct couple all the way. All those 180V tubes should be able to give the swing needed by the 2A3.

[Tweeker]

Take a look at Jeremy Epstein's DC Darling Amp for some direct coupling basics.

[Eusebius]

I wasn't thinking of direct coupling to the output stage, but curiously enough I was considering using 1626 as drivers. If the first stage were direct coupled to the 1626 (max anode voltage 250) that should be fine, particularly with B+ of 390v. If the 1626 stage were transformer coupled to the 2a3s that should be excellent. Presumably one could also use anode loads on the driver stage with a large enough cathode resistor.

[Tweeker]

1626 isnt a DHT.

[Tweeker]

http://boozhoundlabs.com/howto/ has much of the maths you seek laid out.

[Eusebius]

thanks, tweeker! I have Morgan Jones and a few others. That's a handy tutorial site online. Yes, 1626 is indeed not a DHT but I have one or two "nearly DHTs" I don't mind occasionally using tubes like the 27, ECC40, 1626 and 2C22. Not the same but they have their virtues. Still struggling with the maths!