> I dont understand the heater/cathode circuit - where does it get filament voltage from.
Kids today. Connect 2.5VAC across the 2A3 heater pins, "of course". Everybody knew that.... back in 1935!
You will need a separate dedicated 2.5VAC winding for each 2A3. And another 6VAC winding for all your 6SN7s. Hammond has PTs for this.
> d a mic preamp from this schematic. I know it is going to be a lot of level - maybe 35dbm or over.
Max output is 3.5 Watts, which is indeed +35dBm. Over 45V rms 126V peak-peak in 600 ohms. More than enough to explode most input chips.
6SN7 gain is 15. 2A3 needs 45V peak to make +35dBm. So we need 3Vpk 2Vrms at 6SN7 input to make 45Vrms output. Raw gain is 45/2= 21, or 26.4dB. A dynamic mike straight in will make a weak line level. A hot condenser will make more, but how do you stick Phantom on it? A 1:10 mike transformer gives overall gain 46dB, input overload 0.225V. Some hot-music hot-mike situations will overload it (though the smoke from your ADC may be more annoying than the mild distortion).
Input noise with 1:5 or 1:10 mike transformer is fine. Just don't put the gain pot there or effective S/N goes down when you use it. Don't put the gain pot between 6SN7 and 2A3, because the 6SN7 can just about drive the 2A3 if you don't waste any of the 6SN7's effort. Don't put the gain pot at the output because 3.5 Watt pots are hard to find. I suppose in the middle is the best compromise.
It isn't the tubes. It is how you use them.
> More like a program amp.
Yup. My monster console had 2*6V6, would do 10 Watts (+40dBm), was rated very-clean +35dBm. But it had more voltage gain.
