• Low power amp

The circuit is my design although it borrows heavily from Fender Blackface and Tweed amps. It’s a single ended, class A, cathode biased amp that puts out about 0.125W clean and 0.2W max.

There’s a lot of gain in the 12AX7 tube stages. If you’d like the amp to start distorting earlier, reduce the value of R13. The first version of the amp didn’t have R13 at all, and it was nearly impossible to get a clean tone, so any value will work although you probably won’t like the tone at lower values—too raspy. Earlier distortion can also be had by reducing R12 to 1K.

It’s easy to make a 1/2 power mode. The B+ can be reduced by 50% if you disconnect one of the AC wires (red) going to the diode bridge and connect the center tap (red-yellow) in its place. A SPDT switch would make this switchable, although I don’t know if you really need less power.

The Tone control is wired to only reduce frequencies (not boost them) like almost every other tone control network. But it can be changed to an active treble boost by moving the wire from the tone pot wiper from the junction of R13/R9 to the junction of C4/R13. This also increases the signal into the 2nd tube stage, so distortion goes up as well, sort of like a Dallas Rangemaster Treble booster effect.