This project started as a way to find a use for a bunch of really cheap 6J1 pentodes I got on Aliexpress for around $1 each.
I started out with just building a small class A preamp and saw that it performed surprisingly well at low voltages and began to wonder if it would maybe be able to develop enough power to drive a pair of headphones.
I knew that for this to be even remotely possible I'd need an output transformer to match the relatively high voltage and high impedance output of the tube to the low voltage, low impedance signal needed by a pair of headphones.
Using a nice audio transformer seemed like kinda of a waste though, so instead I hacksawed open a bunch of old 12V wall warts. With a turns ratio of 20:1 I figured they'd be close enough. Turns out, they were! I wasn't just able to drive my headphones, I was able to drive them at a level almost too loud to listen to comfortably. Success!
All this success made me wonder if maybe it would even be able to drive the 12" speaker in my guitar combo. Some quick connections with some alligator leads later I was ready to be let down. And I was.
It was audible, but barely louder than the sound my electric guitar made by itself. Which wasn't too surprising considering this amplifier lacked any kind of preamp and fed the guitar signal straight into what was basically a "power" amplifier.
But like most problems in life, this one could be solved with more guitar pedals, so I hooked up an external tube preamp pedal and cranked the output volume on it to drive the "power" amp harder. And it worked! It was way louder than it had any right to be. And I finally had a guitar amp that I could turn up all the way without annoying the neighbors.
Now suddenly convinced that this might actually be something slightly useful I decided to give it it's own enclosure so it could sit proudly on top of my regular guitar amp. I added in a 120Ω resistor for the heater so I could run it off of the same 24V power supply that powered the rest of the amp, which is kinda wasteful but keeps the whole thing really simple. All the connections were made point to point, because that's how all good tube guitar amps are made.
This thing was a ton of fun to build and I actually play it more than my old combo now. And I think you should also build one, it's fun!