My friends Mikey and Wendy gave me this thing they found at a yard sale. They were hoping to sell it for something on ebay, but it turned out to not have any input jacks or knobs. A radio? but no tuning nob or switches. It was full of tubes and had speakers. Clearly it could be used as an amplifier somehow!
I traced out the tone and volume circuitry, and found it in the middle of a chain of four tubes. I disconnected the wire going from the output of the volume potentiometer (wiper contact) to the input (grid) of the next tube. I left the current limiting resistor. Then I just installed a 1/4 inch jack on the back, connecting the sleeve to the ground on the circuit board and the tip to the current limiting resistor going to the grid of the first of two amplifier tubes. And, there it was, a PA amplifier. It worked great with line level signals from my ipod etc, and I got a small amount of sound from a guitar, but it wasn’t loud enough to use as a guitar amp.
To boost the guitar signals I built a preamplifier circuit based on the 12AX7 tube. I found a useful schematic on the “cooperative tube guitar amp project” online at ax84.com. I just copied the preamp section of the basic amplifier design. I drew from the +300 volt supply already on the circuit board which I found by looking up the plate pins of the other tubes and metering it out.
With this addition the amp not only gets nice and loud but distorts when it’s turned up all the way. It sounds great too! Listen to my friend Andy shredding in this video:
The guitar (if I may add) was found in the dumpster behind a local thrift store – making this ear smashing hair thrashing metal machining rig virtually free.