I decided to convert this little code practice oscillator into a guitar amp. Lots of folks turn old radios and phono amps into guitar amps these days, to the point where the prices for these forlorn line-rectifying tube amps are pretty high.
This is a 1954 BUD CPO-128B made by BUD Radio in Cleveland, Ohio. It sold new for $14.45 (that’s $161.00 in 2024 dollars). The oscillator circuit looked like this:
Note that this is the typical “Widowmaker” design – it directly rectified the household “mains” wiring (see “Taming a Widowmaker Part II” https://hardway-vintage.com/2021/11/taming-a-widowmaker-part-ii/). It used a tiny PP-type center tapped output transformer. You can find these CPOs in good condition for $40 or less. Sometimes a lot less.
I added a 12AU6 tube as a preamp, got rid of the old circuitry, and made a new circuit like this:
This is a typical 35W4, 12AU6, 50C5 “widowmaker” guitar amp design, like Valco and Harmony and Silvertone, etc. I elected to keep the original BUD output transformer and not use the center tap on the primary. The amp was quite dark and muddy sounding, so I made quite a few changes to improve the tone. I added an isolation transformer and a full wave bridge and increased the resistance of the filament string dropping resistor to compensate for the higher voltage of the full wave rectified supply. The (half-wave) 35W4 isn’t really doing anything, but I kept it in circuit as ballast for the filament string – otherwise I would have to find a way to drop another 35 Volts.
Note that the 50C5 screen voltage is a bit higher than the plate. Also the plate voltage is a bit higher than specified, but I know a lot of these amps run this way. Not too concerned. The capacitor between the plate and the screen of the 50C5 is a weird kind of tone control; it was originally 0.02 uF – I changed it to 0.005. I also added the bright cap on the volume pot and changed the pot from 3M to 1M.
This was all point-to-point wired and jammed together into the tiny chassis. As expected, there were a few oscillation issues which were fixed with lead dress. The 50C5 tube and the 470R dropping resistor get very hot. Which makes sense.
The amp actually sounds pretty good through the 4” speaker. Pretty clean at low volume, and pretty raunchy at full volume. I would guess 2-3 W. It takes pedals well, especially nice with reverb, and sounds great through a 12” speaker extension cabinet.
