Lacking another big diversion, I spent a good part of this past winter learning how tube amps work and trying to piece together a small guitar amp. I read all about the Fender Champ and even bought an amp chassis and parts from an old Hammond organ to try to make one.
That project got stuck on some still-undiagnosed problem, but keeping my eye on estate sales, I found one where a guy had been fixing and reselling old radios, with tons of related parts and tools. I got there way too late for the great stuff, but I picked up a working AM radio and this non-working tube-based intercom (for free):
The pen is there for size comparison. This is a Fanfare, made by Fanton, probably from the 1950s. I can't find a lot of info on the, but it appears that they worked wirelessly by transmitting a signal through the power line, and used the built-in speaker as the microphone (I couldn't find another mic in the box).
The lever at the top is a push to talk setup, and the knob is on/off volume. Here's the back:
And here is the chassis and guts:
It's similar to the AA4 radios of the time in that the tubes are all running directly off of the mains, rather than from a power transformer. This isn't the safest setup for electrics, but it was a simple and cheap way to build things. I don't have a shot of the underside, but there is a bus ground, and I was able to confirm that at least the case isn't electrically hot. I added a polarized plug as a basic precaution, and may add an isolating transformer at some point.
The tiny speaker was crumbling. The paper box at the top is just housing the push-to-talk lever and related switches. It's a relatively complex switch that I forgot to photograph separately. The 35W4 on the left is a rectifier tube to cover AC to DC, and the 50C5 tubes are both power outputs.
The attractive thing about this for me is the 12ax7 tube, which is the basis for many guitar preamps. After studying many different circuits, I figured I could strip away everything that wasn't part of the audio path and hopefully end up with a reasonable guitar amp.
I'll have to update this post with the actual schematic I ended up with, but it's very simple. The signal comes in through a 1/4" jack, through 1/2 of the 12AX7, out to the volume, back to the other 1/2 of the preamp, then out to just one of the 50C5 tubes. The 2nd tube seemed to be wired more for boosting the wireless signal, so I routed around it and added a resistor to the circuit to keep the electric load correct on the remaining tubes.
The plan worked, and after swapping out a few small components with values more in line with other amps I'd seen, it works! I did initial testing with the bare chassis, the photo above was the first test back in the case.
Now the tiny amp needed a speaker cab. I had planned for this to be a fairly small practice amp, but I found this 12" Peavey Black Widow speaker for only $25 at the local guitar store. Also, the amp is surprisingly loud, so it seemed like a decent-size speaker would be worth a try. It would make the speaker cab more versatile for other amps if I end up completing any of them.
I didn't get any pictures of the bare cabinet, but it's just 1x10 pine from the lumber yard, and uses box joints at the corners. This is how it looked with tolex (black, Vox weave pattern):
It's a bit tall so I can store the amp inside the cabinet and keep everything together. You can see the speaker offset in the baffle panel:
The baffle is plywood I had in my scrap pile. I got the vintage grille cloth from Ax-Man, who just happened to have a huge roll of it at the store.
Here's the back of the assembled cab with amp on top:
The speaker is upside-down to keep the wiring more compact, with storage space at the bottom. The cabinet itself isn't terribly heavy, but that speaker weighs a ton. It's luggable, but not light.
I kept the baffle simple and screwed it on. The cloth isn't as straight as I'd like, but not terrible, and fairly tight.
The overall package sounds pretty good with my old Ibanez guitar and my very mediocre playing skills. It's surprisingly loud, and gets some nice grit in the sound at anything over 1/3 volume.
But at least it plays, and it also looks pretty cool at night (the light only comes on after the tubes are warmed up):
There's a persistent hum that I still need to chase down, most likely a grounding problem somewhere. I might also add a tone control and some kind of attenuation circuit to keep the volume down a bit. Fun project though, and fun to play through.