#1
Updated pics and schem:
Schematic (revision C)
Front angle
Rear angle
Rear
Rear, back removed


I built this in 2012 from parts I had in the junk box after buying a Fender Mini '57 Twin-Amp. I'd never heard of anyone trying to DIY a self-contained tube amp this small, so I wanted to see if it was possible (besides, that tweed box was just begging to become a tube amp). I've built and re-built this thing at least two dozen times, refining things as best I could a little bit more each time.

If anyone decides to build this then I am very interested in hearing about it. Feel free to post your own questions or experiences.

Space was obviously at a premium so I chose the 5C1 Champ circuit and used "all-American five" radio tubes, hence the name "Radio-Champ". The 7-pin bottles fit in the box pretty well and are designed to operate on line voltages. Being such a small box, it doesn't use a power transformer; I did a little circuit trick to make it safe(-ish). Don't ask me what it was; if you can't figure it out on your own then you probably don't need to know. It's safe as long as one doesn't touch anything that's earth grounded while playing. If the power transformer is no bigger than the output transformer then it should fit into the box if the layout is carefully planned. A Stancor PS-8415 might work. A Hammond 262-series should work too but is probably still too big to fit in the box. The output transformer I used was pulled from an old radio; I went through several before settling on this one, mainly because it is unusually small.

It gives half a watt or so and it's fairly clean, loud, and somewhat bright. At high volume it gets a little distorted and a bit more compressed--it's mostly speaker distortion. Plugged into a guitar cab, there's a nice grit at max volume. It's a great general-purpose audio amp.

The small all-range speakers make it sound...not great. It sounds much better with a real guitar speaker. As a general music amp the crummy stock speakers actually work quite well.

I originally used a 6BJ6 remote-cutoff RF pentode as a preamp. RF tubes are typically more microphonic than AF tubes but I've had no problems with this. I eventually tried a 12AU6 sharp-cutoff RF pentode and I liked it a good deal more; it's warmer, clearer and very marginally (but still noticeably) louder. I also tried a 12BA6, which sounds really close to a 12AU6 but I liked the 'AU6 better. Technically you can used these 3 tubes interchangeably in this circuit without changing the ballast resistor (or pin wiring), but the tubes' heater voltages will drop a bit with a 12v tube in place (the 12xx6's drop was negligible, the 50C5's was around 20%).

I made a cutout in the back panel resembling the vintage tweed amps, partly for better sound and looks but mostly for ventilation. The cutout doesn't extend all the way across the panel as it would on a full-size amp; this is to avoid using more screws since there's no place to put extra cleats. This way the rear panel stays in one piece and firmly attached--plus I think it just looks better at this smaller scale.

Ventilation is a bit of an issue. A 50C5 creates a lot of heat and there's a power resistor dropping the excess heater voltage. The ideal value here is 430R 20W, but the closest I was able to get was a lash-up of resistors totaling 450R 20W--better higher than lower... I will eventually replace them with a capacitor, which is smaller and produces no heat. Don't use an electrolytic non-polarized cap here. They don't age well, which would eventually lead to burning out the tubes' heaters. For a 12v preamp tube, use a 410R 20W resistor (or parallel two 820R 10W, which are more common).

One final note... I originally used a mini toggle switch for the power switch, but I always thought it was disproportionately large compared to the rest of the amp so I used a micro-size rocker switch I happened to have. It's 15mm x 10mm and mounts in a 13mm x 9mm hole. Really tiny. It's similar to this or this.

Now, here is what I would do differently, and some other ideas:
*I would try to find a very small isolation transformer to mount in the box rather than make a separate adapter that has to be carried around. On the upside, using a couple of transformers to make an adapter for this thing would allow me to use tubes with standard voltages and eliminate any ballast components altogether. If I were to do it this way, the adapter would contain the ballast resistor or cap and probably even the whole power supply section; then I could have all that crud outside the amp itself and use a DIN plug to supply operating voltages to the amp.

*I would mount the tube sockets on standoffs or re-design the chassis--this would allow for far better air circulation and be easier to build. Even with the cutouts it stays pretty warm. I would also try to keep the original chrome control panel and keep the original handle mounting screws to have it look that much better.

*I would add a drive/distortion control (or at least a "channel" switch).

*I might add a tremolo circuit. I found a diagram for a one-triode trem using a 12AT6/12AV6. Here's a simpler one. These tubes were commonly used in AA5 radios as the audio preamp and detector. Adding another tube, a couple of pots, a footswitch jack, and a handful of parts might require a bigger box, though there are triode/pentode combo tubes available...

*I should probably install a fan. The one I have in mind is about 1" square and 3/8" deep; it came from one of those small cigarette lighter power inverters. The problem is having a 12v DC source to power it. Not a problem if I had built an adapter... Anyway, the fan probably isn't necessary if a cap is used as ballast instead of a big power resistor.
Hope you enjoyed the read. Thanks for looking.
Last edited by Invader Jim at Aug 3, 2016,