As noted in the Fanfare guitar amp and speaker cabinet story a couple of years ago, that wasn't actually the first guitar amp i tried to build. I had bought a small box of parts along with a couple of chassis that had been pulled out of old Hammond organs that would have provided most of the pieces needed to make a working amp.
That project had multiple problems: reusing a partially-cleaned out chassis made working in there messy, i didn't really know what i had for transformers, and all that made it difficult to really trace what was going on to prevent it from passing a signal.
When i got interested in trying again this winter, i decided on largely re-starting. I kept the main board that i had wired, because i felt pretty confident in that part of the build. I set the rest of the parts aside and started with a clean schematic for the Champ, the board, and the power transformer. I was able to measure the output, so i at least knew what i had for the PT. I also had the output transformer, but was less confident in that since i didn't know anything about it.
I used Rob Robinette's site for schematic and layout ideas but wanted more of a head-style cab that i could use with different speakers so i made a more horizontal layout. I splurged and bought Harbor Freight's cheapest bending brake, and it was barely adequate for the ~16ga steel i had in my materials pile.
A little hammering to square up the corners and some paint to cover the dings, and i had a chassis. I spent a lot of time tracing and cross-checking the board to make sure everything was correct. And i corrected some things. The jacks and pots were all cleaned up and reused from the first attempt.
I didn't take many pictures along the way, so here's the mostly finished internals:
Feeling good about thing so far, i carefully followed Rob's Amp Startup procedure. That was going great until i put in the rectifier tube and got no DC voltage at all. That 5U4 tube was old and i don't know how to test it, so i decided to order a new one and get a correct output transformer while i was at it.
When the parts arrived i grabbed my meter and plugged in the tube to find no change. Retracing everything for the rectifier i realized i had one of the 5v heater wires on the wrong socket connection, so that's why the tube never even got warm. With that fixed, the old tube worked fine, so now i have a spare.
The rest of the start up went well, with no drama and good tone, so i moved on to making a small head cabinet for it.
I planned the chassis size to be a little smaller than the surface of the speaker cabinet i made earlier. The small vents on top are probably overkill, but i added a power resistor to drop the voltage a bit, and didn't want that to get hot against the top of the cabinet (it barely gets warm).
I covered it with the left over tolex from the speaker cab, along with some perforated metal from Ax-Man and a new handle.
The controls are very simple: a single input, a toggle switch to turn on the negative feedback circuit, and a combined on/volume switch, with an indicator lamp. I had originally thought about using the switch for high/low input so i could have 1 input instead of 2, but i got really hung up on how to wire that correctly and thought the NFB switch would be more useful anyway.
I was right about that, the overall volume is pretty low, so no real need for the dual inputs and the NFB switch does make a real difference in tone and how the amp distorts.
The back of the amp is pretty standard, but i decided to wire up both impedance options from the output transformer. To keep it simple there's just one speaker jack but a 4/8 ohm switch to match the connected speaker.
Here's the full stack:
It makes a cute combo, maybe 2.5' tall. I doubt that the 12" Peavey Black Widow is the ideal speaker for that amp, but it sounds ok. A little peaky with the treble, but i'm not sure if that's the speaker or the complete lack of tone stack. It's quiet enough for an apartment through about 1/3 volume, and get a really nice growl around 1/2-2/3 volume. Even fully cranked up it's not painful and has a nice grit to it. The distortion kicks in at a lower volume with the NFB turned off.
On bass the volume is lower overall but the tone is good, if still a bit peaky on high notes.
For future changes, i'd like to try sealing the back of the cabinet and cutting a vent slot at the bottom of the baffle. I think that speaker will sound good with bass with a more sealed up surround. I might try the small capacitor "ice pick highs" mod from Rob's site to see if that smooths out the treble. That's about it though, i don't want to use this amp as a platform for endless tweaking, i'm happy having it work as a decent bass practice amp.
For next amp projects, i'm thinking of building a good mid-size bass amp, something like the '60s Ampeg B-15 flip-top.