Fender Vibro-Champ Modded, Part 1!
I bought this amp in great cosmetic shape back in the 80’s. The electronics were a mess, though, and it took some reading and researching to get it working right, even though it’s a simple circuit. I am by no means an expert, but I can read a schematic, solder well, use a voltmeter, and avoid getting shocked.
It served as a guinea pig for a few different preamp circuits, with varying degrees of success, and then I put it away for a long time. Along the way, I cut the baffle for a 10″ speaker, since the original speaker was too small to sound good to me. It’s a 1967 amp, but strictly player-grade, since it had been “serviced” at least a couple of times in the past by someone who really had no clue, and original parts had been swapped out.
I settled on the Naylor 10 because I had it, but that meant finding a new output transformer. The Naylor is an 8-ohm speaker, but the original Fender speaker is 4 ohms, and that mismatch is not a great idea. Fortunately, Classic Tone makes a Champ OT with selectable 8- or 4-ohm speaker taps, and it’s a direct replacement for the old transformer.
Next up was to install a modern grounded power cord, especially since the old plug had mostly crumbled away, and the removal of the dreaded Fender “death cap”. At least I was less likely to kill myself or start a fire now, so that was a useful session on the bench.
So, what did I want the amp to do? At one time, I had a stock preamp circuit wired up. While the clean tones were nice, the breakup was raspy and not at all what I like. I had removed the vibrato circuit because that freed up the second 12AX7, so it seemed like the best choice was a third gain stage. I wanted to hear the amp break up more quickly, with a smoother transition into breakup, and a lot more breakup than the stock circuit could ever provide.
I had gathered together a lot of printed schematics and tube amp books, but the internet has made available all sorts of stuff that was kept secret for years. Marshall and Fender, Trainwreck and Dumble, just about anyone who ever designed and built an important amp has had at least some of their circuits and secrets revealed. In part 2, I’ll show you what I found, and what problems I had to solve along the way.