Up on the bench recently at MTA was a very nice and clean silverface Fender Champ which would not turn on.
This was one of those classic repairs where everything you find makes sense – you clearly see the damage, you find the root cause and the repair is limited to eliminating the root cause and reversing the damage. This Champ got a bit lucky – an oversize fuse (2A) had been soldered in place where the original fuse (1A) socket was, because the socket had suffered physical damage, and that pigtail replacement was someone’s “quick-fix”.
That shortcut could have cost the owner an output transformer replacement, or even the HV winding on the power transformer, but as things transpired, both transformers survived, while the oversize fuse and a number of other components finally burned up first.
Upon opening the amp up, I immediately noted that the cathode bias resistor had burned though (a 5W unit!) and melted the plastic-cased cathode bypass capacitor it was next to. I also saw that one of the high-voltage power supply resistors was also browned-out by overstress.
These are all points along the HV trail to the 6V6 output tube… so out he comes and hits the tester.
This is where things get a bit strange…. At first, it was clear that there were one or more internal shorts to the tube – three out of five of the shorts indicators on the Hickok 600A were dark.
Beyond that, the main fuse lamp (not ivisible in photo) of the tester was glowing – indicating too high of current flow for this type of tube. That settled, I turned back to the amp chassis to begin replacing the bad fuse holder and those burned-up components. A few minutes later, I looked again over to the tester, and the short-circuit indicator lamps were no longer indicating a short. After a few moments of warm-up, the tube no longer appeared bad. Strange indeed! I have seen many tubes that tested good at first and failed when hot — but not the other way around.
So now I understood the failure a little better… and why those parts kept taking too much stress for a limited time during each turn-on cycle. With the larger fuse in there, it probably only appeared that the amp took longer to warm up than it should, but left plugged in and switched on, after a while it would work. At least it did until the resistors and 2A fuse finally burned through from the repeated stress.
I replaced the fuse-holder, the fuse, the 1K HV power supply resistor, and the cathode bias components.
I know that a lot of people are pretty vehement about using original-type carbon composition resistors in older amps, but I don’t do that unless asked to. Yes, they look cool and old-school, and everyone likes to look into a beautiful old Fender chassis that looks untouched since new, but carbon comps almost always drift high with age, and are an inferior technology. Leo used what was on the market and reasonably priced. I don’t think physics supports the idea (or Leo would have ) that your ears can actually HEAR the way a resistors turns electrons to heat, or that they will change materially the way a tube clips or amplifies- but hey, lotsa folks believe in angels, too. I just prefer to put quality parts in that can’t burst into flames when they fail -and leave all the Mojo Magic stuff to the players who still think they can buy their secret way to tone heaven rather than working for it. Having done that, all that was left then was to order a new tube. Once that was installed, checked for idle current and everything play-tested for an hour or so, she was ready to go back home and rock and roll some more – still the Champ