Fender PA100

Not seen one of these Fender variations before. basically looks like a Twin Reverb / Bassman 100 Watt, with a reverb unit and 4 basic inputs. This one lead me on a merry dance to start with and for those of you that work on this range of chassis may want to take note! The fault was hum, noise and very low output. Removing the 4 output valves, 6L6Gs and popping them onto the AVO valve tester, showed all 4 were very tired with bad heater to cathode leaks. Ah, that explains the hum and miserable output. Fitted 4 shiny new valves and powered up with confidence all would be happy. But not at all. After a lot of messing about, I noticed that with an input connected and the volume control for that channel turned up, there was a significant tone ripple, same as the input, superimposed on the HT supply feeding the final amp and phase splitter 12AT6-ECC81 valve. This was coming from the reverb sender valve HT supply. The filter reservoir electrolytics had gone open circuit. This created an odd feedback condition on the phase splitter section that uses the same HT supply, causing strange drive conditions and HF oscillations. With the capdensors changed and a couple of resistors replaced, new valves, all was working well. For those not familiar with these amps, the HT is quite high at around 450 angry pixies. Something interesting for a change.

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