300B PSE Loss of Volume, Bass, and distortion

[David Roberts]

During the first lockdown, I undertook a full ‘nut and bolt’ restoration of a pair of WAD 300B PSE that had been waiting patiently in my workshop for about seven years. The rebuild went well and, apart from some minor issues, which were quickly sorted, with one of the amps when first switched on, I was very pleased with the final result and immensely impressed by the sound. I didn’t manage to hit the hum level quoted in the literature with no feedback (4mV), the best I could manage was 7.6mV, but it isn’t intrusive, and is perfectly acceptable for my listening. Both amps ran perfectly for a year or so, but now one has started to misbehave…

The fault manifests itself as a drop in volume, lack of bass, and an occasional tendency to distort. At first, this occurred after the amp had been on for about 30 mins, but more recently the fault manifests itself at switch-on, which sometimes 'clears itself' and produces full volume, which can be, er, ‘surprising’ to say the least!

I have checked the voltages of both amps: they are similar and in accordance with the instruction manual - certainly well within the quoted 15% tolerance. I have successively swapped the valves with the good amp, switched input cables, switched output cables, inspected, measured and tested various critical components (which resulted in changing C3 and C6 in both amps), but all to no avail. I am now at a loss at what to try next, and any suggestions from more experienced engineers who might have encountered a similar problem would be most welcome.

Many thanks.