Customer brought in a couple of Lux91 300B-based amps, one suddenly stopped working.
First off noticed customer had inadvertently managed to remove and reinsert the 300B 4-pin valve rotated 90 degrees (as a ‘diy’ check swapping valves between units) prior to bringing to me which of course failed to fix anything. Thankfully nothing damaged being 90 degrees out vs 180 degrees out, as HV to the filaments probably would have killed the expensive 300B. Reoriented back to correct position having tweaked the now-slightly stretched pin.
Removed bottom covers to find some very crispy (underrated) resistors in both (so not the problem, just a problem noted for resolution):
These are known as the ‘FCUPS’ boards (Film Capacitor Universal Power Supply) – Pity, not universal enough for our UK 240-250VAC mains – clearly overstretching those resistors!
Couldn’t quite make out the colours but measured 100K and 150K respectfully. Ordered replacements of higher wattage (particularly for the 100K). Meanwhile, on the nonworking unit, the supply to 300B filaments was missing, so no glow, no heat, no thermionic emission, and no sound 😉 This is supplied from a separate board, which had a slow-start function:
Fixed the issue there (at time of writing this I forget what exactly)
Replaced resistors with much higher power versions:
And now a much more respectable (still hot though) temperature.
Checked bias etc. and operating fine before returning to customer.