In article , Mike Gilmour
writes
Cathode resistors on some power amp output valves are purposely underspeced
(wattage wise) to go open circuit if an excessive amount of current is drawn
due to a faulty valve or circuit fault associated with that valve. Often the
resistor is burnt o/c & the B+ fuse is okay. You'd think they'd make it at
least easy to get at to replace though.


Some old Philips TV's used to do this with the frame output pentode a
PCL86 IIRC, they used to put it on two short tags so that if it got hot
then it would melt the solder and "drop off" ..

Fine idea except that the cathode bypass electrolytic then used to take
the AC load and explode putting electrolyte all over the PCB and cause
other bits to conduct better than what they should have done!...

Usual problem was a heather to cathode short as the valves were series
connected heaters via a dropping resistor etc....

--
Tony Sayer