A Traynor YCV40 arrived recently with a failed standby switch and it presented an interesting problem. Seems the switch actually controls a FET (field effect transistor) that controls the B+ voltage, and a shorted power tube will kill the FET.
The fix according to Yorkville was to replace the FET (Q8) a small 10v zener diode (D13) and an integrated circuit (U3). U3 is the little white square in the picture, Q8 is the transistor on the right and D13 is too small to see easily but if you start at U3 and count to the right it's the sixth device, to the right of five resistors. Make sure you check the part numbers on the FET and the IC because in this case the amp I was working on used a different device-note it has only six legs. Make sure you use some fresh heat sink compound under the FET.
Q8 is an IRF 830 FET and U3 is a 4M35 integrated circuit. Don't quite remember the part number on the zener but it's in Traynor's excellent service manual.
Also these amps were built with lead free solder so be prepared.