Peavey ValveKing 100 Overhaul

[physics]

Hi Guys

That cap might have died and taken out the fuse, which would be great as it means everything else is likely fine.


A cap across AC should have safety ratings and/or be polypropylene, which is self-healing. The circuit will work fine without the cap. Overall reliability would be improved with a bleeder resistor across the first filter cap C202. You may need two Rs in series to have sufficient voltage rating for the resistors. They can be 330k or 470k metal-oxide flame-proof 1W each. This will drain the cap at turn-off and it will protect the bridge when you switch to stand-by.

Of course, we all know that the safest stand-by switch is the one disconnected or never switched off.

I did wonder about the cap ratings, this one doesn't have any fancy approval markings like an X or Y film cap usually does. It's on the secondary side of the power transformer, if that makes a difference. What function does the cap play in the circuit? My best guess so far after some internet searches is that it's there to help manage oscillations due to the HV secondary winding inductance and bridge diodes recovery capacitance interacting.

Thanks for the reliability tips, bleeder resistors are definitely on the list of things to do. First I want to get this thing working though, and then I'll go from there. Using the standby switch/hole for something more useful is another possibility. Quick question on that actually: if I recall, one of the TUTs says that until we get up to kilovolts on the plates, standby switches are not needed. From reading elsewhere (Getting the Most Out of Vacuum Tubes by Tomer), the cause of cathode damage when there is no standby is that it gets pelted with ions until the space charge builds up. A quick websearch indicates that it takes about 3kV to ionize 1mm of dry air, if I read correctly. Can it be said then, that the reason we don't need to worry about standby switches is that the plate voltage is not high enough to start ionizing the gas in the tube, meaning the cathode isn't in danger of an ionic hailstorm?