Amp got HOT - now hums like crazy

[greg]

I made the mistake of putting my little home built amp in an isolation box (with no ventilation) for too long while recording. It got very hot, as you might imagine. Right before turning if off for the night it started to hum very loudly. I have not had a chance to look at it yet - and I didn't even check to see if it still actually works (i.e. I didn't play the guitar through it before turning it off).

Is there anywhere in particular I should start looking for the cause? My assumption (and hope) is that the solder heated up and I simply have a grounding short in the power section, but what else could cause this amp to amplify a large amount of hum?

Thanks in advance.

Greg

[Enzo]

I doubt it got hot enough in there to melt solder. Did the knobs melt off?

However, I would not be at all surprised if you cooked a filter cap or two. Id a cap vented, then it no longer will filter, so that would make hum.

Basic troubleshooting here. It is either the power supply or the amp circuits. Verify all the power supply voltages are right and clean, and go from there.

[tonezoneonline]

There isn't much better advice around than Enzo's.If that does not narrow it down start checking some voltages.

[greg]

Thanks for the replies. I'll check it out this weekend and will be back on the boards if I can't track it down. It is amazing to me that I have been able to successfully build 4 amps with such little knowledge about electronics and not yet having developed any troubleshooting skills. I suppose that means I am pretty good at the planning and exectution, but I am looking forward to learning something from this situation.

Thanks again.
Greg

[Shea]

Have you tried replacing the output tubes? I've had amps hum when an output tube started failing.

Shea

[greg]

No, I haven't replaced the tubes yet, but that is definitely the first test I will perform (actually, I plan to test the tubes in another amp first just in case they were damaged by something major, I don't want to kill any new tubes by using them in the problem amp.

Thanks

[Thorny]

If you have it running (humming) look at the power tubes. Are they "red plating" where the actual grey metal plates start to get red hot? Is just one or all of power tubes doing this? That will cause the hum as well, grossly mismatched tube current. If it is only on one side it narrows down where to look. Check for a burnt resistor on the tube socket. If it is only on one side and you move the power tubes and it follows a particular tube - that tube is the problem. If it stays in a particular socket, you probably have some damage to a resistor on top of the socket.

It is late and I am tired, so take that into consideration if I overlook something or say something stupid. Be careful.

[greg]

Finally had a chance to take a look at it.

OK - here is what I found. Swapped out for a new set of tubes. Still hums. The tubes are not red-plating, nothing looks or smells burned. All voltages look good. The filter caps are a 20+20+20+20 uf can which are internally grounded - I can't really check these ground connections out, but they don't appear to be grounded via the little metal legs that slide through the chassis. I did clip in a 40uf cap and the hum went down, so it appears to be the can. Damn - I knew I should have put in individual caps.

Thanks for the advise

Greg

[Enzo]

Actually the can itself IS the common negative terminal of the four cap section. At least one of the metal tabs must be soldered to ground.