Help! Tube Amp cuts out after 2.5 hours

[Blue Finger]

Sovtek mig 100 volume goes to almost zero after the amp has been on for awhile. If you turn off the standby (leaving the main power on) for a few minutes the amp will work fine for a few minutes or, when it cuts out, you can pick it up a couple of inches and drop it and it goes back to normal for a while.

Any ideas??

Thanks.

 

[John Phillips]

A bad joint or cracked resistor in the B+ supply somewhere, almost certainly. At the very worst it could be a bad PT secondary winding, but it's more likely to be downstream of the standby switch.

 

[Blue Finger]

Hello John,

Thanks for your input, I'm not all that up to date on B+, could I email you a schematic and and let you point out the areas I need to check?

Thanks

Jimi

 

[John Phillips]

It should be as simple as to meter round the circuit from the standby switch onwards once you can get it to fail on the bench. If you check the voltages on the tube pins - start with the power tube plates, then the power tube screens, then the phase inverter plates, then the preamp plates - it should tell you where to look more easily than I can from the schematic actually...

If you can't get it to cut out on the bench, go through the amp with a well-insulated tool (a plastic pen is quite good) poking everything until you find the problem.

(I'm sure you know this already, but BE CAREFUL in there - the voltages you're looking for are in the 400V+ region. Make sure your multimeter is rated high enough, and that the probes are too.)

 

[Blue Finger]

Hey John, thanks so much for the help. The other guitar player in my band posted this last night, I'll make sure he gets your reply. Now that he has an idea where to look, I'll bet he can find the problem. We don't have any amp techs close by, so we always try and fix 'em ourselves, if we can.