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I have a Peavey Classic 50 that's red-plating all for tubes. Trying to troubleshoot, but can't trace out where the fault could be. Ideas?
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That sounds like you have lost your bias voltage.
Pull your power tubes so you don;t melt them. Find pin 2 on one of the sockets and see if you have about -17vDC there. You should, but if it is missing, there is the problem.
Look at the schematic. You can see just below the power tubes is the bias supply, and it starts with RAWV- which is -27v. If you follow RAWV- upper left of page, it passes through a ribbon connector and becomes PREV-. Now look on the preamp page right middle, and see the ribbon cable there connecting PREV- to -27. Thus RAWV- is just -27v. Whew...
If your roughly -17v of bias is missing, look for -27v of RAWV- on R16. If that is missing, look at those two ribbon connections. A simple test for -27 itself is the heaters of preamp tubes V1,V2. If they are lit, you have -27v.Education is what you're left with after you have forgotten what you have learned. -
I've tested the voltages on the pin 2 of the el84 tube sockets. I do get a roughly -17vdc. RAWV- is -27vdc. RAWV- on R16 is -27vdc. Everything looks normal.
They drop a few volts when the tubes are inserted. I used a set of 12ax7s and el84s that we keep at our shop for testing so we don't burn out the new tubes. When the tubes are installed, there is a humming noise on the speaker output. Could there be something breaking down under load?Comment
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Sure, maybe C10, maybe R16. But it could just be a cold solder joint or a broken jumper wire as well.Originally posted by goldtop5 View Post
It would be unusual for all 4 power tubes to redplate without a loss of bias voltage. You say that the bias voltage drops a little when the tubes are inserted, how much does it drop? When the tubes start to redplate what is the level of the bias voltage then?
I would measure these at the source of the bias voltage and not at the tubes sockets and shut down the amp after seeing where the bias reading is headed before the tubes really start to overheat.Comment
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The voltage would drop about 2 volts, roughly. They haven't redplated when I tested it. I will test again without the variac and limiter.Comment
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