Peavey Classic 212 with no output

[Johnrcurry]

I am working on a '70's era Peavey Classic 212 (solid state preamp, 2 6l6 power tubes). I'm including a link below with photos of the chassis to help identify the amp:

Classic 212

I haven't been able to find the right schematic for the amp. I've written to Peavey customer support and am waiting to hear back from them.

The amp has no output. The pilot light comes on and the tube filaments glow, but no output. I've got 528 volts on pin 3 of the power tubes, but only 14 on pin 4. The high voltage to pin 4 disappears as it reaches the big blue resistor pictured below:



There is 528 volts on one side of the resistor (the right side in the picture) and only 14 on the other (the left side in the picture). The red wire on the left leg of the resistor goes on to pin 4 of the power tubes. The resistor tested open in the circuit, so I unsoldered one leg and it also tests open out of the circuit. My thought was to replace it. However, in light of its odd value (as I read the bands - yellow, black, orange, silver - it is a 40K 10% resistor) and the fact that I can't verify it from a schematic, I'm looking for some more expert advice as to whether I'm on the right track. I'd appreciate any help you folks can provide.

Thanks

John

[Chuck H]

That looks more like yellow-black-brown to me. 400 ohms.

Different paint hues can be confusing. One companies orange looks sort of brown or the reverse. Are there any resistors with the same blue color in the amp. They may be the same MFG and could be used as a reference if you can find both brown and orange for comparison.

There's a good chance that whatever opened that resistor is still a problem in the amp. Could have been a bad power tube. If it was a bad tube and that tube is in place when you fire up the amp it could blow that resistor again or even something else. Do check the screen grid resistors since a bad tube will often take out a screen resistor when it goes. After repair, bring the amp up without tubes in it and use a variac or current limiter.

[Jazz P Bass]

Bingo on the 400 ohm resistor
5 watt.
If the amp is a Classic B (which it sounds like), that "is" the screen resistor.

[Chuck H]

Well... Probably a shorted power tube then. (but only probably)

[Johnrcurry]

Chuck and Jazz P Bass,

You're both absolutely right. Jazz, that is the right schematic for the amp I'm working on. I temporarily clipped in a 470 ohm resistor (the closest value I had on hand), and brought it up on a limiter, with out tubes first, then with tubes. With tubes I had 361 volts on both pin 5 and pin 4 and the amp worked. All looked well, so I brought it up without the limiter. All went well again. I had 500 volts on pins 4 and 5 and the amp worked.

Why the resistor failed in the first place is still a mystery, and I have to find a 400 ohm, 5 watt resistor. Thanks so much for your help.

John

[Jazz P Bass]

Why did the resistor fail.
Something downstream pulled more than 5 watts through it.

[Enzo]

Those resistors fail when power tubes fail. Very common.