1977 Fender Twin

[pnut5150]

Keeps red plating one tube, does not follow same tube but if i switch out different tubes it may change location..I just have some mix matched tubes in it so they are not matched by any means. 470 ohm resistors are good, it’s a different bias section than I’m used to, it has the hum balance and match pot. The (2) 220 bias resistors are good other than one read higher than the other (250)...

[The Dude]

It seems to me like you have multiple problems. Try this.

1) Remove tubes.
2) Clean and re-tension the sockets.
3) Measure bias at the sockets and make sure voltage is good and stays after warm up.
4) Insert known good tubes or try each tube until you get no red plating on any.
5) readjust hum balance and tubes matching.

I don't think it is your problem, but I would change that resistor that has drifted.

[pnut5150]

I have already retensioned tube sockets and actually replaced 2 of them. I will swap in some more tubes as well. I found a schematic online but couldn’t make a lot of it, I’m used to the bias circuit up by pilot light..this one has me scratching my head

[g1]

Don't think of it as adjustable bias. It's just a little tweak to balance between the push and pull sides.
The actual bias is set by a resistor. There may be a fault (like bad bias filter cap) or someone may have changed the bias resistor to a setting that is too hot. Normally they would be biased quite cold from the factory and you would not be able to get any red-plating from tweaking the "matching" pot.
Can you post the correct schematic?

[The Dude]

Before you install more tubes, check the grid pin of each tube socket and make sure it's making it's way there.

[pnut5150]

I’ll look for schematic and check voltages

[pnut5150]

https://www.google.com/search?client...DRHkxqRoIfYsM:

[pnut5150]

There is the schematic

[pnut5150]

500+ Volts On pin 4 on all tubes..the 2.2k resistor reads good, somebody has changed the 33k resistor on the marching pot, dropping it down to around an 18k resistor.

[pdf64]

So this is the 135W TR? If so the power tube grid leaks should be 47k, the high HT puts more stress on everything. 47k is the limit for 2 x 6L6 sharing.
If the 33k to 0V off the matching pot has been changed to 18k then the bias voltage to the power tubes will be reduced and tubes run hotter.

...Normally they would be biased quite cold from the factory...

I think the original Sylvania STR387 were biased cold but modern 6L6 seem to run somewhat hotter in there and require the bias voltage increasing.

[pnut5150]

After I looked over the schematic a little while last night and everything looked good and then noticed the 33k had been changed out I figured that must be why they are running hot. Somebody has installed new filter caps (F&T and Ruby) and changed that one resistor. Had original Fender tubes in it, one or more were bad it was blowing mains fuse with them in it.

[pnut5150]

Yes the 135 version

[pnut5150]

That was the problem, no more red playing with a 33k on matching pot. Thanks guys y’all always make me look and think.

[pdf64]

Red plating may not be noticeable until maybe twice the plate limit.
The absence of (obvious) red plating doesn't mean that the tubes aren't idling too hot.
To be sure, there's no substitute for checking the plate / cathode current of each tube.
Fitting 1 ohm cathode resistors are very helpful with this.