All the layout drawings you posted show 4 power tubes, but the chassis pictures show 2 power tubes. I wrote out instructions for 4 power tubes, but it's the same with 2 power tubes.
@mresistor - is the -58v wire coming out of the doghouse? I'm thinking this is the yellow wire as noted in the drawing.
Yes, that's it. It goes to the -bias balance pot. In your drawings; PT brown wire is acv bias tap, goes to 1K2 dropping R, then reversed diode, to get -dcv, then blue wire to dog house filter cap board, to 1st 70uF/100v -dcv fliter cap, then 2K2 dropping R, then 2nd 70uF/100v -bias filter cap, then yellow wire to -bias balance pot CT, and also off the balance CT a wire, color not labeled, that goes over to the trem circuit 2M2 R to kick start it. The -bias filter caps, + (positive) end goes to ground because their for a negative dcv supply.
I would pull the 6L6s out and not put them back in until you find out where the problem is with the negative bias voltage.
First check to see if the Output tubes matching potentiometer is wired correctly. You don't show that pot being wired in your drawing. I'm thinking that the -58v wire is not connected to the potentiometer.
https://el34world.com/charts/Schematics/files/Fender/Fender_twin_reverb_sf_135_schem.pdf
Turning the amp on and in standby what is the voltage on pin 5 of the 6L6GCs?
Like mresistor wrote, take the power tubes out until you get the -bias working and balanced. And all these measurements can be done with the amp on but in standby mode.
Your amp has a -bias balance pot, not an adjustable -bias pot. So you might need to adjust the -bias balance pot. You may have enough -bias on 1 side = 2 power tubes, but not enough on the other side = other 2 power tubes.
With the power tubes still out and the amp on in standby;
1. Measure for (negative) -dcv directly at pin 5 of
each/every power tube socket. What do you get? Post all 4 measurements please.
2. Measure at the -bias pot, B1 and B2 in your drawing. 1st, with the pot turned full clockwise, measure -dcv at B1/B2, then with the pot turned full counter clockwise, measure -dcv at B1/B2, post all 4 measurements please. We're looking for the balance pots -dcv swing.
3. Now adjust the -bias pot to get B1/B2 as close to each other as you can. B1/B2 feed each of the 2 power tubes pin 5 pairs -bias. Please post the balanced -dcv measurement.
4. Now double check; with the power tubes still out and the amp on in standby, measure the -bias on the far 2 power tubes at pin 5, closest to the chassis end, then measure the -bias on pin 5 of the other 2 power tubes.
What do you get? Post all 4 measurements please.
If the 4 power tubes are matched, then they should all draw about the same current. When Fender made that amp they were not getting matched tube sets. But they were probably getting tubes that were pretty close in current draw. Close enough so they could find an overall -bias dcv that would keep the hottest tubes from red plating and add a small margin for tube safety. The other colder tubes would just run cooler, but Leo, then CBS, wanted clean, not hot, so it worked. Then they could use a -bias balance pot to balance the output to keep the amp quiet and have a good bass response. (When you balance both sides in a push/pull (PP) amp, it helps give a nice quite, clean(er) output, with good bass/bottom end. As it goes out of balance, you start to loose bass and the output starts to get noisier. The more out of balance, and as the tubes age, the worse these things get.)
Your -bias balance pot could be off, or, your tubes have drifted, or a little of both. That could be why your tube (1 tube?) is red plating. You'll know more after you take some measurements and adjust the balance pot if needed. You probably need new bias caps too, if they haven't been changed yet. What power tubes are in the amp? How old?
And because you'll still need to know how hot or cool the output tubes are running now, even after their balanced, you really should install 1 ohm R's between the power tube cathodes and ground, for easy, safer bias measurements. Our host Doug sells them in his online store.
But you need to find out where the amp stands as it is now.
Willabe
Topic: '75 or '80 Ultralinear Super Reverb redplating (Read 5274 times)