HELP needed from tube amp gurus! b15n distortion and fade problem

Here's a possible hint. That sort of a dull roar (hum) is typical of a mismatch of current in the output tubes.

The output section is pushpull. You have two tubes connected to opposite ends of a center-tapped transformer. The (positive charge) or voltage flows from the center tap (A red wire) outward towards each tube (blue and brown wires). I put a green rectangle to draw your eye to the center-tapped primary of the transformer.



The hum cancels when each tube pulls approximately the same amount of currently.

So my guess is either one of the tubes is changing state and conducting really hard, or practically turning off.

If it's conducting really hard you may see evidence because the plate will turn a cherry red. Actually I think I see this in the video. Check the tube to the right of the Plexi Ampeg logo. It had a red glow when the amp was humming. After you flip the standby off an on, the glow is gone. My guess is one of the 270K bias resistors is intermittent or the tube socket needs retensioned or cleaned.

If the tube has been allowed to redplate for long it may not last for much longer.

Good luck!

 

when was the last cap job done on the amp ?

 

I would also suspect an intermittent power tube.

I would avoid "cap jobs" unless it can be shown without a doubt that a bad cap is causing the symptoms. Cap jobs are just another term for hand job unless it can be shown that a cap is failing. My guess is that 95% of all replaced caps are perfectly fine.

 

As @agedhorse mentioned, it could easily be an intermittent tube as well. You could swap the tube and see if the problem stays with the socket or follows the tube.

Sounds like you may have enough of a background to handle this yourself .

 

That part of the circuit is pretty simple. You might want to read up on how fixed bias works.

If you don't know, rather than simply shorting the caps, it's better to use a shorting jig with a resistor. I use some insulated test leads and 33K, 3W resistor I swapped out of one of my amps. It takes a little longer to fully discharge a capacitor, but no worries with leads sticking to the terminals due to arcing. A lower value resistor would be better, but the 33K was handy .

Confirm the caps are discharged with your multimeter.

 

This one looks rather warm compared to the others.. @Wasnex has a good eye!

 

If it’s an intermittent issue with a power tube, it may not reveal itself with a tube tester. Tapping the power tubes while the issue is occurring might help reveal which tube it is.

Another possibility is a bad standby switch. In this amp, the standby runs through the high voltage center tap to ground. It stresses the switch and eventually they fail. Even when in the switch on position.

Another possibility is a problem with the rectifier tube. If one of the two diodes is intermittent, hum will come and go. At the very least, check the socket tension. If you have a replacement tube or plugin solid state module, swap it in and test.

 

I worked on one about a year ago. The solder joints all looked fine, even under high magnification, yet reflowing a few fixed it.
Tap around with a small wooden stick to see if you can get it to mess up.

 

but a whole new set of tubes (you should have a backup set regardless) and try swapping them. could be the rectifier tube.

 

If you can get the amp to repeat the problem, I suggest looking at both output tubes. Pretty sure it's only the tube on the right that is red plating, but I could be wrong. Also FYI my ears are really what keyed me to the problem. (hint, hint) That sort of dull hum typically means the current in the output transformer is badly out of balance.

If it's just the one tube, I think the best thing to do is simply swap the tubes (with one another) and see if the problem follows the tube or not. Could be a tube, could be a resistor, could be a flaky solder joint. If the problem follows the tube, you know you need a new set of tubes and a setup.

I don't recommend just throwing a new set of tubes in the amp. If new tubes are required, it's a good idea to check the current passing through tubes to make sure the amp is biased to the proper range. This requires bias probes or the installation of bias sense resistors off the cathodes of the tubes.

AFAIK, the amp does not have variable bias, so there also a good chance it will require changing out a resistor in the bias supply if the tube current does not fall in the proper range. This is probably work for a more experienced tech.

If you just throw a set of new tubes in and they are biased to hot, your basically just throwing away good money and risking a major melt down that could take out the transformers.

While it's possible to measure or extrapolate the tube current off the plate and transformer, I don't recommend these procedures for a self-professed novice, due to the lethality of the voltages. I don't even use these procedures myself. Not that I can't use the procedures, I just prefer to use other methods.