65 Fender Pro Reverb weirdness

[Randall]

I got the cleanest unmolested PR (one of my all time faves) in today. The complaint is it is getting too hot, as in not red plating, but can't keep a finger on the chassis on the power side. The bias measures 75mA L and 80mA R! I pulled the tubes and measured -67v on both pin 5, 511v on the plates, and 509 on the screens. Tried another set of tubes, same thing. I also measured -59v on both pin 5 with the tubes in for a brief moment.

Later something changed, but I don't know what. The original tubes settled down to 32mA @ 481v. That is until I put my probe on the right side plate, then plate voltage dropped to 467v and current rose significantly. Doing this same thing on the left side made no change. So now I'm wondering what could be going on here, why would a Fluke drop plate voltage like that, and only on one side? So I measured the OT windings and got 74 ohms L and 84 ohms R.

I then removed the tubes and re-flowed the bias circuit from the board up to the sockets, and hit all the socket connections as well. It is now behaving as one would expect, but some of this makes no sense to me. It would certainly seem that the tubes were losing bias voltage, but that is not backed up by my measurements.

Anyone?

[mozz]

Change the bias capacitor at least.

[The Dude]

It might also be a good idea to clean and retension the output tube socket pins. Possibly, all of your removing/installing tubes "restored" some connections. But, I wouldn't leave it to that.

[Leo_Gnardo]

What mozz & The Dude said ^^^. A few more, don't neglect the possibility of flaky output tubes, try some fresh ones. Besides the bias cap replacement, how's the resistor on that bias circuit board? And is it well attached to the rectifier? Bias voltage jumping around, could be bad connections there. Also, how are the caps and resistors in the drive circuit and their connections? Plus the 1k5 resistors to pins 5 of your output tube sockets, and their connections? Gotta put a stop to these random bias voltage changes first.

[The Dude]

And while we're at it, there's also the possibility of an intermittent/failing coupling cap from PI to output tube(s).

[Randall]

Everything measured good, except I didn't look at the PI coupling caps. The bias cap was a fresh looking Sprague that tested good. I made sure all connections were solid when I resoldered them, and did clean the tube sockets. Customer came and got it as soon as I told him it was working, so I may never know. I did run it on the bench for about 2 hours and it was solid, and no amount of wiggling or tapping around or tube rocking made a difference.

Wish I'd have thought of the PI caps.

[Enzo]

Put your scope across the speaker, see if the amp breaks into RF oscillation when you see that B+ drop and current rise.

[Randall]

I think I see where you are going with that Enzo, scope induced? Well, it stopped doing it after I re-soldered everything. It was very stable after that, and now it's gone home. Guy was in a mad hurry, as he is a snowbird heading north in a day or so. REALLY good sounding amp, btw.