'65 Gibson GA25RVT - Bias supply issues?

[Turbo7MN]

I picked up the amp a while back for $100 in realy rough shape (it was in several pieces actually ). Anyway, I put it all back together and it's been working fine for a while but recently it developed a problem where a few minutes into playing it would make a "popping" noise for a few seconds, and the output would drop considerably and there was a lot of extra distortion. After it did this a couple times I watched the tubes (JJ 7591s) when it happened, they kind of "lit up" and after a little while the plates started turning orange, I could tell there was more heat coming from the tubes as well.

So today I opened it up, got it all hooked up on the workbench and sure enough, as it makes the "popping" sound the bias supply drops from -19 VDC to -0.7 VDC. Not good. I also noticed the plate voltage drop considerably but I'm assuming this is from the extra current the tube is pulling.

The filter caps in the bias supply were replaced before I got the amp so I'm assuming it's not those. My guess is the diode is on its way out. This amp is the biggest rat's nest I've ever encountered and working on it is not an enjoyable experience so I'd like to see what you guys thought before I either replace the wrong component or unnecissarily tear the whole thing apart so I can get to the rest of it.

This amp doesn't match the GA25RVT schematics on the internet, it's closer to the GA35RVT schematic and the bias supply is nearly identical.

http://www.schematicheaven.com/gibsonamps/ga35rvt.pdf

Thanks in advance.

-Darren

[Rob Mercure]

Sounds like your bias supply has become a low frequency relaxation oscillator - while I suppose it's possible that the bias supply diode would become intermittently a higher/lower resistance diodes generally tend to be good, open or shorted. Yeah, these are rats nests but can sound good when functioning well. First I'd not trust caps that I didn't replace myself when confronted with bias supply problems so I'd probably just replace the "whole kit and kaboodle" - the diode and the pi filter section and the 330k dropping resistor - cuz the marginal cost of the components is far less than the cost of the OT/PT/output tubes.

Rob

[Turbo7MN]

Yeah, I think I may as well redo it all. What component(s) do you think went bad to make it do this? I'm probably going to replace it all, I'm just curious. Whoever had it before me really made a mess of it (horrible solder joints, speakers wired for the wrong impedance, random cap wired across the output tube plates, etc.) but I fixed everything that was obviously bad and the bias supply looked alright physically.

It is a good sounding amp though. Really warm cleans. I was pretty mad when it crapped out .

Thanks Rob.

-Darren

[Enzo]

The last thing I would suspect is the diode, but anything is possible. Failing filter cap first. Is this voltage right at the bias supply or at the tube sockets? On the other hand, I am a believer in "parts are cheaper than labor" so in less time than it would take to test the diode, a new 5 cent diode could be in its place and then yuo are sure it is not the diode.

Does the bias still go away when the power tubes are removed?

If both are affected, then I would be looking at the supply too.

Welcome to Gibson amps. They rarely match up to the schematic of the same name. AS you found, other Gibson models often match better, not only that, the Epiphone amp lines are mixed in there as well. So a GAxxx might really be an EAyyy.

[Turbo7MN]

I checked it at both actually. Since this has one of those odd transformer phase splitters I measured at the bias supply too just to make sure I wasn't losing it at the transformer. I was getting a little tired of holding the meter probe in there waiting for it to happen but sure enough, "pop, pop, pop" and the voltage fell like a rock like it did when I measured at the tube pins.

Also, when it was working for a little while today I noticed that cranking it (driving the output stage harder) also seemed to trigger the problem.

Anyway, I'll order all new parts for the bias supply tonight. Unfortunately I'll be out of town until next weekend so I won't have a chance to work on it for a while. I'll let you guys know how it went when I get it done, thanks for the advice.

Any other ideas though, please post them.

-Darren

[Turbo7MN]

This thing is cursed.

I rebuilt the whole bias supply. New resistors, new caps, new diode. I also finally got around to replacing all the power supply filters while I was in there.

I finished it up tonight and hooked it all up. I was enjoying probably the sweetest sounds I've ever heard from my strat for about 10 minutes and all of a sudden the volume drops and there's the distortion again. I'm 100% stumped.

Also, I noticed a really nasty hum that wasn't there before. When I went to measure B+ it blew the fuse in my meter. Someone before me installed a 3-prong cable but never removed the death cap so I clipped that while I was in there. Could that have screwed something up? As far as I can tell I've done nothing different on this amp besides rebuilding the bias supply than any other amp I've got. The electrolytics I used were on the shelf for a few months before I used them, could that have anything to do with it? Any other ideas here?

-Darren

[Turbo7MN]

Alright, so I've been playing though it most of the night now. It's behaving much different than before. It doesn't seem to happen at all while playing anymore. When I shut it off sometimes I can faintly hear the "pop" and when I turn it back on it acts like it's going to crap out but it comes back and all is well. What's weird is once in a while now I'll get a very strange and very loud noise out of it right when I turn it on, it's hard to describe besides being an "electronic" sort of noise if that helps, that varies in pitch for a few seconds and then goes away. It doesn't seem like any "normal" kind of oscillation.

I wish I could figure out the hum though. I noticed it gets noticeably better when I turn the reverb off via the footswitch. When I use the vibrato it also affects the hum so it's got to be somewhat early in the preamp. I'll have to give it a runthrough with the scope. I also wish I knew why my meter blew. Seems like it's more likely to be a short in the meter more than anything else, I might just grab a new one from Radio Shack to see what's up.

Any thoughts or suggestions on any of these problems would be very much appreciated. I want to have this amp fixed and sold by this week so I can go pick up my new Telecaster on Saturday.

-Darren

[Turbo7MN]

Okay, I grabbed a meter from Radio Shack today just to see if my old one was indeed screwed up, the new one reads fine so that's not a problem.

So I'm just trying to get rid of the hum now I guess.