Ampeg Gemini VI - Can't Crank It

[escottm]

I trying to ressurect a 60's Ampeg Gemini VI which was stored in the original owners garage. I have replaced all the electrolytic filter caps and the coupling caps. A couple of the large power resistors were falling apart and I replaced them too. I changed out the 2 prong cord for a 3 prong.
The amp works, tremolo and reverb work, but the amp only sounds good on very low volume. Turn it up to 3 or 4 and I have a nice clean sound. Turn it up to 6 or 7 and I can hear the beginnings of a great tube amp blusey, growley distortion, but only for a short time. When the amp starts to distort, it also starts to sputter, the good distortion is replaced by ugly sounding stuff and then the volume cuts out. To recover, you need to turn the volume back down to 3 and wait for several seconds without playing. Then the good clean sound comes back. It almost sounds to me like a coupling cap issue, but I have replaced almost every cap in the amp.

Any ideas?

[Diablo]

You might try replacing tubes one at a time to see if things get better.

[escottm]

also my first thought

Yeah,
That was my first thought. I got a set of brand new tubes from flip top and plugged them all in. Same symptoms. On further thought, this is a lot like a parasitic oscillation problem. If turning up the volume caused an oscillation above the hearing frequency, then I would imagine the outcome would be exactly what I am hearing from this amp.
I have an ocilloscope, but not a lot of experience with this kind of problem.
Anyone know a good method for looking for parasitic oscillation with (or without) an oscilloscope?

[Jazz P Bass]

Monitor thr B+ voltage when the problem occurs.
If it drops drastically, you may have a bad output transformer.
I would also suggest monitoring the mains current that the amp is pulling.

[escottm]

535V pretty steady on the B+. Playing it drops occasionally to 524 and once I saw 516 for a brief moment. Does not seem to coincide with the breakup of the sound from the guitar.

[davohilts]

I'm working on my Gemini 6 right now and am wondering how you replaced the electrolytic caps. Stock there was a 4 x 40mfd x 500 volt cap can I believe with 2 of the 40's tied together for 80mfd. That leaves 2 other 40mfd caps in the can and then 2 20mfd caps which must have been single axial caps wired into the circuit. I is a fixed bias amp that has a 15K adjustment pot on the bias circuit. When you replaced the bias cap (100v x 100mfd) did you install it properly with the + going to ground? Have you checked the bias of the 7591 tubes? Don't know if you have this sorted yet but you might try these things and see how it works.
Dave

[stokes]

Could be an oscillation caused by a bad cap.Had a Deville on my bench a while back that was acting similar.Was a bad filter cap in the preamp.Amp was less than a year old.I've seen an inordinate number of these newer Fenders with bad filters,fairly new amps.I attribute it to a bad batch of those asian caps Fender uses.Sometimes in these Ampegs,if you use axial caps in place of the can cap,you have to be careful where you ground them,they will cause problems if not grounded properly.Its also possible that you got a bad cap from the supplier.New filter caps should be bought up to voltage slowly with a variac before applying full voltage,its called forming.You can usually get away without forming,but if you got a cap that was sitting on a suppliers shelf,unused for a number of years,pre-forming is a must.

[leydenjar]

If you're using individual caps instead of a can cap, attach the ground (-) of the new caps where the old ones were.

[olddawg]

Not to be condescending, but have you verified that the speaker is functioning properly? The voice coil might be grabbing when you overdrive the amp.

[stokes]

Where are your power tubes idling?Is that 535vdc on the plate of your power tube?Its quite high.

[kleuck]

The only Gemini VI schematic i found shows 500 volts caps, so 535 anywhere is absolutely not normal.

[Mr Johnny Birchwood]

Check your power transformer. My gemini VI had a bad power transformer, went up in smoke at one point and it was a giant chunk of carbon on the inside. It would work for a minute and then the carbon would get warm and start conducting, and I ended up with B+ voltages all over the place, as well as 120vac sitting live on the amp chassis, which blew fuses when I installed the grounded cord. I had similar problems with fading out and found a couple of bad resistors in the ps that would get warm and start to go open causing the amp to fade out after a few minutes. If you replaced resistors in the ps you want to make sure they are of a sufficient wattage rating.