Vintage Ampeg Gemini I sudden rustling turns to feedback

http://www.ggjaguar.com/gem1.jpg

1) Turn on the amp, let it warm up, turn the volume up about 1/4 of the way on both channels
2) Take a NON-METAL object (my choice - bic pen) and tap each tube individually. One of them will make a signifigantly louder ping noise than the others. Replace this tube, and you are probably good to go. If it's a tube in the power amp rather than preamp section, you need to replace the pair of them.

If this doesn't work, bring it somewhere to have the tubes tested.

If you don't have a notch, spray the cleaner into the little gap by the contact points and do the same rotate thingy. If you can't get to the back of the pot at all, take the knob off and spray right where the center shaft meets the pot and do the same thing.

If this doesn't work, abandon all hope and replace the offending pot.

theres the pot, theres the knob :)

There's 7 tubes. I have no fucking clue what they are. They don't seem to match all that well, though. There's two big ones, then three smaller ones, a slightly bigger one and a small one. They're all dim orange. No blue cast.

There is also one dial/switch on the back, but I have no idea what it does. It's not an on/off switch, it has a smooth turn like a volume knob or something. But since there are two of those on the front, I doubt it's a volume knob.

Experiment: turn on with nothing plugged in, all knobs turned down and wait for static...
I just turned on the amp with nothing plugged in and let it just sit there... it sat there for a while and then there's a little click (sounds similar to when you drop a needle on a record)... then another click... and finally static. And eventually that static became loud and I turned it off.

As soon as I turn it off, if I turn it back on, there is NO STATIC. No matter how loudly it was getting before. The static comes after electricity goes through it for a few seconds.

I turned that knob on the back all the way opposite where it was before and, while it is still popping and crackling occasionally, so far it has not erupted into persistent static and I have not had to shut it off yet.

Any idea what this mystery knob might be?

Also, the fuse on the front that pops out—how do I know if that fuse is still good? Otherwise the amp would be dead?

I'm afraid to tap these tubes to test for "ping" as someone suggestd for fear they will burst and I will be electrified and/or burned.

Going on 10 minutes... still crackling like an old record, but no static eruptions yet.

Powering and controlling the amp are seven tubes as follows:

two 7591,
one 6CO7,
three 12AX7,
one 7199.

The reverb tank location is not like Fender's later designs with the unit at the bottom of the cabinet. This reverb tank is located on the underside of a panel below the ends of the amp's tubes. A switch-able foot-pedal for Trem and Echo is standard equipment and held in place inside of the amp cabinet by a rectangular metal rod.

Is it possible that the wrong tubes are in here or in the wrong order but the thing still works? I remember when I brought it to the shop the guy wasn't really sure if he had anything that would work. I have no idea if he just stuck tubes in randomly or what. But, I can pull them out and compare to what's listed above, right? It should be written on the side of the tube, I'm guessing.

Any tips on how to pull out the tubes? Do I just yank it down while holding onto the glass? Or do I have to pull it from the base or something?

Sorry to shout, but you will blister the bejesus out of your hand if you don't give them time to cool down.

Don't be worried about tapping the tubes. I'm mean and nasty, but not mean and nasty enough to lead you into peril. Also, if you are going to pull out the tubes, number them first to avoid putting them back in in the wrong order...

Seriously, try tapping the tubes. Don't be frightened. It'll all be OK.

And with no problems. I played for almost an hour.

Do I seriously have to let the tubes warm up for 45 minutes before I can play the thing?

Sadly, this amp is way too loud for my apartment and way too tricky for playing live. Sucks, too, because it sounds amazing. What's weird about it compared to most amps I'm used to is how varied the volume is. If I hit the strings loud, there's a big response compared to most modern amps which tend to keep things within a certain range.

still, but intuition is that it's tubes, not pots that are causing the microphonic feedback.

correct, a bad fuse means the amp wouldnt turn on.

"Experiment: turn on with nothing plugged in, all knobs turned down and wait for static..."

try this again, but keep a close watch on those tubes. see if they change color. do the tap test. what you DO NOT want is the big grey metal plates in the tubes (the most visible part) to start turning red. Dim the lights a bit so you can see any color changes. The very inside of each tube should be glowing nicely, but the outer metal plates shouldnt. If any of these plates are glowing red (it'll start like a spot in the middle, like it's blushin), turn the amp off. If you have a replacement tube, try it, but if not I would strongly suggest getting a tech to look at it. It's more likely than not something inside the circuitry causing it to do that, and the tube can overheat and blow very quickly, causing a short and be very dangerous.

#3, the big dark mass in the upper part of the tube is the plate. your tubes should all look *something* like this.

I did play for about an hour last night with no incident, as I said, after having let the thing warm up for about 45 minutes before I started playing it. The only time it squealed was when I turned up the Echo with my guitar right in front of it, which is normal feedback. I immediately turned it off and then a few seconds later turned it right back on to see if it would still squeal, but it didn't. I'm thinking it just needs to warm up a long time, but I will double check to make sure the plates in the tubes don't turn reddish tonight.

I wonder what a distortion pedal would do to this thing.

that extra pot looks like a hum balance pot on the schematic. probably best to keep it right around the middle.

You mean the dial on the back of the amp? Just looked it up... you're right!

What does a hum balance even do?!

it's probably the tube that controls the reverb/echo. it's either the 6cg7 or the 12ax7 for reverb. not sure which that is there. but i'm pretty sure it's one of those that's bad.

So that warm-up time is too long? It worked fine after I let it warm up for 45 minutes. I think it only squealed because I was standing right in front of the thing with my guitar facing it as I turned up the Echo. I mean, people put their guitars in this position for the sole purpose of obtaining feedback.

I'm going to hook up a big muff and a crybaby to it tonight. See if I die.

you will not die regardless of how high you turn the gain up on the big muff. It might sound awful, but you wont die, believe me.

also, im nto sure why it worked ok after you let it warm up a while. once it's warm enough to work ok, it should be fine.

also, microphonic feedback is worst when you're right near the amp. reverb helps feedback of all kinds, and it would surely make this worse.

Something else I discovered:

1. Big Muff really controls the amp better for some reason.

2. Unfortunately, Big Muff is a sucky useless pedal for anything but power chords.

i'll trade you that crummy big muff for a really spectacular tuna melt tremolo!

It will clean up yer Big Muff tremendously, and make single notes purr like kittens. Admittedly, angry kittens, but still...

I prefer the distortion channel from almost any amp over this Big Muff. I've never been good with pedals, though, which is sad because whenever I see a cover band with really amazing guitar tones that sound very much like the original sounds, they inevitably have a pedal board full of shit with half the lights on at any given time.