69 Gemini G12 Help

[jazisded]

Hi Everyone,

Just came into a 69 G12 Gemini amp dirt cheap and took a chance on it, as I was looking for an Ampeg restoration project. Might have gotten a little more than I bargained for since I found out this amp uses PCB's. Joy!

The amp looks to be all original with Amperex 12ax7's and Magnavox labelled 7591's. Doesn't appear it has never been serviced. Ever. Don't think a solder joint has been heated since it was born in New Jersey.

I'm going to order a cap kit from fliptops (the amp motorboats when turned on), but it looks like I also need 2 4meg PCB mount pots for the Treble and Intensity controls and also 2 switches for Polarity and Power (the shafts are broken off) and it doesn't appear they have them. Any ideas for a source for these?

Aside from doing a Cap Job and putting a 3 prong power cord on it, what else should I be doing right off the bat on this amp? I don't have much experience working on a PCB amp or Ampegs in general. Is this even going to be reliable when serviced?

I have no clue what kind of speaker is in it either - it just says "Blo Out Proof" 100 Watt. Ha! Bet someone was sore they blew the original Jensen. Ever heard of it?

Finally, I'm a new poster here but have lurked for a long time. Hopefully, I'll be able to finally contribute something with this project.

[stokes]

I have a couple of older Gemini's-tag boards instead of PCB's.For the switches,I used a standard on/off toggle in place of the Polarity/standby switch.With the 3 prong power cord you dont need the polarity anymore.New caps and a good speaker and the amp should be fine,maybe new power tubes,on one of my Gemeini's I re-wired it back in the late '70's for 6L6's,but it still has the original Amperex/Bugle Boy preamp tubes from '66 or so,and the amp saw a lot of use over the years,including about 3 years of being abused by a bass player I played with.

[jazisded]

The parts came in from Fliptops, and I changed out the power cord, filter caps, bias caps and the electrolytics in the tremolo section. Also replaced the power switch and polarity switch (for cosmetic use only). The amp now works on channel one and two, but the Reverb and Tremolo effects don't work. So that's the next part of the project, along with finding replacements for the 4 meg Treble and Intensity Pots. Still haven't located a supplier for those.

Any ideas on how to troubleshoot the Trem/Reverb effect? I'm thinking perhaps the footswitch isn't working since they are both dead. Is there a way to test the Trem module before I just order another one?

[jazisded]

One more question, I just realized that there isn't any reverb Pot on this amp -- How does one control the amount of reverb that gets dialed in?

[olddawg]

[The next part of the project finding replacements for the 4 meg Treble and Intensity Pots. Still haven't located a supplier for those.]

Are the shafts broken? You may be able to take the pots apart and repair them. Even if the shafts are broken you may be able to remove the element from the obscure pot and install it on another old pot. With restorations it is always good to have a bunch of old pots and switches, good and bad, laying around. They don't have to even be from guitar amps. Many times you can rebuild what you want from junk parts especially if they are CTS. If the shaft is broken or whiper bad, just replace it if the element is good. Just make sure you use the correct solvents, cleaners, and lubricants.

[jazisded]

Olddawg,

Thanks! That's a great idea as the shafts are snapped off. These are PCB mount Pots, and I doubt I will be able to find an exact replacement. Perhaps I can replace the shaft with something similar and save the original unit. I think I'm going to troubleshoot the Trem and Reverb and then pull the board those Pots are on and see about taking it apart.

If that fails, I think I just found an NOS Clarostat part at Angela that I *think* I can use, although it won't be PC mount and I'll probably have to rig it up with jumpers. I'd rather salvage the original part as you have alluded.

I was mistaken at first glance about this amp - something must have happened to the main PCB at one time as a small rectangle was cut out by a tech and jumpers were run around the affected area. Looks like that area was connected to the output stage, so I bet something gave out in a spectacular way. Couldn't have been that bad though - still has the original Magnavox labelled 7591's and Amperex 12ax7's so the tubes survived ok.

It's gonna be fun to get this old dog running full steam again.

[olddawg]

More than likely the shaft and wiper out of the same brand of pot from that general time regardless of the mounting structure will work with the surface mount case and element assy. As for the hole cut out. This is frequently done because the boards are made of cardbard and resin or other paper based composition that turns to charcoal when a component burns over it. The charcoal is of course carbon and being conductive has to be removed. It is a very common situation. The jumpers should be fine if done correctly. To get things working you might want to just slot the snapped off pot posts with a jewler's file so you can turn them with a screwdriver. Also, any pot with the right values and taper can be hardwired into the amp. It just depends on how staight you want to keep your restorations. In my mind it's about functionality. I save the old parts, but I put in whatever I have to do to make my amps reliable and functional. Most of my amps have been rescued from various junk piles and look like they have been through WWIII. I love playing clubs with a sweet sounding but really beat up and ugly old amp.

[jazisded]

olddawg,

Thanks for that great info. I haven't encountered a burned PCB board before so its good to learn that. Seems a disadvantage of the design. It was pretty easy to swap components on though. Anyway, I'm going to try and come up with replacement shafts for those pots.

Love getting old wrecks and fixing them up. I used to do old tube Hi Fi's. I'm hoping to fix up a bunch of different guitar amps until I find a few I really love. Until then, I'll fix em and flip em and hopefully learn lots of things in the process. Side benefit and Karma bonus: Getting old amps gigging again.

[jazisded]

Well, I spent a little time this weekend trying to get the reverb and vibrato working, but haven't had any luck. Channel 2 does work and sounds fine, the effects just don't work. I swapped the 12ax7 and 6cg7 tubes and that didn't improve the situation. Any ideas on how I should proceed?

[52 Bill]

Originally Posted by jazisded

Any ideas on how I should proceed?

With the reverb on, when you shake or rattle the tank do you hear the springs through the speaker? If you do, then you know that the tank's output transducer and the amp's return circuit are working. If not, then start by checking the reverb tank and cables for broken wires or bad connections. Test each transducer coil with a meter for continuity.

Check the tremolo oscillator, is it running? If not check the phase shift caps and resistors for off values. There usually is cathode bypass cap on the oscillator tube be sure to check that. If the oscillator is running, then trace the output voltage through the depth control to the output tube grids. See if the control is ok and check the grid resistors. Also check to be sure that the phase inverter coupling caps are not leaking voltage to the grids of the power tubes.

Hope this helps.

[jazisded]

Well, I seem to be moving very slowly with this project. Finally got back to it today and got the trem working. Looks like it was a bad pot (one of the ones with a broken shaft). Cut a groove in it and used a flathead to rotate it while spraying deoxit inside. Magic! Nice to know the trem module is good.

Now, here's where my inexperience shows:

Next thing I did was to check voltages on the tubes. I have a problem with the phase inverter (7199) section. Voltage at pin 2 is supposed to be 120vdc and I got 232(whoa). The other pins checked out ok. So I thought I might have a cap leaking DC, so I changed it out and it made no difference. Put back the original, and now I have 232 pin 2 and 243 on pin 3, so I must have botched something. Sound is now very weak and garbled when I plug a guitar in.

Also have high vdc on pin 1 of V7 (12ax7). Supposed to be 130, and I have 184.

Sheepishly, I ask for your guidence.

-Tony

[jazisded]

ok, I'm really a dummy. I had a 12ax7 in V3 instead of a 7199. I am back to where I started - the amp plays the way it did before, but I still have 232vdc on pin 2, but now I have 40 volts on pin 3.

Here is the voltage chart from Joe Piazza that I am referencing: http://web.archive.org/web/200104180...arts/g-12.html

[jazisded]

ok, got the reverb working. Took apart the tank and found broken solder joints on the two green wires. Soldered them up and put it together and we've got surf guitar! Wow - what a deep reverb!

[jazisded]

Bump for help on the high voltage on pin 2 of the 7199 Phase Inverter. Still not really sure how to proceed with this. Could it just be a bad tube? I am getting 232 vdc on pin2 and the voltage chart states it should be 120vdc...

[darrellcheng]

check readings on resistors before this point, make sure they are correct value, and not cracked or discolored

[jazisded]

ok - sounds like a plan. Do I have to take the resistors out of the circuit to accurately measure them?

[darrellcheng]

first read the values of the resistors, use something like this:
http://www.dannyg.com/examples/res2/resistor.htm
then you can try to read them in circuit, if they read correct, then it should be fine, unless they are visibly damaged/discolored. If they are not reading correctly, then take em out. Also, check the coupling cap and pretty much all e-lytic caps by making sure they aren't passing DC. IF you have a cap checker, you can check the values too.

Also Do you have a correct schematic? Make sure there were no mods or incorrect values replaced.

[olddawg]

Originally Posted by jazisded

ok - sounds like a plan. Do I have to take the resistors out of the circuit to accurately measure them?

I find the easiest way is to just lift one lead. Just be meticulous or it can look sloppy afterward. Unless you are into a restoration for a collector I find that it is easier just to replace most caps. They're inexpensive and you have to have them out anyway. IMHO cap checkers lie to you too often.

[darrellcheng]

Old Dawg, cap checkers lie? What would you suggest as a reliable test method for caps then?

[olddawg]

Originally Posted by darrellcheng

Old Dawg, cap checkers lie? What would you suggest as a reliable test method for caps then?

Like I said before, replacement. Generally, as in the case of your trem ciruit, there are only a few. Sometimes you can just jump one with a known good cap. I gave up on cheap in circuit ESR meters and such a long time ago. Test equipment does exist that can accurately test caps but it is expensive, complex, and you have to take the cap out anyway.

[luthierwnc]

The trems work the opposite of a Fender. The trem has to be grounded to work since that is actually the cathode of the drive tube. If the footswitch is croaked, jump it short with an alligator clip and see if that helps. It will also change the voltage on that triode.

Skip

[jazisded]

A month later and I just got back to this - I took out one leg of the resistors around the 7199 and measured them - they all checked out within spec. I swapped the 2 .1 coupling capacitors between the 7199 and 7591's. Swapped the other 2 .1's going to the 7199 and also the .2 cap - no voltage change. Anybody have a hint for a weekend warrior tube tech who's out of ideas? I'm still getting 230+ volts on pin 2 of the 7199.

[olddawg]

Did you ever find an accurate schematic?

[jazisded]

I am going to get the aspen pittman book later today to see whether my schematic is correct. I spoke to a friend who has it last night and it appeared to be the same. I want to see it with my own eyes though.

[olddawg]

Originally Posted by jazisded

I am going to get the aspen pittman book later today to see whether my schematic is correct. I spoke to a friend who has it last night and it appeared to be the same. I want to see it with my own eyes though.

Is it this one?
http://www.unofficialampeg.com/

Go to schematics at the top and scroll down to the G12.

Try shorting the footswitch connection at the board.

[jazisded]

Yes, that is the schematic I have been using. The voltage chart I have been using is located here: http://web.archive.org/web/200104180...arts/g-12.html

That is from Joe's homepage in 2001 - I haven't been able to get to his site in a long time, so I used the wayback machine.

[olddawg]

Originally Posted by jazisded

Yes, that is the schematic I have been using. The voltage chart I have been using is located here: http://web.archive.org/web/200104180...arts/g-12.html

That is from Joe's homepage in 2001 - I haven't been able to get to his site in a long time, so I used the wayback machine.

Is the problem just that the trem isn't working at this point or do you have other issues? I would suspect the optical device if you have replaced all of the associated caps and verified the resistor values.

[jazisded]

I can't believe i didn't update this thread after getting the amp working. I think I was so excited I forgot all about it.

The problem with the high voltage on pin 2 of the 7199 was the wrong resistor value was connected to that pin (Wonder how that happened). Once I added the correct resistor as per the schematic, the voltage came down and everything was fine. The Trem works, reverb is good, voltages are all within spec and it sounds pretty good. I popped in a pair of EH 7591's and the amp sounds very strong.

The one thing I have to resolve still is finding some new shafts for the broken pots. I've ordered a Weber Blue Dog Ceramic for it, and I'm hoping to have a great amp on the cheap.

Thank you to everyone who helped me with this project. I really learned alot and I couldn't have done it without you.

Regards,
Tony

[olddawg]

Originally Posted by jazisded

I can't believe i didn't update this thread after getting the amp working. I think I was so excited I forgot all about it.

The problem with the high voltage on pin 2 of the 7199 was the wrong resistor value was connected to that pin (Wonder how that happened). Once I added the correct resistor as per the schematic, the voltage came down and everything was fine. The Trem works, reverb is good, voltages are all within spec and it sounds pretty good. I popped in a pair of EH 7591's and the amp sounds very strong.

The one thing I have to resolve still is finding some new shafts for the broken pots. I've ordered a Weber Blue Dog Ceramic for it, and I'm hoping to have a great amp on the cheap.

Thank you to everyone who helped me with this project. I really learned alot and I couldn't have done it without you.

Regards,
Tony

Great score...Fliptops will probably have the exact replacement pots for a couple of dollars each.