Ampeg Gemini 2

[marcoloco961]

Hello everyone. Hope the New Year finds you all well. I have one of these Ampeg Gemini 2 that is blowing fuses violently. It is at a friends shop and he is pulling his hair out. He has replaced the original can cap but it did not help. I am going to look at it tomorrow to see if I can find anything. Anyone ever have this problem with one of these amps. Suggestions on where to look first would be much appreciated. Thanks in advance gentlemen.

[eddie25]

Assuming it's still blowing fuses with no tubes in...

How about tube socket carbon build-up causing a short? Seen that on an old Ampeg.

[Stevem]

I have repaired tons of these and the power transformers are not the greatest.
When the amps filters start to go the PTs run very hot and then fail once the cap gets so shorted that is blows the fuse a few times.
so long story short you also need a new PT, and Fliptops can provide you with one.
Also note that if you want the amp to work better than stock in the 120 HZ hum department, have your tech install all seperate Atom brand caps so you can inprove on the amps grounding layout.
You can leave the amps toasted can filter hanging there for a stock look if that matters.
Glue the seperate filters in place with black automotive silicone gasket maker and ground the filters that deal with the preamp section supply back over at the input jack.
You should up grade the first filter from its stock 30 uf @ 600 volt valve, to two 150uf@ 300 volt filters with a 180K 1/2 watt resistor across each cap.
Another nice mod while you are in the boiler room is to replace the 4 stock diodes in the HV section with UF5408 fast recovery types, one or more of the stoke ones may have blown anyway just due to the shorted filter issue anyway!
Good luck.

[marcoloco961]

Thank so much, will start there

[Leo_Gnardo]

Another nice mod while you are in the boiler room is to replace the 4 stock diodes in the HV section with UF5408 fast recovery types, one or more of the stoke ones may have blown anyway just due to the shorted filter issue anyway!

I'd start with the rectifier diodes first (cheap), also suspect the bias components. For the hi voltage rects I've found UF4007 to be plenty sufficient (1 amp rated) but if you want to go ahead with 3 amp rated '5408's.

Recently I had to repair a Gem II where someone had already replaced the PT, and the new - secondhand - one was also defective. After looking at Fliptop's price, I got a PT intended for Vibrolux Reverb from Ruby/Magic, perfectly good transformer for a rock bottom price, removed the lay-down bells, & replaced 'em with standup bells, and installed that.

Note - to "prove" the PT was bad, I de-soldered all the secondaries, and the PT still drew 3A of current with no load at all. Sure sign of a baddie.

Output transformers all too often fail in these old Ampegs and that's another place to look for trouble. Open windings, shorted windings, feh. But if you have one that works, the originals do sound excellent and you've saved a bundle.

There's also the possibility that someone has stuck 6L6 or 6V6 into output tube sockets intended for 7591. Or vice versa. 60's Ampegs used one or the other, sometimes the tube chart is wrong. Look at the wiring and you'll know in a couple seconds what to use. Adding screen grid resistors and control grid "stopper" resistors on repair is always recommended.

[marcoloco961]

i pulled all the tubes and disconnected the ht and heater secondaries of the PT from the rest of the circuit. it still pops the fuse immediately after you throw the switch. Is that a fried PT for sure?

[martin manning]

As long as you're sure there is no short on the primary side...

[Leo_Gnardo]

It's lookin' bad for the bad news bears all right... Yes unless there's a wiring short on the 120V end of that trafo, it's scrap metal. This is one of those rare times you can use your nose to figure out what's up. WITHOUT power connected, smell the transformer and if it smells burnt it probably is.

[Stevem]

Not to pick on people, but if your friend at that shop where the amp is getting checked out can not deduce when a PT is bad then he`s not much of a tech, be it tubes or solid state repair work !
I can see why he might be going bald as you posted about him pulling his hair out.

If you have him do the mod I posted about in regards to changing the first filter out for two, the two new caps go in serise with each other, not parallel.