So I recently picked up an Ampeg Gemini 6 from a friend and it blows the power fuse upon power up. The fuse goes pretty fast. I pulled the power tubes and the fuse still goes. The amp has been sitting for a number of years and I realize I should probably charge the caps, but I do not know the proper procedure for that. As well I checked the MR760 rectifier diodes,one is reading around .145 volts and the other is reading .004 volts. Now heres another question: Do these diodes need to be tested out of circuit? or are they reading low because they are in circuit and being affected by the power transformer winding? To the best of my knowledge, I should be able to read them in circuit with no problem. I do not own a variac, so I dont have the ability to bring the voltage up slowly....I would appreciate any help. Thanks.
The transformer can influence the readings, but the .004 reading is pretty much a telltale sign of a shorted diode.Originally Posted by randomair
When you tested the diodes, did you reverse the meter leads and get any readings? Either way, I'd suggest removing or at least clipping out one side of the diodes and re-test them out of circuit. I think that you will find that one, if not both are shorted.
hey bill thanks for the reply...sure enough i pulled one side of the rectifier section and its shorted. do you know of a suitable replacement for these or are they still available? thanks for your help...
before i go out and just replace this part. need i be concerned with anything else in the power section being this amp has been sitting for a while? or should i just go for it and hope that does the trick? thanks again.
Sometimes diodes just fail, sometimes they are taken out by another component, like a bad tube or capacitor.
I'd recommend starting by replacing the diodes and powering up the amp through a light bulb limiter. From here you can find out if anything else is wrong, without doing any additional damage. This will also save you from replacing any additional fuses as you troubleshoot the amp.
I think that MR760's are still available, but you probably can use a more common 1N4007 to replace it in that amp. I'd replace all the diodes so that you have all one type in the circuit.
That 1N4007 won't last long, the MR7xx are 6 amp rectifiers. If the circuit needs that. MR760 is the 1000v one.
Any 6 amp rectifier of suitable voltage rating would work just fine.
My experience has been that those diodes have very substantial surge ratings - hundreds of amps - and are usually failed on their own rather than taken down.
But if this is a tube amp, then I cannot imagine why someone needed 6 amp rectifiers. My schemastic shows .6A diodes, not 6A diodes. And for that the 1N4007 would indeed be perfect.
well i replaced both of the diodes and the ampeg is going strong, save for the tremelo and the echo not working...that is another demon. i will try putting some different tubes in this bad boy and see what happens. kind of a sweet deal considering someone just gave me this amp being they didnt want to deal with it...cost me 3 bucks to get running. thanks for your help all...
Last edited by randomair; 04-30-2008 at 08:10 AM.
so like stated, i did get the gemini up and running, sounds pretty decent, but the tremolo and echo are not working. i was just going to simply swap out the tubes to see if this would make a change but as i was poking around inside the chassis i noticed something peculiar. there is some type of small box(about the size of a relay in an old stereo)labeled "cell" on top, "neon" on bottom, and also says Tm-1 on the body. there two diagrams on the side, both are small circles with lines going in either direction. no idea what this is??? but there are two wires coming from it that are soldered to the intensity switch. there are two other leads on this mystery box, one goes back into the box and the other looks as if the lead from it is broken off. but there is no broken lead left for evidence. does anyone have any idea what this little box may be? or where that other lead may need to go? thanks.
i guess i figured out it is indeed an optocoupler that i am looking at...question is, can i use a newer style fender stock if i need to replace it? or should i try and track down an original optocoupler? from what i have read, this is a possible cause of the tremolo failing to operate...
i have isolated a bad .47 cap in the drive circuit of the reverb...can i substitute a cap of a lower or higher value just to see if the drive circuit works? as well i am still in question about which opto-coupler to use for the tremolo circuit. below is a link to the schematic: thanks.
http://www.schematics.ca/uploads/PDdownloads/gs15r.gif
Yes, just make sure that the voltage rating is high enough to cover the voltage available at the plate of the drive tube. The frequency range of the signal going to the tank will be changed with the value of cap that you use, but it will let you know if the circuit is working or not. Make sure that the tank itself is ok. Check the wires inside the tank, check the transducers for continuity, etc.
I've never tried to use a Fender OI in place of an Ampeg, so I don't know if it will work or not. Are you sure that yours is bad? Is the oscillator circuit running? In 90% of these amps, the cause of no tremolo is bad caps in the oscillator circuit.
thanks for the reply, i think i am good with the cap issue on the reverb now...i will go ahead check those caps in the oscillator circuit for the tremolo. as far as the OI being bad?? it is hard for me to say at this point, but i do know the OI is completely encapsulated so i am not sure how to test it out of circuit. and like i said in the previous post, there is one lead that appears it should have a wire connected to the end of the small coil, but the wire is gone and there is nowhere for me to reference where it goes visually. on the schematic it is the positive side that goes to the 1m resistor and splits to the 1.5m resistor going to the tremolo circuit. there is a wire that goes to the circuit from the + side but at the actual OI the wire is hooked to a small coil. the small coil has a wire going into the OI and on the other end of that small coil it appears there is a broken jagged wire connection that goes somewhere??. now on the neg side, small coil there too, wire coming from circuit on one end, an additional connection into the OI, then another small wire that goes back into the top of the OI. hard to describe. what i will do is post a photo and try and explain it differently. at any rate, i will test the caps in the tremolo/oscillator circuit and hopefully i will find fault in there somewhere.
There should be 4 wires comming out of the OI box. Two that go to the neon light bulb, and two that go to the photocell/light dependent resistor. It sounds like the two photocell wires that go to the intensity control are ok, but the others are missing? A photo would be helpful. Check the flip-top site to see if they still carry a replacement OI. If there is a photo of one there, it might help you to see what yours should look like.
Hi, I am working on the same amp. Teh power supply caps are missing. Does anyone know which ones are supposed to be in the metal can? Where are the outer ones? I'm guessing there should be a paper cap fastened to the chassis someweher. Accroding to the schematic, there are 5 caps in the power supply. plus the one for bias ( I found that one)
If anybody knows, or has a picture of how the components are laid out, I'd be really grateful. Even a photo of the board / chassis would be grat.
Thanks!
the original cap can was a 4 x 40 x 450v with two of the 40's tied together to make 80 mfd
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