Ampeg Gemini Output Transformer

[Dylan W]

Turns out the output transformer was really cooked.  The output tubes are, thankfully, doing just fine.

CJ sent back a beautifully rewound transformer. The amp has new filter and cathode bypass caps.

The dissection thread is here.  Can't say enough about the work he did. http://groupdiy.com/index.php?topic=56321.0

Hooked up to an extension cab, the amp is back with a vengeance 

 

[Dylan W]

Unfortunately, we've uncovered a new problem with the amp. With the stock Jensen attached, we get a loud squealing (oscillation/feedback?) as soon as the tubes warm up.

DCR measures a healthy 6.9 ohms, but the voice coil is rubbing badly. I wonder whether the coil melted first and that cooked the output transformer?

 

[lassoharp]

May want to double check the grounding on the OT sec lo side and make sure its secure.  I've seen amps squeal when that connection isn't solid.

 

[CJ]

try reversing the leads that go to the plates,
could be the neg feedback is backwards,

 

[Dylan W]

try reversing the leads that go to the plates,
could be the neg feedback is backwards,

This was my first thought, but it's only with the Jensen speaker in the combo. An external speaker works fine.

 

[CJ]

try running a ground wire from the speaker frame to the chassis,

 

[CJ]

Hey Dylan, what kinf of 7591a tubes are you usin?

got a gemini 2 with the 15 jensen, needs a couple of tubes, wondering which to buy, JJ, EH, RCA, Sylvania, etc

thanks!

 

[usekgb]

CJ.....  The JJ's work great in the Gemini.

 

[Dylan W]

try reversing the leads that go to the plates,
could be the neg feedback is backwards,

Belated update: this did the trick. Still doesn't explain why it worked fine with another speaker.... but this one's a wrap.

 

[CJ]

cool!

let me know if yo wanna try the Gemini 2 OPT, same 7591a P-P output stage but with a much bigger core and more interleaving, however, a smaller core means a more saturated sound and a simpler winding structure might cut down on high end brittleness, so you might be happier with the Gemini 1 OPT,

i will check my winding print on the xfmr i sent you and see if i flipped the leads somehow,

cj

 

[Dr.Banzai]

sounds like the amp is marginally stable if changing speakers put it into osc. The other thing is if it only squealed with the onboard speaker, do you have a bum tube?