First Amp Repair - Ampeg J12-T Jet II - possible blown OPT

Greetings friends, I've decided to try my hand at amp repair. I have an Ampeg J12T Jet II that supposedly has a blown OPT.
Schematic here. I've removed the OPT from the chassis and measured it, and I'm getting different readings for each side of the primary. Red to Blue reads 157 ohms, while Red to Brown reads 108 ohms. Bad? Replacing it with something like the Hammond 125E seems like a reasonable fix to me.

The EL84 tubes are Cathode-Biased and may be running a little hot; plugging the data off the schematic into the RobRob bias calculator shows them at 105%. I'm thinking of changing the Cathode Resistor to 150 ohm to cool the bias a bit, but that will raise the b+.

Does anyone have experience with this amp? any advice?


Apply 12 to 24 volts ac supply to half of the primary and see if the other half develops a near 12-24 ac voltage. If too low, the transformer needs to be replaced.
Due to winding thickness, the inner layer always will have lower DC resistance, unless wound differently.
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I assumed 310v p-k, that's 10.4v lost to the cathode resistor and ~7v lost to the OPT, given a b+ of 327v.

Installed 470 Ohm screen grid resistors, up from 100R, that will reduce the current a bit.

If I can read a datasheet correctly, 2 EL84 tubes in Class AB Push Pull with 300v on the plates and screens will yield 17w at full power. I think the stock OPT for this amp is just under-rated. That's what the tech told my buddy back at the shop, but that the amp wasn't worth the cost to repair. Now I've got it :)

Guitar amp transformers are designed for 80 Hz and they will look smaller in size compared to a normal 50 Hz transformers of same output power.
You will have power issues only if you use a 7 string guitar. But The amp will sound just brighter.

You cannot assume output power based on your method of calculation.
Feed a 1000 Hz sine wave signal and measure the transformer output with a 50 watts 8 ohms load resistor for the speaker.

If you feel the output is not that great, the stock speakers could be the culprit. Replacing the speaker with more gain will yield more sound output.
Try connecting the amp to a better speaker.
The original speaker is 8 ohms, 20 watts, 12". Generally the ware house speakers are not good for guitar.
They will work, but may not be loud and the tone will not be appealing.
Try with a external guitar speaker. You will be surprised.
Guitar speakers play a vital role in the final sound.

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