Rebuilding a 1967 JMP 100 watt Marshall

I was tasked the pleasure of rebuilding this rare old amp. The owner wanted it completely cleaned up and dead stock. When I first saw the amp it was not in good shape. The input stages were cascaded instead of two separate channels. The bias circuit was replaced with an adjustable bias circuit that did not provide enough negative voltage. The original filter caps, due to their arrangement, had shorted against the bottom of the case, and presumably replaced long ago. The handiwork of the replacments was not very good. Electricians tape and silicone were being used to stake the newer caps and prevent shorts. The output transformer is non-original, with fewer laminates, but certainly not much younger than the amp. There were no screen resistors, the lamp was dead, and the front plexi panel was badly discolored.

I restored the preamp and bias circuit to stock, replaced all small electrolytics. I replaced all the power electrolytics and solved the shorting problem by lowering the standoffs about 3/8" and staked the capacitors with much more secure lead dress and soldering under the perfboard and hand tied lacing on top. The front panel was replaced with a repro. The original panel was retained in case it ever proved interesting to anyone. The knobs were replaced with NOS Marshall knobs. The only modification from stock is that the polarity switch was rewired as a half power switch at the customers request.

The amplifier sounds very rich. It is not as crisp as a 1959 model. It is very organic and warm, with a fair amount of sag and a very thick lower midrange. The power transformer is certainly very odd in that it has two secondaries, each stacked on top of each other to provide about 530 volts to the plates. The stock filtering scheme is also underfiltered compared to the 1959 model. It is likely that this model originally shipped with KT66 tubes, but is currently fitted for EL34s.

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