This project has taken months but after tracking down Graham Sopp, the guy who designed the original transformer for the JMP-1, I'm delighted to announce that the PML-TX01 upgraded transformer for the Marshall JMP-1 is now available, featuring laminates made with a higher quality material.
You don't need to be a rocket scientist to notice that the original transformer in the JMP-1 is not stressed. Just take a peek at the regulators and you'll notice an obvious lack of heatsinks suggesting that the current draw from the supply lines is far from excessive. Measuring the input voltages to the regulators indicates that the voltage drop across them is not too much and of course we all know that the valves take very little current.
So why then, does the JMP-1 transformer get so hot?
Well, it's due to the fact that the transformer's laminates oscillate. That's right. They oscillate so much that apart from the hum, a considerable amount of heat is also produced.
I spent a lot of time looking at the mystery of the Marshall JMP-1 humming transformer, refusing to acknowledge any issue with the laminates as I just didn't want to believe it. Once the penny dropped though, it all made sense.
It took a while but my first batch of PML-TX01 transformers is now here and while not perfect, it does perform better than the original Marshall TXMA-00014, with a little less hum and considerably less heat.
Although the laminates are made of a different material, thus lowering heat dissipation, the flux density of the device is the same, which means that the transformer could still hum.
Since heat generation is reduced however, we can now install a dampening material such as sponge or foam to the top of the transformer without fear of cooking the device. In tests, hum has been completely eliminated.
A WORD OF CAUTION
The transformer is soldered to the now very old, double-sided main board in the JMP-1. Please take care when removing the original transformer. It's a relatively heavy device and the through-hole plating isn't exactly the best quality. The last thing you want to do is strip it!
My PML-TX01 transformer is a high specification replacement for the TXMA-00014 transformer of the Marshall JMP-1 only. It is NOT suitable for other pre-amps such as the Marshall 9001.
If you have any questions about my PML-TX01, please don't hesitate to contact me or you can just