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Marshall JMP-1 Hum

Many users of this well known pre-amp, will be familiar with the infamous Marshall JMP-1 hum! Emanating from the transformer, the hum is caused by oscillating laminates which don't just produce hum but also can generate a huge amount of heat, thereby potentially reducing the life expectancy of your transformer.

Months in the making, I'm delighted to announce my new PML-TX01 replacement transformer for the Marshall JMP-1.

A replacement transformer for the Marshall JMP-1, my very quiet PML-TX01
And here it is... my PML-TX01 transformer for the Marshall JMP-1

Looking just like the original TXMA-00014 and being pin-for-pin compatible, my PML-TX01 is identical except for one feature; the material used for the laminates is of a much higher quality. This single unique aspect of the PML-TX01 reduces the likelihood of laminate oscillations, excessive heat and mechanical hum.

My PM-TX01 is of course pin-for-pin compatible with the original Marshall TXMA-00014
My PM-TX01 is of course pin-for-pin compatible with the original Marshall TXMA-00014

In a quiet recording environment, the Marshall JMP-1 hum is super-annoying and at last, a solution is now available. If you have hum issues with your JMP-1, then I strongly suggest that you consider this little upgrade.

This item regularly goes out of stock, I'm afraid but... I encourage customers to back-order. Unfortunately, the crappy e-commerce plug-in I use, only tells the links (like the one above) that the item is out of stock. What 's the bloody point of that?!?!?! So if you want this, then please just visit the PML-TX01 page on my e-store here.


I regularly receive two questions from those interested in my PML-TX01:

  1. Is the PML-TX01 replacement transformer for the Marshall JMP-1, 240V or 120V?
  2. Are the voltage selector components diodes, ferrite beads or just fancy wire links?
Marshall JMP-1 voltage selection
Here are the link locations to select the voltage rating of the primary windings for the transformer in a Marshall JMP-1. The example shown is set to 230V.

The answer to the first question is, just like the original TXMA-00014, my PML-TX01 has two separate primary windings, each rated at 115V. So…

  • EUOPEAN / UK VOLTAGE SELECTION. Wired in series, the primary becomes a single 230V winding. To wire in series, connect ONLY LK2.
  • USA / CANADA / JAPAN VOLTAGE SELECTION. Wired in parallel, the primary becomes a single 115V winding. To wire in parallel, connect LK1 and LK3.

The original links were JUST WIRE LINKS, so you can use wire.

JMP-1 Input Voltage Selection Schematic
This representation might make things a little clearer. Apart from the higher quality material used for the laminates, the PML-TX01 is a direct drop-in replacement for the original Dagnall TXMA-00014.


The above is a simple schematic REPRESENTATION of how the links are connected to configure the primaries for either 230V or 115V operation. The physical configuration of the voltage selector links however, is a bit unconventional and suggests that the transformer's primary windings are mirrored with START and FINISH for each winding not being where one might expect! The STARTs, for example could be the the two middle pins and the FINISHes, the two outside pins. Normally, transformer windings would be laid out START - FINISH, START - FINISH. I thought it might therefore be helpful to provide an illustration of the actual (physical) layout of the power input in the Marshall JMP-1 (below).

Marshall JMP-1 Input Voltage Physical Layout
Here's the actual layout of the input voltage side of the Marshall JMP-1. Note that the start and finish of the transformer coils aren't where you might expect.


One might ask why the two individual primary windings are put in parallel for 115V. Why not just use one winding?

Well, a system uses a certain amount of power. Power is the product of voltage and current: P = V x I.

You can probably see now that if you half the voltage, you'll need twice the current to deliver the same amount of power. Placing the two primaries in parallel does just that, it doubles the current going into the system. 🙂

And lastly...


So as I've just intermated, the power requirement of a machine is the same whatever the supply voltage. Since power = current x voltage, if you're halving the voltage, you'll be doubling the current.

Marshall JMP-1 fuse
Don't forget to select the correct fuse rating for your region.

If running at 230V, the JMP-1 fuse should be 80mA (230V x 0.08A = 18.4W).
If running at 115V, the JMP-1 fuse should be 160mA (115V x 0.16A = 18.4W).

'T' stands for 'time delay' so use a time delay fuse.