Marshall TSL MMOT Upgrade


Installing a Mercury

Magnetics FatStack

Output Transformer

in a Marshall TSL100


DISCLAIMER: this upgrade involves sticking your hands into your amp, which even when unplugged can contain a dangerous amount of electrical charge. I can take no responsibility for any damage done to yourself or your amp by attempting this upgrade, if you don’t feel confident doing this contact your local Amp Tech and throw a pile of money at him and I’m sure he’ll happily do it for you.

First of all you need to go to Mercury Magnetics and get yourself a new transformer, in this case you’ll want the ‘MAR100-OM’. Then wait a week or two for it to arrive then the fun begins, for this operation you’re going to need the usual ‘taking things apart’ items like a screw driver and some kind of socket set.

If you don’t already have a method to discharge the capacitors you’ll need to build something, for which you’ll need…

  • Soldering Iron
  • 2x 4K/10W resistor
  • 18 gauge wire
  • An Alligator Clip
  • A DMM Probe
  • Some heat shrink if you are feeling fancy

You’ll need to solider them together Clip -> Wire -> Resistor -> wire -> Resistor -> DMM. Like so..


Now we need to get the chassis out of the amp. First remove the back grill.


And here is the old transformer.


Remove all your tubes, you’ll want to keep all your tubes in the same order they came out.


Next, flip the amp over onto its front and undo the 4 screws underneath that hold the chassis in.


Be careful when removing the chassis as the bottom is completely exposed and you don’t really want to go poking your fingers in there just yet.


This fella here is the bottom of the Original Output transformer. But before you get all excited and pull all the wires out its probably a good time to get your discharging on..


Attach the clip from your discharging cable to the chassis. Then using the DMM probe, touch each of the pins on the bottom of the transformer. This should (disclamier-tastic!) bin the dangerous charge from the capacitors.


Now we can detach the wires from the old transformer, then using your screwdriver and socket set you can unbolt the old transformer and remove it.


(left) Old vs New (right). Drop that bad boy in and lets wire it up.


You should have gotten an wiring diagram with the new Transformer. For this model (MAR100-OM) going into a DSL/TSL amp you’ll need to wire it up in this order.



If all has gone well, you can put it all back together again and put your tubes back in. Leave the back panel off for now as you’ll need to check the bias on your power tubes before you go off thrashing your new trannie.


Here is also another great guide for fitting one of these transformers in a DSL50.

Big thanks to the fellow rockers at the Marshall Amp Forum.

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