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With the amp finished and sounding great, many amp builder starts to think, “how can I make
it better”? Well, the below mods can help you define “better” …

Here are a few cool mods for experimentation …

1. Change the value of the tone control “slope” resistor (R9, between T20 and T22) –

This kit uses a value of 33K where as the original JTM45 and Fender 4x10 bassman used a
56K in this position. We've included both values for fun.

2. Change the value of the bright channel coupling cap (C4, between T12 and T13)

–

This kit uses .022, the original “plexi” used .0022. The smaller cap allows less bass into the
signal chain, thereby improving the tonal variety when mixing the two channels together.

3. Change the value of the tone control treble cap (C7, between T20 and T23) -

This kit uses a 250pF. By increasing to 500pF, more midrange will be available. Put these two
on a switch and it becomes a midrange boost.

4. Change the value of the phase inverter cathode resistor (R15, between T34 and T35)
and replace V3 with a 12AT7

– by changing the phase inverter to a 12AT7, the low end from

the amp becomes much more solid. This is a great mod for players that use pedals in front of
the amp. Replace R15 with a 22K / 1W resistor and install the 12AT7.

5. Change the phase inverter to a low gain 12AU7

– For those wanting to play the amp

with more preamp drive but without so much output volume. replace R15 with 22K / 1W
resistor and then install a 12AU7. The output stage volume will decrease giving more room to
turn up the preamp.

6. Install a different master volume

– By upgrading from the stock master volume to a post

phase inverter master volume (PPIMV) type, the amp will sound more natural at lower volume
settings.

7. Install a “cut control” in place of the presence control

– A cut control functions as a

very high frequency adjustment after the power tubes. This will allow better control of the
“brittleness” often associated with an amp at full volume.

8. Replace the 5AR4 / GZ34 rectifier tube with a 5R4 rectifier tube

– changing to a much

lower efficiency 5R4 tube will give a massive amount of sag. Very cool sounding.

9. Link together the first gain stage cathodes

– The JTM45 had the cathodes of the first

gain stage tube tied together. This made the two front panel “channels” very interactive with
each other when mixed together. Move the wire attached to T4 to T6.

10. Operate the amp with 6L6 / KT66 / 6550 power tubes

– for the most part, these tubes

can be directly installed into the amp and all is well (after rebiasing). There are a few other
changes that can be made to “fine tune” the installation of these tubes.

11. The more obscure mods

– effects loop; reverb; variable B+ control; balanced line out;

they are all possible … some more difficult than others.

For any questions regarding mods, contact:

[email protected]

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Mods