JTM45 ri mods

[Necrovore]

I found a website that had a good bit of info on modding the JTM45 ri's. I don't know if this is of any value to you guys but here is what I was able to copy.

Preamp Mods
These amps use a cheap, single-sided pcb which makes servicing or modifying them difficult. The first mod is the only one that necessitates removing the pcb.

V1 Cathode Mod

The JTM45 was copied from the 5F6-A Fender Bassman circuit. The cathodes of the first preamp valve share an 820 ohm resistor and 330µF bypass capacitor. Plexi amps have separate cathode circuits on V1: 820 ohm/330µF on the'Normal' channel and 2k7/0.68µF on the 'High Treble' channel. Since Marshall uses the same pcb for the JTM45, 1987X (50W plexi) and 1959 (100W plexi) reissues, this mod is fairly easy to accomplish. The most difficult part is accessing the underside of the pcb for soldering, because the pcb is still held in place by the wiring once the mounting nuts are removed.

1. On V1, cut the wire between pins 3 and 8 at pin 3. Unsolder the remainder from pin 8.

2. Solder a 2k7 resistor and a 0.68µF capacitor in the positions marked (X) towards the left hand side of the pcb. For some reason, my amp already had the 2k7 installed.

3. Connect V1, pin 8 to the pcb track connecting the 2k7 and 0.68µF cap. I used 16/0.2 stranded wire.

4. Put the pcb back in position & try it out.

For what it's worth, I left my amp like this for the time it took for the soldering iron to warm up again. Not surprisingly, the 0.68µF cap makes the amp incredibly trebly. I didn't like it at all.

Taming the Bright Channel

A major improvement can be made to the 'High Treble' channel by removing the 220pF or 470pF bypass capacitor from the 470k resistor. This is the capacitor on the trace connected to pin2 of V2 by a green wire. Rather than unsolder it, just cut the leads. Removing this cap will eliminate the harshness, but it will still sound different from the 'Normal' channel at less than full volume because of the 100pF cap on the volume pot.

Some people recommend changing the bright cap on the volume control instead, but I only found this out later. Also, swapping the 470K resistors for 220K will reduce the effect of the cap across the resistor. This may also make the volume controls more interactive.

More Gain

Some Plexi and metal face Marshalls have a 0.68µF bypass capacitor on the cathode resistor of the second gain stage. This is the 820 ohm or 1k resistor connected to pin 8 of V2 by a yellow wire. Solder the capacitor to the leads of the resistor.


Biasing for EL34s
There wasn't enough adjustment on the bias pot to allow the use of EL34s, so I soldered a 150k resistor across the 47k resistor. This shifts the adjustment range enough to allow either EL34s or 6L6s to be used.Soldering parts on top of others may seem sloppy, but it's much easier than removing the pcb. At some point I'd like to try KT66s, which is what the originals would have had, but that will have to wait.

Noise Reduction
Marshalls always seem to be noisy, but there are some improvements that can be made. The heater wiring can be redone, with the wires twisted tightly together and kept away from the audio wiring. If you really want to go to town, find all the earth connections and make star-earths for the ac and dc. I've toyed with the idea of replacing the pcb with a point to point board, but not for the "superior" tone compared to a pcb - that's marketing crap. The only reasons to do it are to make it easier to service/modify and because I'd be doing a lot of rewiring anyway.