JTM45 mods

Refer to the JTM45 schematics from Tube Town.

I decided to do the following mods after reading up a bit on the subject in various forums.

  1. To tame the treble in my TT JTM45 I will change the mixer resistors (R10 and R11) from 270k to 470k. This shall remove the harsh treble.
  2. Add a simple master volume. Basically adding a 1M A pot between treble mid lug and C9
  3. Add a PPIMV.
  4. One wire mod. I.e. running the two channels in series and thus cascading the gain in V1.
  5. Perhaps add dip switches to select different values of bright caps on the two channels.

 

1. 470k resistors

I replaced them and I think its not as trebly anymore. I will have to do more tests at high volume. Kids were at sleep and I focused more on testing the PPIMV.

After further tests I really think the amp now behaves much more pleasant. Ice picks are gone.  I just heard something interesting from Pete Thorn (some live stream he just ran) that the bright cap on the volume pot affects less the louder you go with the volume. So normally below 5 it can be rather ice picky. With that in mind, my experience here can have been influenced by the PPIMV. Although I did set the PPIMV to fully off and still I thought that the treble was not harsh any more.

 

2. Simple master: I think I will skip this after installing the PPIMV.

 

3. PPIMV

This adds a 250k Audio Log Stereo Potentiometer between R22 and R24, as well as R23 and R25. Works very well. I ordered a length of shielded cable from TT but the wires was so thin I decided it was useless. Instead I took leftover cable from the original build and twisted the wires best I could. Seems ok.

I added a cutout from Tube Depot JTM50 Black Flag, which shows clearly how a PPIMV is installed.

PPIMV Tube Depot

 

4. One wire mod (OWM)

Something tells me that I might skip this as well. With the PPIMV I could crank both channels to 10 and get some nice overdrive while keeping the volume low. Time will tell if I will try this. I found one idea where an OWM could co-exist with a normal 4 channel circuit. By just pluggin in an empty guitar cable plug in one of the holes then the mod could be activated. Without the plug, it would be as normal.

Perhaps I will explore the JTM45 Dirty Shirley schematic (Friedman) and see what they have done. From a quick glance it looks like it has a OWM in the first stage.

 

5. Bright caps

It’s hard work to remove the amp from the cab so this will have to wait a while.