Review of my JTM45 build

[Axeman010]

JTM45 Specification

Valves : Gold Lion KT66's, JJ ECC83S (V1 tested in all positions) & JJ GZ34

Transformers : Output 45/50W 8K , Choke 3H and JTM45 Mains Transformer

Caps : Sozo Mustard.

Marshall Components Kit and Turret Board from Valvestorm


Amp being put in to Marshall 4*12 Lead Cab
Amp dimed and guitars used a 1979 Fender Start and 2000 Gibson Les Paul Custom.


First impressions are how versatile the amp is (mixing the two channel gains produces really good sounds) and how loud it is! Once the amp was finished I could not stop playing the thing with just a guitar play direct in to the amp. There was a tiny bit of hum once the amp was turned up high but nothing intolerable and not noticeable when playing guitar.

I then tried a range of my DIY pedals in front of the amp including moded tube screamer, booster, wah (Whipple) and delay( Rebote 2.5). All worked absolutely great with the exception of the delay which produced some low level hiss when engaged, which i have never experienced with any other my valve or SS amps. Not sure what's causing this yet but once again its not noticeable when actually playing.

With the Strat the sound started to break up (with heavy playing) at about 4.5 and the Les Paul at around 3.5. Backing off on both and the sound cleaned up nicely. However for home and small venues this is quite loud. Couldn't get near a lead sound as way too loud in a confined space.

So today fitted the Lar-Mar PPI-MV. On full MV the amp has lost none of the original sound that I love. Still playing around with the PPI-MV and gain levels at the mo but favourite setting so far is MV on about half and gains adjusted so that I get a hefty crunch with the guitar volumes on full. This way guitars clean up easy without losing too much volume. I found with the channel gains up near full the sound became very distorted in a fuzzy sort of way, a very useful sound if that's what you like. But I prefer to use my tube screamer to get my lead sound. So in other words I have a 3 useable sounds. Clean - guitar backed down, crunch - guitar full and lead with tube screamer.

Had considered some other mods including zero loss loop, Larry's ground pattern and shielded cable from the inputs to the board. But I'm not sure I'll do them as I am enjoying the amp the way it is right now.

Just waiting for the Amp Cab to be delivered now as I'm useless at woodwork.

Overall very impressed with this amp and would like to say a big thank you to George and all the contributors on this forum.

[Elad E]

pics or it didn't happen!

just kidding, though I really would like to see the inside or WiP pics after seeing the initial teaser pic on the other thread.
very interested in the UK made iron you used. if I understand correctly the OT is Drake style (8K)?

[Axeman010]

Here's the gut shot !!! Prior to the PPIMV being installed.

All the transformers are from Danbury and yes I believe that they are made to the original Drake specification.


Have been playing the amp quite a lot now and found that the excessive Fuzzyness when using channels linked and hi gains can be avoided by turning the bass all the way down. I am therefore now considering moding the amp to reduce the overall bass. I understand there are quite a few methods of doing this listed on the forum.

[neikeel]

Have been playing the amp quite a lot now and found that the excessive Fuzzyness when using channels linked and hi gains can be avoided by turning the bass all the way down. I am therefore now considering moding the amp to reduce the overall bass. I understand there are quite a few methods of doing this listed on the forum.

Easy ways:
- cut the bass early (smaller V1b coupling cap from 0.022uF to 0.01uF
- smaller PI input cap (same 0.1uF down to 0.047uF)
- smaller PI output caps (0.1uF down to 0.047-0.022uF to taste)

I just turn up the presence, treble and mids, dial back the bass and ride the guitar controls and apply PPIMV very sparingly.