JTM50 Modulus Amplification

[Nicolas S]

Hello to everyone .
After reading a lot on that forum, i think it's time to show my own build .

It is a Modulus Black flag jtm 50

I tweaked some values to be as close as possible electrically speaking, to an original .

The specs are normal Modulus JTM 50 with Dagnall transformers but :

-27 k 8 ohms NFB
-no bright cap
-Filtering is 64UF mains, 16UF screens, 32 Uf PI, 32+32 preamp (not original jtm50 values but works great, tight bass ).
-I put some Mustard also
-Original Marshall grounding scheme .
-LARMAR PPIMV

It took me a lot of time to get it sounding right, i've redone some solder joints, tweaking some values many time .

The result is a great sounding amp even at low volume, and with the 1*12 cab it is also movable quite easily

The speaker is a 1981 G12-65

Some shots :

 

[Nicolas S]

I didn't manage to post pictures, the server ran into problem, i try again later

 

[Dblgun]

Sounds very nice, looking forward to the pictures.

 

[playloud]

Looking forward to the pictures too!

If you upload the pics to Imgbb.com, it generates BBCode you can copy into your message with the pics.

 

[AndyD]

Looking forward to seeing pictures. As a side note, 64uf would be considered an absolute maximum for mains filtering with valve rectification. It’s typically 32uf .

 

[Nicolas S]

 

[Nicolas S]

 

[Nicolas S]

 

[paul-e-mann]

@Calvinr thats super nice, great job and congrats!

 

[Nicolas S]

Looking forward to seeing pictures. As a side note, 64uf would be considered an absolute maximum for mains filtering with valve rectification. It’s typically 32uf

yes, for the moment i stay with diode rectification, i feel that the limitation due to tube rectification was a tradeoff and SS rectification allow to increase the filtering so that what Marshall did to improve later amps ?

 

[neikeel]

64uF as SS rectifier is fine. As you no doubt know Marshall increased filtering gradually over the years. 68 JMP50 had mains 100uF, screens and pi 50 and preamp 32/32. Your values are very similar to the 100w amps of that era, tweak until pleased as one of our senior members here uses as his tag line.

 

[Nicolas S]

Some pics of the cab with the G12-65 . I asked Modulus for green on black tolex and the result is beyond my expectetions, the build quality is top notch for me .

 

[AndyD]

Looks terrific!

 

[playloud]

Really nice work! The cab looks fantastic

I guess you've landed on specs somewhere between a JTM50 and a later "1986" (bass 50W).

"Tweak until pleased" indeed, or as I prefer to say: "chase that tone dragon!"

 

[Nicolas S]

Th

Looks terrific!

Thanks

 

[Nicolas S]

Really nice work! The cab looks fantastic

I guess you've landed on specs somewhere between a JTM50 and a later "1986" (bass 50W).

"Tweak until pleased" indeed, or as I prefer to say: "chase that tone dragon!"

I settled on jtm50 with ss rectifier but slightly different filtering. I think it sounds good until the next move . What's I really like by tweaking is you never know how it will sound and it keep you exciting, but I want to stay close to what Marshall did during those years so it's just little things.
Maybe I would try to change the NFB to 47 k 8 ohms.

 

[Nicolas S]

I settled on jtm50 with ss rectifier but slightly different filtering. I think it sounds good until the next move . What's I really like by tweaking is you never know how it will sound and it keep you exciting, but I want to stay close to what Marshall did during those years so it's just little things.
Maybe I would try to change the NFB to 47 k 8 ohms.

And thanks for the kind words

 

[Mcentee2]

Glad to see another Modulus JTM50 build, I did mine in Jan onwards last year and documented it here, still going strong.

I went for the head, but your cab looks fantastic, Modulus do great cabs.

I kept the 47k feedback resistor to 8ohm tap, and stuck with the supplied 32uF to all stages, sounds fine to me, I would have been very wary of going to 64uF for the plates as I installed a tube/ss toggle.

I added a few mods to start with, and as most folks have said here in the past - "great, do that to start but I bet it ends up you just use your favourite settings all the time anyway", so true!

The one mod I do use a lot though is a bright cap toggle between 100pF/600pF/4800pF

The v2a bypass cap I never really use, and neither do I particularly like the 33k/500pF tone options - so much is I am thinking of changing that toggle option to go for a more original Bassman flavour of 100k slope and 0.1uF bass cap as well as the 56k/250pF treble cap standard I like.

Oh, and also thinking of trying it without a mixer treble peak cap.

Hello to everyone .
After reading a lot on that forum, i think it's time to show my own build .

It is a Modulus Black flag jtm 50

I tweaked some values to be as close as possible electrically speaking, to an original .

The specs are normal Modulus JTM 50 with Dagnall transformers but :

-27 k 8 ohms NFB
-no bright cap
-Filtering is 64UF mains, 16UF screens, 32 Uf PI, 32+32 preamp (not original jtm50 values but works great, tight bass ).
-I put some Mustard also
-Original Marshall grounding scheme .
-LARMAR PPIMV

It took me a lot of time to get it sounding right, i've redone some solder joints, tweaking some values many time .

The result is a great sounding amp even at low volume, and with the 1*12 cab it is also movable quite easily

The speaker is a 1981 G12-65

Some shots :

 

[Nicolas S]

Glad to see another Modulus JTM50 build, I did mine in Jan onwards last year and documented it here, still going strong.

I went for the head, but your cab looks fantastic, Modulus do great cabs.

I kept the 47k feedback resistor to 8ohm tap, and stuck with the supplied 32uF to all stages, sounds fine to me, I would have been very wary of going to 64uF for the plates as I installed a tube/ss toggle.

I added a few mods to start with, and as most folks have said here in the past - "great, do that to start but I bet it ends up you just use your favourite settings all the time anyway", so true!

The one mod I do use a lot though is a bright cap toggle between 100pF/600pF/4800pF

The v2a bypass cap I never really use, and neither do I particularly like the 33k/500pF tone options - so much is I am thinking of changing that toggle option to go for a more original Bassman flavour of 100k slope and 0.1uF bass cap as well as the 56k/250pF treble cap standard I like.

Oh, and also thinking of trying it without a mixer treble peak cap.

Yes, Modulus Amplification supply really nice and accurate kits of classics amps.
It was a real pleasure to build and then to ear the Amp coming alive.
For the price at the time it was the best value I could find .

I thought about 33k 500pf tone stack but you say you didn't like it with this Amp?

 

[Mcentee2]

Yes, Modulus Amplification supply really nice and accurate kits of classics amps.
It was a real pleasure to build and then to ear the Amp coming alive.
For the price at the time it was the best value I could find .

I thought about 33k 500pf tone stack but you say you didn't like it with this Amp?

I am a home player with a strat and tend to gravitate to more fendery clean/crunch sounds, the 33k tone stack is just a bit too mid rich for me most of the time - I'm not fond of tubescreamers, Klones etc for similar reasons. However, it does help if you tend to really want a JMP or similar cranked tone with a Les Paul though, split v1 cathodes along with v2a bypass all are things that lead to same destination. My "most of the time" flavour is the other way, gz34 rectifier, shared v1 cathode, no V2 bypass, 0.22 coupling caps, 100pF bright cap, 47k on 8ohm NFB and 56k/0.1uF/0.22uF/250pF tonestack.

I built a Modulus Supa Deluxe as a practice before this one and love it's cleans, so am going to test out that tonestack in the JTM50 on a toggle - original 5f6a bassmans apparently had that tonestack in - 100k/0.1uF/0.02uF/250pF (although a lot of published schematics show it as the 56k/0.02uF/0.02uF/250pF version).

Also hence my thought to try without that 500pF mixer bypass cap as well.

The great thing is about all this is that you can wire in what you want with these amps, then take it out again until it is as you want it