JTM45 build. Authentic Tone!

Marshall Amp Discussion

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RockinRocket
Posts: 628
Joined: Sun Nov 20, 2011 5:23 am

Re: JTM45 build. Authentic Tone!

Post by RockinRocket »

I'm interested in why duplicate the tone the amp makes now? 50 years later after the CC pots measure out of spec, filter caps lose there spec, CC resistors drift and the whole 50 years of break in.

The recordings of those amps most folks try to duplicate were recorded when those amps still were in spec and sounded new!

I want my clones to sound they way they sounded coming of the production line 50 years ago!
MHProd
Posts: 167
Joined: Tue May 07, 2013 6:47 pm

Re: JTM45 build. Authentic Tone!

Post by MHProd »

gsb wrote: Mon Mar 27, 2017 4:26 am Hi !

I build also a replica jtm45.My OT is Shinrock von German winder-Drake with 8k.What do you meen 1k for Screen -where you put resistors on -Pin ?
Are you used electrolyt cap on V1 or poly.? If i understand you used 100p bright cap an 560 mixer cap on bright channel and nothing on normal channel just like original schematic.
Thanks gsb
Exactly what Roe said. One side at the end of 8K2 (where the choke is) > 1K > screens

The 1uF on V1 is an electrolytic. And yes, 100pF on the pot, 560 bright channel mixer. Channel 2 is stock.
MHProd
Posts: 167
Joined: Tue May 07, 2013 6:47 pm

Re: JTM45 build. Authentic Tone!

Post by MHProd »

RockinRocket wrote: Mon Mar 27, 2017 11:35 pm I'm interested in why duplicate the tone the amp makes now? 50 years later after the CC pots measure out of spec, filter caps lose there spec, CC resistors drift and the whole 50 years of break in.

The recordings of those amps most folks try to duplicate were recorded when those amps still were in spec and sounded new!

I want my clones to sound they way they sounded coming of the production line 50 years ago!
My originals had the filter caps changed. Resistors in my amps didn't drift all that much (CF, not CC, for the most part...).

Not convinced 2 months of hard playing vs 50 years of break in make any drastic change in tone, not for the better anyway.

Remember the whole point of the post was to explain my process of coming up with a close sonic clone of my originals. Not claiming people should or shouldn't do the same to achieve the idea of what they think a 45 sounds like.

I tried to document the main differences I heard between the amps and tweaked the circuit from there. Just sharing my findings.
gsb
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Location: GRAZ AUSTRIA -EUROPA

Re: JTM45 build. Authentic Tone!

Post by gsb »

Hi!
Thanks for information about screen resistors -I think its about that on some amps screen voltage is equal to plate and with aditional resistor you drop some voltage on screens.Have you voltage card for your build ?
thanks gsb
RockinRocket
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Joined: Sun Nov 20, 2011 5:23 am

Re: JTM45 build. Authentic Tone!

Post by RockinRocket »

I was just pointing out how lots of folks (Metro) try to get the tone of a 50 year old amp. Not right or wrong of course.

I find it interesting how you got to where you feel it sounds right. I am skeptical though. Changing bias on tubes, the parameters of the gainstages don't make sense to me.

Keeping in mind I don't agree with how George Metropoulous builds his clones either to get that "sonic" quality either. CC through out and Ceramic disks.

Nor do I agree with Roe about the Pots measuring high possibly being the last major piece of the puzzle. They certainly do change the frequency slightly.

I have done a few tests of my friends Smallbox Metal Panel. I built a tone clone with Iskras, Mustards, Dogbone ceramics and Merren transformers. All the right parts except pots I guess. That got me sonically very very close. The original had its electrolytics replaced for a apples to apples here. It wasn't until I dropped in a original used Drake 784 139. Bingo. The Merren are the best New transformers but I really feel that the 'smoothness' comes from the break of years and years of the output transformer being pushed. The whole amp did break in nicely after years of playing and never did get the chance to compare again, as the Smallbox got sold.

I never measured the pots in the original because it never occurred to me then that they would drift like Roe has stated. But looking at Duncons simulator I don't see much difference that could be audible by my ears.
MHProd
Posts: 167
Joined: Tue May 07, 2013 6:47 pm

Re: JTM45 build. Authentic Tone!

Post by MHProd »

I know what you're saying... I built the clone as close as I could and I was quite far away from the tone of my originals.

I just let my ears guide me. Shaping and matching the tone and drive response. Ultimately, I think the key component is the OT. Not because it has had 50 years of use (most didn't) but because I don't think today's materials are quite the same. IMO, there's way more going on than an extended break in period. One way to sum it up would be to think of the combination of the mods is just a way to compensate for the lack of mojo in the OT. Not exactly the same, but pretty close !

Also, I have heard many plexi clones (especially the ones using merren's iron, although was quite impressed with classic tone too) sounding pretty damn close to originals. Not the case with 45s! The RS/103 OT clones all seem to miss the mark (Merren's being the closest to my ears).

Anyway, my point is: try it for yourselves guys!! As long as you have the trannys, it's just a matter of a few resistors and caps.
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Littlewyan
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Location: UK

Re: JTM45 build. Authentic Tone!

Post by Littlewyan »

I built myself a JTM50 using Classic Tone transformers and mostly modern glass. It has two Mullard ECC83s in the V2 and V3 positions (I wanted their distortion characteristics). I will admit that it falls short of the mark when compared with original Marshalls but at the same time I'm using Celestion's Heritage G12M speakers, not original G12Ms. I was considering increasing the value of the mid pot as I felt that the amp is lacking mids when compared with originals but for now I've left it as is.
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