JTM 45/100

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JTM 45/100

Post by wavesofffear »

This is a recent build, but thought it might be worth sharing...

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It's a clone of the "Dickinson amp" (serial number 7206), thought to have belonged to Hendrix and which Marshall used as the basis for their 100JH reissue a few years back. The circuit is basically a stock JTM 45/100 - "version 3B" according to Roe Fremstedal's excellent history of the Marshall Super Lead - with a later tonestack (33k/500pf), 10k dropping resistors and extra preamp filtering. There's some debate about when these changes occurred, as the amp didn't surface until the early 2000s.

I used Marstran/Heyboer transformers and, as you can see, quite a few NOS parts. Shipping everything over here was a quest in its own right.

After a fair bit of experimenting, I ended up with GECs for the power tubes, Mullards in V1+2 and a Valvo in V3.

The stock HT for these amps is very high (~560V). The reproduction PT had a lower voltage tap (490V), but curiosity got the better of me and I discovered I preferred the original voltage (so much punch!) After blowing a JJ - supposedly the most rugged current production KT66s - I realized I'd have to fork out for the original GECs....

Most importantly, the amp is a powerhouse. Incredible rock tone. It's obviously loud as anything, but in practice, I run it into a Boss Tube Amp Expander, which then feeds a 2x12 cab with Pulsonic T1281s.

I have a headbox - built by Sourmash in the US - but still need to cut the holes to install the correct Vox vents. As it is, running the amp outside a cabinet keeps it cool, and lets me keep an eye on the tubes!

I've laid the vents themselves on top here so you can get an idea.

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Re: JTM 45/100

Post by FuzzMonkey »

Nice work. Would love to hear it.
"There's no creativity without vulnerability."

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Re: JTM 45/100

Post by olegmcnoleg »

Wow, there's some talented folks on this forum.

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Re: JTM 45/100

Post by Dharmajester »

That looks seriously great but I'd be inclined to overlook the authenticity factor and use a standard 100 watter top vent for the box. Big bottle 66's chuck out a fair amount of heat and obviously far more is generated than in an AC30 ( and they get hot enough )..

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Re: JTM 45/100

Post by wavesofffear »

Dharmajester wrote: Mon Aug 01, 2022 3:41 pm That looks seriously great but I'd be inclined to overlook the authenticity factor and use a standard 100 watter top vent for the box. Big bottle 66's chuck out a fair amount of heat and obviously far more is generated than in an AC30 ( and they get hot enough )..
Fair point. I might be more inclined to put a small fan inside and preserve the look though (superficial I know).

I've also made a slight concession to longevity by running it with a variac at 230V, which keeps the B+ at 560V (and the variac is plugged into an online UPS, so this is stable +/- 1V or so). I have the tubes biased rather conservatively at about 55% of max plate dissipation, but this keeps the B+ up around 580V at full mains - it's a balancing act!

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Re: JTM 45/100

Post by Cdog »

Nice build bro, and cool circuit! Good luck with those teething issues, I hope those new tubes last as good as they sound. There are a bunch of other techs and builders on the forum who are generous with their expertise, its great to see your build. That cab is hot too, congrats.

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Re: JTM 45/100

Post by wavesofffear »

Got the cabinet sorted just in time for Akl gearfest! Here's the final product:

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Re: JTM 45/100

Post by robthemac »

It sounded fucking massive.

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Re: JTM 45/100

Post by Bg »

it weighed fucking massive.

Next time I want to hear it ;)
So, is that low alcohol or no alcohol at all? mmmm, no alcohol, do you want to try it? Noooooooooo.

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