JTM 45/100

Amp tinkerers and people who aren't afraid of 700 volts or so in here....

Moderators: Slowy, Capt. Black

Post Reply
User avatar
wavesofffear
Stagg
Stagg
Posts: 48
meble-kuchenne.warszawa.pl
Joined: Sun Nov 21, 2021 1:08 pm
Has liked: 116 times
Been liked: 50 times

JTM 45/100

Post by wavesofffear »

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

Image
Image
Image
Image

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.

Image
Image

User avatar
FuzzMonkey
Gibson
Posts: 633
Joined: Fri Nov 06, 2020 7:25 pm
Has liked: 521 times
Been liked: 415 times

Re: JTM 45/100

Post by FuzzMonkey »

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

www.fantailaudio.com

User avatar
olegmcnoleg
Vintage Post Junkie
Vintage Post Junkie
Posts: 5595
Joined: Sat Dec 21, 2013 11:26 am
Location: Awkland
Has liked: 874 times
Been liked: 744 times

Re: JTM 45/100

Post by olegmcnoleg »

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

User avatar
Dharmajester
Vintage Post Junkie
Vintage Post Junkie
Posts: 1609
Joined: Sun Apr 05, 2015 8:44 am
Location: Dunedin
Has liked: 280 times
Been liked: 385 times

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 )..
“The music business is a cruel and shallow money trench - a long plastic hallway where thieves and pimps run free and good men die like dogs. There's also a negative side."
Hunter S. Thompson

User avatar
wavesofffear
Stagg
Stagg
Posts: 48
Joined: Sun Nov 21, 2021 1:08 pm
Has liked: 116 times
Been liked: 50 times

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!

User avatar
Cdog
Vintage Post Junkie
Vintage Post Junkie
Posts: 4418
Joined: Wed Feb 07, 2007 3:01 pm
Location: Christchurch
Has liked: 3587 times
Been liked: 735 times

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.

User avatar
wavesofffear
Stagg
Stagg
Posts: 48
Joined: Sun Nov 21, 2021 1:08 pm
Has liked: 116 times
Been liked: 50 times

Re: JTM 45/100

Post by wavesofffear »

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

Image
Image
Image

User avatar
robthemac
Vintage Post Junkie
Vintage Post Junkie
Posts: 8910
Joined: Sun Apr 28, 2019 3:47 pm
Has liked: 1130 times
Been liked: 1456 times

Re: JTM 45/100

Post by robthemac »

It sounded fucking massive.
Jops wrote: Sun Sep 18, 2022 7:46 am Spring is the comic sans of reverbs anyway.

User avatar
Bg
Site Admin
Posts: 43475
Joined: Fri Sep 05, 2003 12:13 am
Location: Auckland
Has liked: 2281 times
Been liked: 4015 times

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.

Marshmallow
Vintage Post Junkie
Vintage Post Junkie
Posts: 1181
Joined: Sun Jan 10, 2016 7:46 am
Location: Auckland/Tokyo
Has liked: 209 times
Been liked: 980 times

Re: JTM 45/100

Post by Marshmallow »

wavesofffear wrote: Mon Aug 01, 2022 2:38 pm This is a recent build, but thought it might be worth sharing...

Image
Image
Image
Image

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.

Image
Image
Awesome work man!

Post Reply