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Marshall JTM45 Reissue Mods

Thanks to George Metropoulis at Metroamp.com I'm working on tweaking my newest project which is a Marshall JTM45 Reissue gotten at Guitar Center on trade for my beloved JCM 800 50 watt combo split channel with reverb vintage 1986. In case you're not familiar, the JTM45 is Jim Marshall's first amplifier which was modeled after the Fender Bassman 5F6-A or better known as the '59 Tweed Bassman.

Why then, do you ask, would someone want a JTM45 instead of a Fender Tweed? For starters, the difference between the "Marshall sound" and the "Fender sound" is well documented and far reaching. Marshalls are known for their huge, roaring bottom end with wailing mids and glassy bright highs, think Hey Joe, Wind Cries Mary, Red House, Purple Haze,  Bluesbreakers Beano sound, while Fender's are known more for their clean tone, think Beach Boys, Dick Dale - Surf music, clean Blues and Jazz.

About this particular reissue, I believe by the serial number it was made the first week of 1999. George ay Metro made this killer clip of his JTM45 vintage clone version which sounds a lot more robust and full of sustain and harmonics than mine currently does, so I'm hoping with some small tweaks to gain more complex harmonics and sustain.

Until I can sell this one or start upgrading it piece by piece to full vintage spec, George suggested I replace the signal caps on the circuit board with Sozo caps (5) .022uf (4) .1uf (those are the little square gray blocks on the PCB board) and replace the Output Transformer which is hugely responsible for the character of the tone, sustain and roar associated with Marshalls. He had one meticulously made to vintage specs and which the top discerning critics rave about. I'm considering the swap since mine seems to lack just a tad in the sustain area but rocks in bottom end and highs. So first, I'll work on the caps and see what difference those make and then move on to the Output transformer. I also ordered a new set of tubes.

I will be posting some before and after clips here soon!

Meantime, here is a fantastic Premiere Guitar Magazine article about JTM4 clones vs. originals roundup which ranked George's Metro Amp up top of the list. Surprisingly, the PCB equipped Marshall JTM45 reissue came in second place. Though the one tested had EL34 output tubes instead of mine which has been upgraded from 5881/6L6 tubes to the beefier KT66 tubes as were the first used in mass production models by Jimi Hendrix, Clapton, etc when there was a shortage of 5881/6L6 tubes in England. However, the very first JTM45 amps made came with 5881/6L6s which are stock in reissues.
See 45 Degrees of Tone: The JTM45 Roundup

By the way, these are pure tone machines, I kid you not. The instant you hear one at stage volume, you just know this is THE sound that made all the great rock songs of days gone by from early Jimi to Clapton to Zeppelin. I brought this one below to an open mic last night for it's first test run. An old hippie dude complete with silver spray painted space boots plugged in his vintage Strat when I briefly walked away and starts ripping on a very loud "All Along The Watchtower" at this mid sized club. So I went to the back of the room and just listened. Man, if I would have closed my eyes I couldn't tell if it was Jimi or if it was Memorex. Then there's a guy with his expensive reissue Vox 2x12 Combo next to it which sounded like a piercing little buzzing toy next to the JTM45. Let me tell you, the amp is rated at 30 watts but with the KT66 power tubes into a 70 watt vintage 30 speaker, this amp rocks loud and strong and can blow the doors off small to mid-sized clubs.

In fact, to get the most out of it, I probably need a power attenuator to be able to dime it and wrench it's best tones at lower volumes because there were some walking in who said they could hear it all the way down the street and there were around 100+ folks in the room. I had the am set at around 7 but was using a Fulltone OCD overdrive with the volume barely on at around 8 AM to be able to keep the amp up loud but choke off some of the volume with the pedal. This is because the 45 doesn't start naturally sustaining till you hit around 8-10 on the main volume where you're up to around 55 deafening watts.

Example: A 30 watt amp = 30 watts at around 4 but on 10 that same amp is really 55 watts. So if you have a 50 watt amp, it's more like 90 watts on 10 and a 100 watt amp is more like 185+ watts on 10. If you're playing anything above 30 watts and want natural amp sustain which occurs between 8-10 on a vintage Marshall, you need either a master volume control so you can crank up the amp's preamp stage to overdrive while still being able to turn down the main master volume, or you need an attenuator as mentioned above. Otherwise, you won't last long before either the club owner kicks you out or the cops show up. Some do install a master volume control though others scoff at the idea and say it sucks tone from the amp. For the blues/classic rock gigger, the JTM45 is the ticket.

By the way, if you're considering a vintage spec Marshall, you might first want to check out George's site above or the MetroAmp Discussion forum where you can find tons of info. Better yet, instead of trying to mod one of these PCB board versions to sound like an original, you can get an entire kit with the original style turret board, components, vintage transformers for under a grand and build it yourself in a few evenings or have George build it for you for a few hundred more (well worth it if you're not technically inclined). He can also install a master volume and has kits for all the vintage Marshall amps in 30,50 & 100 watt versions. For the ultimate in vintage sound, get one of his top of the line heads, which have some rare original vintage parts in them at Plexireplicas.com.

I have no affiliation with George except from being a member on his forum and hopefully a long-term customer of his products, amp kits and full builds and having gathered a wealth of info from George and the others who most generously share their time and knowledge to tone hounds such as myself. :-) Back soon with updates! Tubes just arrived! Let's see if those improves sustain at all... See Phase 1 and Phase 2 and the Lar/Mar PPIMV Master Volume Kit Pics so far!

DISCLAIMER:
DO NOT attempt to bias your amplifier or work on anything else inside unless you are qualified to work on electronic equipment. There are LETHAL voltages inside of your amp that can kill you, kill you dead even when the amp is unplugged! If you do decide to do so, please make sure to fully discharge and drain your filter capacitors so that it is safe to work inside.

Phase I
(See Phase 2 Cap upgrade below first set of pics...)

Phase 2

Notice gray Lego block caps replaced with big yellow Sozo capacitors on PCB board

Notice black line on cap. That is ground side. Tested old caps for ground side and arranged accordingly to reduce noise.

Melted stock pot. Had to replace with Radio Shack 5k Linear tape and orange drop till new stock Marshall pot and PPIMV Master Volume kit arrives.

 

Lar/Mar PPI MV Kit (Master Volume Kit Mod)
See Pics Below

For those who live in an apartment and/or are tired of the neighbors calling the cops, or want to have the natural sound of the amp dimed but at lower club volumes with their non-master volume amps, this cool little innovation is the ticket! You just order a Lar/Mar PPI MV Kit from Metroamp.com at the above title link for $8 and also order some 5.6k resistors since they don't come with the kit because not everybody needs them who already has them but mine didn't. You'll need those as well.  Also, there is a full umpteen page thread about this mod on the Metro Amp forum here which should answer just about any question you may have. If not, please post any technical questions there as there are many experts who can answer your questions about your specific amp.

Here's a demo of the Lar/Mar PPIMV installed on a Marshall Super Bass I found on youtube. See all the gear he used to make the clip here

Once installed, which takes about an hour for a newbie, you can crank your volume up as loud as you want while keeping the master volume down at whisper levels with no noticeable tone loss! I had a Hot Plate before but that sounded terrible at low levels, very grainy and the tone just sucked. There may be some tone fanatics out there who say otherwise about the PPIMV but one thing that's great about this is if you turn the master volume up to 10, it bypasses the circuit completely so if you want the original operability, it's all still there unscathed. I was amazed late last night after the install how I had the amp up to 8 right around the sweet spot of crunch and singing leads but had the master so low you couldn't hear it in the next room, yet the amp was wailing! Not like that fake crunch you hear on a lot of the later amps with the diode clipping and master volume. This is real tube breakup only down low! So even in a medium sized club where the most you can put it is up to around 5-6 with an overdrive such as a Fulltone OCD, I can now turn it to the sweet zone at 7-8 and just back off the master volume some and I'm exactly where I want to be while not getting the club folks ticked off.

This variation on the post-phase inverter master volume circuit was tweaked by Larry and Mark on the Metroamp forum. Hence the name. It's based on the Type 2 master volume described by Ken Fischer, but enhanced and refined. The PPIMV lets the phase inverter tube overdrive with the preamp section just like it does when the amp is cranked up. The other type chokes before the phase inverter tube so you don't get as much color at low volumes. PPIMV is the most realistic way to get a loud sound at low volume by far.

This type of master volume replaces the bias resistors. More info here for actual diagram: LAR/MAR PPI MV WIKI PAGE

The only thing the diagram doesn't show is the 2 x 5.6k swamp resistors attached to pin 5 of the output tubes to help limit oscillation. The trick to installing them is to keep the resistor leads as short as possible on the tube socket lug and on the red or black wire that goes back to the dual Master Volume pot. There's another good pic on the forum here on this page

See wiring diagram below pics

Wire is shielded but has a foil shield outside each wire independently. It's good stuff and wire is pre-tinned!

Where bas resistors used to be. Front 2 holes facing are for left side black and right red wire. Either top hole for ground wire. See left for more.

Orange & Green wires on left and right of resistors going to output tubes pin 5 removed from PCB and tubes make room for red-black wires to dual pot.

Careful not to bend wire too much on top to pot or it will snap off as I did first time! Pain in the butt to do this twice.

Run ground wire under PCB and up through other resistor hole. You can insert ground into either hole. Just run across both holes to be sure of ground.

Remove one speaker jack to make room for 250K dual master volume pot

Remove speaker jack, carefully bend speaker lead wires back in case of reversal to original.

Dual pot with 2 x 2.2meg resistors, all the rest of the wires. Lots of tight little work! Patience Required!

Make sure ground wire doesn't touch pot. All grounds go to left lug and tough to fit all in hole so some wrap around.

See 5k6 swamp resistor. Green ring closest to lug, gold end attaches to wire. Keep leads as short as possible and cover in shrink tube.

plastic lock ties to keep wires together. Included with kit.

Overall pic. The black wires all part of the kit. Try to keep not too long or too short.

Notice shrink tube at edge of red & black wires into screen resistors holes. This is to cover any excess shielding. Shielding connects to pot lug only

Sprague .1uf Orange Drop cap used on presence pot.

 

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