My new Marshall JMP

masonlewisdiesel said:
That is at our switch/round house yard, its one of the RCL units in our fleet. If I recall its a gp39 dash 2? 3? Dont quote me on that, do you work for the railroad?
The pedals i'm using curently? To be honest, i'm a simple man. Tuner, 70's thomas organ wah($89 guitar center special), Fuzz(sandspur ppcb), waza blues driver, Uni-vibe, and ive been trying different reverb pedals out. So far everything i've tried gives me too many options, or sounds to digital. Looking at a holy grail to try next.
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I do! Celebrating my 20th year soon, those old gp units are very familiar faces to me.

Your guitar rig is almost identical to mine as a young lad.

I recommend the gravitational reverb sounds great and does subtle to longer reverbs.
 
Ive serviced 3 1964 amps. They all slightly differed in spec. All had the 1202-164 power trans and around 380v B+. The B+ dropping resistors were either 10k or in one case all 4 were 8.2k Iskras. My question is does yours have a 500p or 250p red tubular dog bone on the volume pot? And the tone stack treble cap? 250 or 500p dog bone? These amps are unique and awesome. Most have been altered, would love to know these minor details. Beauty BTW.
 
UPDATE:
Thanks for all the replys on this! been wrapped up with work so much, had to put hobbys on hold for a couple weeks. I am definetly going to do filter caps soon. This thing sounds killer! got some better photos and slid the chasis out to take 2 short videos of the insides. Im surprised how clean this thing is! lucky me!
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EZS said:
Ive serviced 3 1964 amps. They all slightly differed in spec. All had the 1202-164 power trans and around 380v B+. The B+ dropping resistors were either 10k or in one case all 4 were 8.2k Iskras. My question is does yours have a 500p or 250p red tubular dog bone on the volume pot? And the tone stack treble cap? 250 or 500p dog bone? These amps are unique and awesome. Most have been altered, would love to know these minor details. Beauty BTW.
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Hey! sorry for the delay, I just uploaded a couple short videos of the guts, I will get you some high res photos sometime tonight!
 
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jjjimi84 said:
I do! Celebrating my 20th year soon, those old gp units are very familiar faces to me.

Your guitar rig is almost identical to mine as a young lad.

I recommend the gravitational reverb sounds great and does subtle to longer reverbs.
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right on!! You an engineer? Yeah the effects are strongly influenced by Jimi. As i've gotten into doing my own pedals i'm working on "the perfect fuzz" which is a rabbit hole I wish I didn't enter! I also am wanting to do my own uni-vibe. I got the PPCB electra-vibe pcb and 2 more pcbs in my desk. kinda feel out of my league for that one. Thanks i'll check that reverb out!!
 
masonlewisdiesel said:
Hey! sorry for the delay, I just uploaded a couple short videos of the guts, I will get you some high res photos sometime tonight!
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First thing that popped up in the videos are that 2n on the lead Vol and no mounting brackets on the OT, 784-139 good. The 500p dog bone has been swapped on the 470k mixer too. 56k/250p tonestack which is correct. Guessing a PT swap as voltage taps have been moved and are different colors.. Screen Grids should be 1k not 4.7k. Nothing wrong with any of these obsevations just a quick visual if your new to JMP's. Cool amp.
 
EZS said:
First thing that popped up in the videos are that 2n on the lead Vol and no mounting brackets on the OT, 784-139 good. The 500p dog bone has been swapped on the 470k mixer too. 56k/250p tonestack which is correct. Guessing a PT swap as voltage taps have been moved and are different colors.. Screen Grids should be 1k not 4.7k. Nothing wrong with any of these obsevations just a quick visual if your new to JMP's. Cool amp.
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Duuude! I don't know how your reply went right over my head, although it was around the holidays so I may have been a little incapacitated.
Thank you for getting back to me, so if i'm understanding correctly. these changes are not too significant/harmful? Whats the real deal with transformers? I've heard numerous things from the penut gallery over the years as an example: "too bad, replaced transformer(s), wow too bad... It was a nice amp". Roughly 50% of people have said that, i'm new to the 70's Marshall game, so I truly dont know how that affects value. I should add though that those commentors dont seem to have any fact or info to suppport their claim. The other 50% of people seem say something similar to your comment. They also seem to be more educated/experianced with the subject. So, what do the changed do to ultimate "value" of the amp.

Sorry for the run on paragraph! Also, I am indeed crazy. Because i'm never gonna sell it.
I'm going to check out what you mean about the OT mounting brackets
Thanks @EZS !
-Mason

EDIT: Any amp specific fourms you recommend? I really wanna try a low watt build(.5-2W). I'm on the marshal fourm, any other suggestions?
 
I lloked at the first video again. Screen grids may be 470r not 4.7k. Many of these 50w of the era did not have any screen grid resistors. Later US imports w/6550's started implementing the now standard 1k. Ive owned a few 50w without screen grids and I ended up with blown HT fuses and tube failures, but the sound is glorious. 470r would give great tone, and probably safe enough. 1k is just safe. Measure your B+, Turret at the 2W 10k/yellow wire attached. See if its around 390-410vdc or up 460-480vdc. That will dictate the power tranny swapped. Also the 2n on the vol pot will give a little gain grind vs 500p stock. I have no idea why people snip that cap. Not Marshall players. Taming their $20 overdrive into a glorious Kerrang British amp.
 
And.... the circuit sounds great with both high or low voltage power trannys. My friends 73 has a high voltage swap and it rips. If it is a higher voltage tranny, you should use 2 10k dropping resistors and your PI node should be around 320v. Marshall later omitted 1 of those resistors, as in your amp. But Im guessing yours had 2 originally as that amp was reworked. Even low voltage amps had 2 10k, as in my JMPs. Really gets them spongy in low voltage amps- 380ish B+. But ideally try to get your PI supply node around 320'ish and the preamp will sound fantastic.
 
EZS said:
I lloked at the first video again. Screen grids may be 470r not 4.7k. Many of these 50w of the era did not have any screen grid resistors. Later US imports w/6550's started implementing the now standard 1k. Ive owned a few 50w without screen grids and I ended up with blown HT fuses and tube failures, but the sound is glorious. 470r would give great tone, and probably safe enough. 1k is just safe. Measure your B+, Turret at the 2W 10k/yellow wire attached. See if its around 390-410vdc or up 460-480vdc. That will dictate the power tranny swapped. Also the 2n on the vol pot will give a little gain grind vs 500p stock. I have no idea why people snip that cap. Not Marshall players. Taming their $20 overdrive into a glorious Kerrang British amp
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Pulled the chasis out after work, Some things stand out to me a little more after reading what you've said, as well as some more observation...attached is a few photos. 5w 470R, you are correct sir. And the voltage measurement, take it at the turret right next to the Bias trim, correct? with the 2 yellow wires. that I THINK go to the first cap and the trans. Regardless, that is the test point your referring to, right? Any other mesurements since I have it out? whats the deal with the 2 resistors that I circled in red? Thank you!!
 

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EZS said:
And.... the circuit sounds great with both high or low voltage power trannys. My friends 73 has a high voltage swap and it rips. If it is a higher voltage tranny, you should use 2 10k dropping resistors and your PI node should be around 320v. Marshall later omitted 1 of those resistors, as in your amp. But Im guessing yours had 2 originally as that amp was reworked. Even low voltage amps had 2 10k, as in my JMPs. Really gets them spongy in low voltage amps- 380ish B+. But ideally try to get your PI supply node around 320'ish and the preamp will sound fantastic.
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Resting Voltage Power on Standby off(RUN) 472VDC +/- a volts, and your totally right about the new transformer, aging on the leads, wire size, etc.
 

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Yeah, typical JCM800 2204 aftermarket tranny. Pretty much all thats available. But,,, thats ok. Add another 10k in series as labled and that pink wire is the PI node. 320ish is the sweet spot tuning a Marshall. IMG_0064.jpg 1964.gif
 
EZS said:
Yeah, typical JCM800 2204 aftermarket tranny. Pretty much all thats available. But,,, thats ok. Add another 10k in series as labled and that pink wire is the PI node. 320ish is the sweet spot tuning a Marshall. View attachment 89161View attachment 89162
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Hooked it up with the schamatic too!! been trying to locate that! So as it sits right now. i'm at 380+. adding the 10k 2w in series like you laid out(thanks by the way) should help bring it down closer to target? for looks, it would be sweet to pull the board and install in the through holes. bet lets get real...nice little point to point set up is more likely. suggestions on parts source? like Antique Electronic Supply(tubesandmore.com) or valvestate? This isn't like one of those 4chan jokes or whatever where some dumb kid gets tricked into thinking his brand new macbook is water proof and "tests"(Ruins) it on youtube, right? haha
 
Lifting board is easy. Unscrew pots and input jacks from front panel and the board will lift enough. No unsoldering needed. Use a desoldering sucker and be careful of the semi fragile trace pads. Since your just removing a jumper, cut jumper in half first then remove 1 piece at a time for ease. Dont get cold feet now. Or do some top board hack.
 
EZS said:
Lifting board is easy. Unscrew pots and input jacks from front panel and the board will lift enough. No unsoldering needed. Use a desoldering sucker and be careful of the semi fragile trace pads. Since your just removing a jumper, cut jumper in half first then remove 1 piece at a time for ease. Dont get cold feet now. Or do some top board hack.
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Yeah, looking at now. Your right. Plus I would like to do a small wattage build or kit at some point soon, good time to practice. While I got it red tagged, is anything else I should do? Probably do the Caps at the same time
 
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