Does this JMP 50 look ok?

[RWMusic]

Recently acquired another Marshall. Just want to start by saying that so far it's the best vintage Marshall that I have. Very aggressive with lots of gain, which I like. It has a master volume installed, which I assume is a PPIMV? it also had a 3 prong power cable installed.

I use it with my Fryette Power Station and with the Master Volume all the way full, and Volume 1 on just 3-4, it already has hair metal style gain. I can then set the master volume at different spots to get bluesier tones and different textures of breakup.

I have a few questions I'd like to have answered:

1. Does it generally look ok at a glance? Nothing sketchy looking? Is there anything that might be contributing to the high gain?

2. When I have the master volume all the way full, but Volume 1 and 2 are off, I still get a loud clean signal from the amp, enough to fill a basement. Is this normal? Is it possible that the master all the way up is sending more signal to the front, pushing it harder?

3. Is the master volume a post phase inverter style?

4. I've been worried about the polarity switch after reading about "death caps" and getting shocked and all of that. The polarity switch doesn't seem to be doing anything when I try using it. With me having zero electronics knowledge, it looks to me like it's disabled inside. I also don't seem to see a "death cap" like I've seen in other pictures. Is this true? Is the polarity switch disabled with the death cap removed? And does the power cable look to be properly installed? I don't want to be fried while playing.

 

[Matthews Guitars]

The NFB resistor has been parallelled with another, reducing the NFB resistance value which increases NFB. Two 100K resistors in parallel, giving 50K.

All four 68K input resistors have also been parallelled with other resistors. This will make the amp less stable and more susceptible to RF inteference. It's not really about input sensitivity. Not really a mod that does much or makes much sense.

Some of the original capacitors have been replaced on the board, and values changed as well.

Of course it has an MV mod. But it appears to be a post-volume control, pre-phase inverter master volume. Not the type most expected.

 

[myersbw]

You have quite a few mods to the original circuit there. It is not a PPIMV...it's a typical Master Volume style (signal pulled off the Treble pot and to the Phase Inverter). Input resistors are lowered at the jack, post-PI caps have been increased (the piggyback caps) which will push more low end into the output tube circuit. You have 1 ohm resistors located at the cathode of the output tubes to measure tube current (i.e. check bias). But, the orange wire nut...even at the input...is a no-no for me. My guess is someone got a bit lazy versus properly soldering in a change correctly. Not a bad looking amp! I'd certainly have it cleaned up a little and the power caps have some major age on them (I'd have those and the bias circuit caps replaced).

Now, since it's so riddled with changes...I'll give ya $50 USD and also pay for shipping! LOL!!! JK!

Nice score!!! As for the PPIMV...for that era, leave the pre-PI master. I much prefer that route in the early 70's and, should you need anymore, add a pedal to taste. But, as noted above...take it in to a local tech for some TLC...caps will cost you a little, but worth it in my opinion...others will disagree. To me? Old caps are a ticking bomb and you never know when you'll go. Yes, new caps can have issues (as new tubes, etc, etc). I've done quite a few cap jobs...none have returned to me in the past ten years and most folks bringing me amps are now good friends...they'd tell me.

Congrats again on the amp!! My fave era!!!! By the way, much of the piggyback component installs are fine to leave if you like what you hear. To me, this style of build is nice as you shouldn't lose resale value in my opinion. All those changes are easily reversed if desired or selling down the road. THAT amp there...is a keeper!!!

 

[RWMusic]

Hmm, curious what the intended result of these changes was, like what was the previous owner hoping to achieve? The master volume being pre phase inverter is interesting. I have a 76' that I posted about before, but it's master volume sucks. Turning that one down maintains zero gain and you end up with a barely broken up sound with it turned down, but on this 72' turning down the MV still has a bunch of gain and is very satisfying to play which is why I figured that must be the difference between a PPIMV and a pre phase.

How about the polarity switch and "death cap". Am I safe there? Won't get shocked?

Like I said, I really like the sound of it, and it's the best one I have so far. More gain than both the 76' JMP 50 and 81" 2203. I feel like I can easily get the EVH, Lynch, DiMartini tones I like.

I'm usually one of those that have a "if it ain't broke don't fix it" outlook. Probably won't take it in to be serviced until something doesn't sound right.

 

[Pete Farrington]

I think you’ve got an unrealistic expectation of what can be determined via a few photos over the internet.
It’s a 50 year old item of mains electrical equipment, which has been incompetently tinkered with.
I suggest you get it appraised, in person, by a competent person, to at least verify that it’s electrically safe.

From what I can make out, the end seal looks to be missing from the input stage 680nF cathode bypass cap. As such it will be subject to degradation from the atmosphere.

It may have the one wire mod, cascading the V1 stages. If so, both channels won’t be simultaneously usable.

 

[RWMusic]

I think you’ve got an unrealistic expectation of what can be determined via a few photos over the internet.
It’s a 50 year old item of mains electrical equipment, which has been incompetently tinkered with.
I suggest you get it appraised, in person, by a competent person, to at least verify that it’s electrically safe.

From what I can make out, the end seal looks to be missing from the input stage 680nF cathode bypass cap. As such it will be subject to degradation from the atmosphere.

It may have the one wire mod, cascading the V1 stages. If so, both channels won’t be simultaneously usable.

You're right, I guess I do. I'm a player, I'm not a tech, nor do I have any knowledge about the internal workings of tube amps or electronics. All I know is, that after lurking on these forums, whenever someone has similar questions, everyone tells them "Post screen shots of the guts and we'll be able to tell you what's going on". Maybe that's a waste of time if I'm just going to be told "you have unrealistic expectations about what can be determined with pictures."

In what way has it been "incompetently tinkered with"? Why is it incompetent?

The problem with many amp techs where I live (Chicago) is that it's a "drop off" situation. You leave the amp with them, and then who knows what they end up doing. You can't sit down with them 1 on 1, open the amp up together, ask questions in real time, etc. If I was going to go to an amp tech in person, that's what I'd want because I'm the type of person who wants to have a general understanding of what I have, especially in the case of vintage amps, where a lot of money has been spent, and the item is valuable.

A lot of what I've read so far reads like a foreign language to me. It's fine to explain in technical terms, but like I said, I'm not an electrical engineer, so it would be nice to explain in layman's terms. Why would changes like this be made in the first place, and what is the resulting effect on the tone and/or function of the amp?

I also noticed, and this might not be a big deal, that the bottom right corner of the chassis looks wierd, like there is a crack in the chassis or some type of glue to hold the back and the side together. None of the other corners have that.

 

[Matthews Guitars]

I would start by having your technician remove the four resistors strapped across the four 68K input resistors. Those aren't making the amp any better. They are making it more susceptible to self-oscillation and picking up RF interference.

The NFB mod (100K resistor strapped across another 100K resistor) may or may not be to your liking. One end can be lifted, and you can decide which way you prefer it.

 

[RWMusic]

Oh one more thing that came to mind. I noticed on other pictures of early 70's Marshalls, that one side of the board has a square black thing attached. On this one it's missing, but the screw for it is there. What does this thing do, and is it normal for it to not be there?

 

[neikeel]

I will add my summary (which is consistent with most of the above).
It has a replaced power transformer (the 1202-324 is later and has higher voltage than the original 1202-164 that it came with). Commonly these amps had incorrectly wired bias circuit with hot switching of bias - killed a lot of PTs.
The diode block and snubber caps have been removed (as is also common as they get blamed for the PT failure!).
Replacement diodes are a hack repair (see Pete's comment above)
Someone has added grid stoppers to the output tube (5k6 resistors) and 1 ohm resistors on the output tube grids but they don't appear to be of the close tolerance 1% type so might be 5% off.
The paralleled NFB resistor is not ideal but functional, not sure why the PI coupler mustards were replaced, failure v unlikely so maybe different values use in past (cutting old caps and grafting also qualifies as hack).
Odd to parallel those resistors on the input grids as minimal change 68k with 100k = 40k so hardly worth bothering with, you could go down to 10k and still get decent input sensitivity without RF issues.
The V1b coupler is usually 0.0022uF and has been replaced with a 0.022uF (fattens up the lead channel at the expense of cutting tones) often a mod for cascaders
The MV as mentioned is a pre-PI MV and when turned up fully is like an NMV.
I cant see whether there is a one-wire mod or internal channel jumping in there, it is not uncommon to get some volume leaking through with volumes on zero so more investigation needed.

Having said all that if you like it and it sounds good as long as it is safe, why not just play it?

 

[RWMusic]

I do play it, I just am curious to know what I have. I purchased the amp for 2 reasons. I liked the way it sounded, and as somewhat of an investment. I know the price on these will never go down, only up. Even with mods.

Still looking for an answer regarding the polarity switch and "death cap"

 

[neikeel]

The 'death cap' has been removed, can't see the polarity switch wiring clearly, maybe out of circuit, just used as a tie point? I noticed the extra presence cap piggy backed too.

 

[Spanngitter]

Haven't checked all the pictures but am I the only one thinking that this amp has only a 2 prong power cord?
I think replacing this with a modern safety standards 3 prong connection is mandatory before even thinking about turning that unit on.

 

[Chris-in-LA]

Haven't checked all the pictures but am I the only one thinking that this amp has only a 2 prong power cord?
I think replacing this with a modern safety standards 3 prong connection is mandatory before even thinking about turning that unit on.

There’s 3 wires, the middle wire is green and is soldered to a filter cap ground.

 

[RWMusic]

I will add my summary (which is consistent with most of the above).
It has a replaced power transformer (the 1202-324 is later and has higher voltage than the original 1202-164 that it came with). Commonly these amps had incorrectly wired bias circuit with hot switching of bias - killed a lot of PTs.
The diode block and snubber caps have been removed (as is also common as they get blamed for the PT failure!).
Replacement diodes are a hack repair (see Pete's comment above)
Someone has added grid stoppers to the output tube (5k6 resistors) and 1 ohm resistors on the output tube grids but they don't appear to be of the close tolerance 1% type so might be 5% off.
The paralleled NFB resistor is not ideal but functional, not sure why the PI coupler mustards were replaced, failure v unlikely so maybe different values use in past (cutting old caps and grafting also qualifies as hack).
Odd to parallel those resistors on the input grids as minimal change 68k with 100k = 40k so hardly worth bothering with, you could go down to 10k and still get decent input sensitivity without RF issues.
The V1b coupler is usually 0.0022uF and has been replaced with a 0.022uF (fattens up the lead channel at the expense of cutting tones) often a mod for cascaders
The MV as mentioned is a pre-PI MV and when turned up fully is like an NMV.
I cant see whether there is a one-wire mod or internal channel jumping in there, it is not uncommon to get some volume leaking through with volumes on zero so more investigation needed.

Having said all that if you like it and it sounds good as long as it is safe, why not just play it?

So I'm assuming the switched out power transformer would heavily lower the amp's value? Given today's prices, what do you think a fair price for this amp would be with the replaced transformer? I see the early 70's amps going for roughly $4k and up these days. I didn't pay that price, but am trying to determine if I overpaid anyways.

What is the significance of the new transformer having a higher voltage?

My 76' JMP 50 has the correct 1202-164 transformer. it also happens to be my least favorite and already has holes drilled and new tube added. I guess I could always swap the transformers on them if I ever want to sell.

 

[neikeel]

The voltage of the 324 would be 460vdc on plates of outputs.
The 164 varied getting lower from 425/430 in late 60s to 360/370 mid 70s.
You could swap but you’d have to do it really neatly otherwise it is just another replaced transformer.
No idea on prices in US - sorry

 

[RWMusic]

Makes me realize that vintage amps are too much of a risk. I got these all locally where there was no opportunity to open the amp. It was all based on if you liked how it sounded and you bought as is. Not sure what I'm going to do with these since I bought them as investment in addition to playing. I wanted to be able to flip them in the future for profit. Not sure if I'm going to be able to do that with 2 modified JMP's. I bought them for less than they normally sell for, but with the mods and replaced PT, I guess that doesn't matter.

Should have just went with a replica, or Metropoulos Metroplex/Friedman BE100.

Disheartening.

 

[Spanngitter]

Dealing with vintage amplifiers is not different from dealing with other vintage stuff and e.g. there is countless vintage car enthusiasts where the new investment did turn out as a plaster cluster and/or shiny outside, rotten inside. There is a learning curve if you want to go down that rabbit hole of vintage amplifiers and the majority of people doing so has paid its apprentice's due. There is gread replacement transformers available which (when the replacement is done correctly and neatly) do nearly not impact the value of the amplifier or even maybe raise it. As an unexperienced contributor (as you are) I would not recommend to look for an vintage replacement part, instead check out what you will get from e.g. Heyboer, Pacific Transformers or Mercury Magnetics. So not all hope is lost, its possible to restore these amps to the vintage gems they deserve to be.