1969 Marshall JMP Super Lead 100W SL/A 4095A

vomheimlin

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Just curious what some of the experts might notice about the wiring and components of this amp. As far as I can tell, the only thing that isn't stock is the plug receptacle and one of the power tube bases next to it. I've owned the amp since 2002, but I've hardly played on it. I'm guessing some components (ie capacitors) are probably due for replacement after all these years. The amp does have its tiny place in rock history, which I can get into later on I guess.
 

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Seanxk

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Nice amp and welcome to the MF.

Looks like a bit of work on power valves, inc a new base and screen resistors ( green ones original )
Possible power transformer is a replacement.
New switches and obviously the Bulgin has replaced an earlier IEC socket mod.
Odd marks on inner chassis but appears factory rivets and filter caps, i would just test these before replacing them.
 

vomheimlin

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As for the history of this amp, it's by no means a museum piece. I bought it for my son, then a teen, back in 2002. A friend of mine, George Lynch, helped me with some things as I got it cleaned up and ready for Xmas. He signed the inside of the head case and 4x12 cab for him to give it a nice touch. He also used it, along with my other amp (Marshall 30th Anniversary model 6100LE pictured) and some other gear, to record on his upcoming CDs at the time. One was on Lynch Pilson's "Wicked Underground" and the other was on Lynch Mob's "REvolution".

Unfortunately, my son never got the bug to play like me when I was younger. Career and family kind of soaked up all of my free time, so I've hardly played at all in over two decades. Things are a bit different now due to COVID-19, and I'm about ready to dive back into the hobby. That said, I can honestly say that the amp sounds incredibly different with George plugged in as opposed to me. No surprise there though. Tone is unique to the individual, no question about it.

Edit:
My memory appears to be slipping a little. Apparently we did the cab for my son's birthday, and the amp head for xmas. Thank God for the pictures (which I included as a collage - had edit my son's name out for privacy). I'll always be grateful to George for the kindness he showed my son. He even introduced him to several rock stars at NAMM one year, including EVH :D

Lynch Pilson: Wicked Underground


Lynch Mob: REvolution
 

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neikeel

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Hi
Not bad generally. Obviously recovered at some stage.
I see replaced vol 2 pot, treble pot
Non-standard cap on volume 1 pot
Nice to see the C1998 OT still there (they were being phased out 1970/71).
Shame that the mains socket was literally hacked out probably needs a backing plate to secure it.
Replaced V7 socket and screen resistors as pointed out by Sean. No idea why those score marks on the chassis. From the rewiring and missing clamps the PT is a replacement.
So not too shabby at all.
Nice bit of history too!
 

TAZIN

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Nice amp. Most of the changes to the amp have been outlined already but I believe the Treble potentiometer is original. One thing you should check is the cathode resistor for preamp valve V2a. It may be a 10K resistor instead of a 1K.
 

neikeel

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Nice amp. Most of the changes to the amp have been outlined already but I believe the Treble potentiometer is original. One thing you should check is the cathode resistor for preamp valve V2a. It may be a 10K resistor instead of a 1K.

I think that you are right on both counts. I've seen that mistake once in person (on a plexi 69 superbass) and a couple of on line too.
 

VERVEHAMMER

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Very, very, nice.
:DThank you for posting!, Vomheimlin.
 

vomheimlin

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I'm curious to see if I can get this thing back a little closer to stock. I'm by no means an electrician or amp guy, but I learn quickly. So forgive me as I get myself acquainted with your terminology and abbreviations.

For the screen resistors, am I correct that only the mini green ones need to changed out? I'm assuming the correct ones should be the "white ones @ 1k 5w 5%"? Does anyone have any idea why whoever changed these out would use the "Welwyn 1k ohm 2.5w" then?

As for non-standard cap on volume 1 pot, what was originally on there? Again, any ideas on why the cap that's on there now was chosen as a replacement?

Can someone also show me where the cathode resistor for preamp V2a is located? I have a hunch, but don't want to assume. So I'll load up another pic. And how do I tell the difference between a 10k and 1k? Again, if it's in fact a 10k on there now and should be 1k, why would someone do that?

Thanks for any help. This should give me a start on things, and appears the simplest for me to tackle at this point. The mains input will never be right, but I have a replacement bulgin for it. The PT info is likely correct too, but I won't attempt that until I understand more things.

Lastly, I added some photos and links and more info in the prior post about the history. Hope everyone enjoys it.
 

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neikeel

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There is a UK seller with these 1M log CTS/RS potentiometer exactly like the originals. The 50w are ok with 2.5w Welwyns but I would generally go with 5w if you can find them.
The resistor in question (cathode resistor for V2a to be precise) was 820R and went to 1k about the time they dropped the 0.68uF bypass cap over it so you could use either. It is a mistake (the solder has the original dye on it - as mentioned I have corrected it before - probably colour blind assembler who could not tell difference between black/brown/red and black/brown/orange) as it biases the second gain stage very cold (a 2203 uses a 10k cold clipper as its cascaded second stage but has another gain stage after that, this does not as it is followed directly by the cathode follower tone stack).
Look on the board and you will see two big 0.68uF 160v mustards. This resistor sits underneath (electronically in parallel) the innermost cap.
Be very careful with the 0.68uF cap as they are very expensive now. Use chemwick fluxed solder braid to draw the solder out of the joint before trying to remove it or you will wreck the cap.
So, yes, I would find an 820R Iskra or NOS carbon comp to span the board. I have some here, but I am in the UK.
 

TAZIN

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I believe those RS/CTS potentiometers are still available today from AB Electronic. They are Series 45, Type 45 potentiometers.
 

vomheimlin

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Some more follow up:

The OT and choke do appear to be factory installed, as the nuts bolting it the chassis are still colored red. But the wires leading from the OT to the old style ohms selector have newer (shiny) solder joints. The OT doesn't have any identifying marks externally on it anymore either. There's just a residue mark on the end bells, where a sticker used to be.

The PT may in fact have been replaced as stated previously. It was at least removed anyway, as the red marks are missing from the nuts which bolt it to the chassis. I also noticed that literally every solder joint for its wires going to the voltage selector, power/standby switches, fuses, and a cap (near the main fuse) appear newer (shiny). It was mentioned a bracket was missing, but I'm not sure what that looks like or where it would have been. Can anyone show me that? Finally, the only external identifying markings I can see on the PT is on the end bells, which appears to have "S 85" stamped in black on it maybe. Does anyone have any idea which PT this might be?
 

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junk notes

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Just curious what some of the experts might notice about the wiring and components of this amp. As far as I can tell, the only thing that isn't stock is the plug receptacle and one of the power tube bases next to it. I've owned the amp since 2002, but I've hardly played on it. I'm guessing some components (ie capacitors) are probably due for replacement after all these years. The amp does have its tiny place in rock history, which I can get into later on I guess.
:welcome:
Awesome brother glad you are here!

Although he did not get the bug, your son is fortunate to have you as his Dad, brother!
(I have Cromanic in constant rotation!)

Nice amp you got from George. Thanks for the backstory on the amps, and thanks for sharing. :yesway:
 

Seanxk

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The OT still looks good and there would be a sticker just there, so that's a plus in it's favour.
The PT ( Drake? ) is what Marshall would supply if you asked them for a replacement in the 80's, so still a plus.
 

vomheimlin

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Can anyone inform me how to test all the mustard caps (ie 0.68uf, 0.10uf, 0.022uf, 0.0022uf), as well as the big blue cans, WITHOUT removing them from the circuit?

Also, please let me know what tools I will need. I've seen some people talk about a DE-5000 handheld LCR meter, but I'm not sure if this what I'll need.

I just want to make sure everything in this old amp is within spec and working properly. It hasn't been fired up in over a decade. Thanks all.
 

Matthews Guitars

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Test a capacitor without desoldering it
Let just face it.

You just cannot test a bad capacitor inside or outside a circuit board by measuring its capacitance value with a capacitor meter or a multimeter. Because in such a situation mentioned devices lead you into false reading and you may not be able to actually tell if the capacitor you tested was actually a bad or right one.

Why?

  • The reason is, when a capacitor is inside the circuit board, there are a lot of other components in series or parallel with it. So you get the equivalent reading, not the actual one.
  • When the capacitor is outside the board, sometimes a bad capacitor may give you a proper capacitance value on the multimeter or capacitor meter.
No doubt, multimeter or capacitor meters are used to measure capacitances. They just cannot be trusted to tell you if the capacitor is bad or good whether it is outside or inside the circuit board.

So, how can I test this bitch?

There left one choice we can use to test a capacitor, and that is by measuring its equivalent series resistance (ESR).

In summary, the best solution to test a capacitor without actually desoldering it is either using an ESR meter or smart tweezers. Both work the same and are fine to use. But the ESR meter is preferred for through-hole capacitors, and the latter one is preferred to test SMD capacitors.
 

Matthews Guitars

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If the yellow epoxy ends of the mustard caps are undamaged, those capacitors are probably OK. If they're cracked around the barrel of the device, they're likely to be bad.

You will want to bring that amp up SLOWLY on a variac. It should take you at least half an hour to bring it up to line voltage, with it having been that long since the capacitors were charged.

The worst thing you can do for the big reservoir capacitors is leave them in an uncharged state for many years. So they may well have gone bad. That's fine, they were designed to last 20 years and at this point whether they're good or bad they don't owe you anything.

Symptoms of bad reservoir capacitors: They run hot. Or their shrink wrapped label has over-shrunken. Or they have lost their black plastic insulator disc on top of them. Or they have a swelled vent plug or they have already vented. Or they're spewing goop out of them.

Simply plugging the amp in and just seeing if it runs is not advisable. Not with it having not been powered up for ten years or so. If you do that, there's a fair chance that the reservoir (filter) capacitors (the big blue ones) may fail in seconds.
 

vomheimlin

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But that still doesn't answer my question. These mustards have such tiny ratings, which tool would someone use to measure the ESR? And how do you do it in circuit?
 

Seanxk

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But that still doesn't answer my question. These mustards have such tiny ratings, which tool would someone use to measure the ESR? And how do you do it in circuit?

ESR at these ratings is really difficult, but Mustards don't leak and they don't drift, they can break but that's about it.

Your filter cap's look fine too ( just look for bulging near the terminals ), borrow a meter and try them in situ, but remember some will not read as the can says, it's the ESR you're after.
 

Matthews Guitars

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Any time I see an amp come across my bench that SHOULD have mustard caps in it, but doesn't, I'm reasonably confident that they were robbed and replaced with newer caps because the dishonest technician who last worked on the amp figured nobody would know. Mustards don't go bad if they aren't cracked at the ends. Hardly ever.
 

Ivan H

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Maybe elaborate a little on what you mean by a "bad" capacitor. Regarding the coupling caps in our amps, the worst thing is "leakage" (cap leaking DC voltage). These type caps block DC voltage (won't pass it through the cap) while passing AC voltage (our signal). We most often see leakage occurring in the phase inverter output coupling caps, where there is a large positive voltage on one side of the caps (PI plate voltages) & the output tubes negative bias voltage on the other side of the caps (so a large potential difference across the cap). You can often check for this DC leakage with the caps in situ, using the DC voltage function of your multimeter. This means working on a "live" (powered up) amp, so all due caution & precautions. The V1 & V2 coupling caps are easily checked for leakage. V1, just check for positive DC voltage on the end of the caps that are connected to the volume controls. Even a few volts would indicate some leakage. Same with the V2 coupling caps (tone stack caps), check for DC voltage on the ends that connect to the tone controls.
The PI to output stage coupling caps are a bit more tricky as they have the PI plates high positive voltage on one side & the negative bias voltage on the other side. Check the negative (bias) voltage at the junctions of each of these caps & the corresponding bias splitter resistors (or at the output tube control grids). You should have pretty much the same negative voltage at both. A few volts difference could indicate a leaky cap. To further investigate you would need to desolder & lift the cap leads from the bias splitter resistors, then check for positive DC voltage on the now free standing cap leads.
Leaky PI output coupling caps are particularly a problem in that the positive PI plate voltage leaking through the cap is superimposed onto the negative bias voltage, thus lowering it, which in turn shifts the output tube's bias point, making the tube run hot. Leaking PI coupling caps can very easily destroy output tubes.
While mustard caps can go leaky, they are usually pretty good in that regard. Hope this is of help. Cheers
 
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