JMP 50 circa 1973 Circuitry

[Chappy]

I'm looking for some help from the technical experts here on this site reference a JMP 50 head that I acquired back in 2005 in Canada.

I purchased this unit from a pawn shop along with a 2x12 cab. Unfortunately I was unable to obtain any info on their history. At some point in time someone removed all of the tolex from both units and varnished them The outside tags are missing and the chassis label has been modified and is vague.

I believe that the head is a 1987 lead model but I discovered a discrepancy when comparing the components to a 1987 schematic (1987 GW Unicord dated July 70). In addition I scoured the web looking for other gutshots and articles, including the Marshall 101 files.

Circuit items leading me to believe it is a 1987 Lead:
1. Split cathode resistors and caps on V1
2. 0.022 coupling caps between PI and output tubes
3. 100K FB resistor with no cathode cap on V2 as opposed to 47K FB with the V2 cap.

The biggest discrepancy is that my circuit has a 56Kohms slope resistor paired with what appears to be a 250pf cap in the tone stack circuitry. Based on all of the research I have done these should be 33Kohms and 500pf for a 1987 Lead. The parts look authentic (when compared to parts in images of other heads that I found) but it appears that these parts were added after the board was initially created. The lacquer is missing from the turrets.

The Marshall 101 document indicates that Plexis and Bass heads used a 56K resistor and 250pf cap in the tone stack and that Leads used 33K and 500pf valued parts. All of the pictures of other heads that I have found support this. Additional point of note is that the 2045 cab had professional grade JBL speakers installed on a different baffle board.

I speculate that this head was a 1987 Lead out of the factory, but was then partially modified into a Bass head (based on tone circuit modification and JBL speakers). If you zoom in on the attached image of the chassis tag you can see the words LEAD underneath the black marker cover-up. So calling out to the tech experts. Do I have a modified 1987 Lead or was this originally a Bass model? Could Marshall have changed the tone stack parts on the production line, as they do look period correct?

Additional mods:
Someone butchered the back panel and changed AC input. Screen resistors were added to the output tubes. One volume pot was removed and repaired or possibly replaced. I changed out all 3 Daly power supply caps. The bias supply is upstream of the standby switch.

Steve

 

[neikeel]

Looks like a reasonably tidy 1987 board with 250pF/56k (bass/PA/Organ tone stack mod
The board type is typical for 71/72, unusual for 73 - but that is what the serial says (E suffix)

 

[Chappy]

Thanks for your reply Neikeel. I wonder if that mod was done at the Marshall factory as an afterthought? Re-purposing some surplus 1987 boards as PA or Organ boards to fulfil a Canadian order. I just cant see why someone other than a Marshall employee would go through the trouble of blacking out the word "Lead" on the chassis label.

 

[william vogel]

I don’t know why Marshall would have done it without lacquering the solders afterwards. Most likely the amp was modded by an individual who wanted the specific circuit that’s in the amp. To my knowledge there’s no Marshall circuit with those specific components. It’s a mixture of a 1987 and a 1986. The organ amps didn’t have the capacitor across the mix resistors. I bet it sounds good. No bright cap too. Nice.

 

[Chappy]

Thanks for your comments William. You are probably correct. If Marshall had done it they would have changed the coupling caps after the PI as well. It sounds nice but the overdrive sounds a bit rough to me and it seems to have too much fizz. I adjusted the bias on the output tubes and checked the output signal with a scope. Most of the crossover distortion is gone, and I have the tubes biased around 75% max plate dissipation. I suspect that something else may not be quite right. The plate resistor on V2A has drifted up to 123K, and the cathode resistor on V2B has drifted up to 106.8K. I am going to put a scope on that part of the circuitry to see if something is amiss. I can easily clip on higher value resistors in parallel to bring those two back down within spec and make the comparison.

Steve

 

[william vogel]

Thanks for your comments William. You are probably correct. If Marshall had done it they would have changed the coupling caps after the PI as well. It sounds nice but the overdrive sounds a bit rough to me and it seems to have too much fizz. I adjusted the bias on the output tubes and checked the output signal with a scope. Most of the crossover distortion is gone, and I have the tubes biased around 75% max plate dissipation. I suspect that something else may not be quite right. The plate resistor on V2A has drifted up to 123K, and the cathode resistor on V2B has drifted up to 106.8K. I am going to put a scope on that part of the circuitry to see if something is amiss. I can easily clip on higher value resistors in parallel to bring those two back down within spec and make the comparison.

Steve

What’s the B+ voltage at idle coming off the rectifier?

 

[Chappy]

B+ is 420V (at the plates of the output tubes)
Vbias at -38V to give Ip = 43.7mA
JJ tubes

Edit:
I will go re-measure the voltages when I get a chance including the B+ upstream of the choke.

 

[stickyfinger]

I've never been able to find the 250p dogbone. A bunch of 470p 20% showed up for sale 4 or 5 years back and that was the only time I found those.

I think it's possible it might be a factory mix up on the tone stack.

 

[neikeel]

I don’t know why Marshall would have done it without lacquering the solders afterwards. Most likely the amp was modded by an individual who wanted the specific circuit that’s in the amp. To my knowledge there’s no Marshall circuit with those specific components. It’s a mixture of a 1987 and a 1986. The organ amps didn’t have the capacitor across the mix resistors. I bet it sounds good. No bright cap too. Nice.

Yes, although to be clear the organ did have mixer bypass. It was basically a bass amp with 100pF on vol 1 pot. It was the PA that had neither.

 

[neikeel]

Lack of dye on those very specific joints makes a non-factory mod most likely.
I’ve got some 250pF Lemco dogbones.

 

[Chappy]

I stumbled across a site called amparchives.com. Here are pictures from a PA head on that site. The chassis label indicates that it was tested 2 months prior to mine in 73. It appears that all of the turrets have the original lacquer, but note that model type on the sticker has been scratched out (similar to the sticker on mine) and replaced with "PA" in ink. So is this just a coincidence or is there still a possibility that Marshall was re-configuring amps at the factory for a short while in 73? Wasn't this also right before they switched over to the through hole circuit board style?

 

[stratburst]

I’ve seen a few of these over the years. Difficult to know for certain but if it happened at the factory it’s possibile they were incorrectly stamped and the label was updated accordingly during inspection. Based on what I know about Marshals manufacturing processes in the 60’s I doubt they ever converted circuits on the line however. I do remember reading somewhere that Rose Morris did perform changes in their distribution channels in the 70’s.

 

[neikeel]

I’ve seen a few of these over the years. Difficult to know for certain but if it happened at the factory it’s possibile they were incorrectly stamped and the label was updated accordingly during inspection. Based on what I know about Marshals manufacturing processes in the 60’s I doubt they ever converted circuits on the line however. I do remember reading somewhere that Rose Morris did perform changes in their distribution channels in the 70’s.

That red amp has 1985 nomenclature and the board is typical PA (0.1 output couplers, 250/56 tone stack, no mixer cap or vol pot cap.) so just a label change.

 

[Super Blead]

A bunch of 470p 20% showed up for sale 4 or 5 years back and that was the only time I found those.

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[AtomicRob]

The biggest discrepancy is that my circuit has a 56Kohms slope resistor paired with what appears to be a 250pf cap in the tone stack circuitry. Based on all of the research I have done these should be 33Kohms and 500pf for a 1987 Lead.

The important question is, how does it sound? My favorite gigging amp is my '73 1987 lead model which has (almost) that same mod - 250pf and 51K slope resistor. Has a couple other mods too because it's running KT66 tubes. When I first got it many years ago I briefly considered "restoring" it to correctness but thankfully didn't do that because it just sounds amazing as is. I don't know who modded it, or when, or whether that 51K was picked after trying a bunch of values or if it was just whatever was in the parts bin, but it works!

 

[Chappy]

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Thanks for the link. I already replaced my 250pf cap with a modern 500pf cap but I may pick up some of those.

 

[Chappy]

The important question is, how does it sound? My favorite gigging amp is my '73 1987 lead model which has (almost) that same mod - 250pf and 51K slope resistor. Has a couple other mods too because it's running KT66 tubes. When I first got it many years ago I briefly considered "restoring" it to correctness but thankfully didn't do that because it just sounds amazing as is. I don't know who modded it, or when, or whether that 51K was picked after trying a bunch of values or if it was just whatever was in the parts bin, but it works!

After further scrutiny I believe the amp is working as it should and it now seems to sound fine with the 250pf and 56K resistor. The fizz issue that I posted earlier may be partly caused by the attenuator that I am using. I went ahead and replaced the tone stack parts with a 500pf and 33K resistor. After the switch I wasn't able to discern much of a difference in sound compared to before. Just a slight difference in how the TMB controls work due to the shift in the frequency response. I may go ahead and put the 250pf and 56K resistor back in.

Just out of curiosity, does the label on your chassis have the word "LEAD" covered over with magic marker similar to the two labels in the images posted earlier in this thread?

Steve

 

[MikeyV]

Nice piece. I have the same amp and it looks nearly identical. Same features, laydown PT, cream colored turret board, etc. I think it's the same month, May.

 

[Chappy]

Very nice. The serial number on yours is only 16 digits higher than mine!

 

[neikeel]

Nice piece. I have the same amp and it looks nearly identical. Same features, laydown PT, cream colored turret board, etc. I think it's the same month, May.
View attachment 111237

Looks tidy. It’s had some resistors replaced with carbon comps (220k output grid leaks, 33k tone stack slope, 820R V2a cathode resistor and the tone stack capacitor). All features you see on amps converted to lead but clearly has the lead stamp. What value is the channel one coupling cap (22nF or 2n2nF?)