JMP 50 circa 1973 Circuitry

[MikeyV]

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?)

Thanks for the info! Would the CH1 coupling cap be the second mustard cap from the right, the first small one whose value can't be read?

 

[neikeel]

Thanks for the info! Would the CH1 coupling cap be the second mustard cap from the right, the first small one whose value can't be read?

First one is channel 2 (normal coupler 0.022uF always) second is channel 1 (bright) which is 0.002uF on a lead amp and 0.022uF on all the others (inc later 2204/3) amps

 

[nickfox]

In addition I scoured the web looking for other gutshots and articles, including the Marshall 101 files.

Beautiful amp! What are the Marshall 101 files you're talking about? Do you have a link?

 

[Chappy]

I found that info on the net way back in 2006. I just tried to find the original source but the links seem to all be dead. Try a search on <Marshall Circuits 101>. The doc is available on Scribd and the Metropolous forum. I dont have a Scribd account and when I dowload the pdf from Metropolous I get a corrupted file. I have attached two word docs that I made (converted to pdf) from the original source material. I dont know who the originator was and I am in no way taking credit for this work. If anyone has an issue with me posting this I will promptly take it down.

Steve

 

[LoudStroud]

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

You're about halfway there to having the EVH/ Jose mod...Split cathode on the two inputs, Bass tone stack, no bright cap. Add a 250-330uf cap across cathode of V2a, replace the 47pf "fizz" cap in phase inverter to 100pf and the P.I. .022 coupling caps to .1 and you're all the way there. Not only does it sound EVH when cranked, but it's a very versatile circuit. Great clean tones. Worth trying.

 

[Chappy]

You're about halfway there to having the EVH/ Jose mod...Split cathode on the two inputs, Bass tone stack, no bright cap. Add a 250-330uf cap across cathode of V2a, replace the 47pf "fizz" cap in phase inverter to 100pf and the P.I. .022 coupling caps to .1 and you're all the way there. Not only does it sound EVH when cranked, but it's a very versatile circuit. Great clean tones. Worth trying.

Thank you for that. I can do all of those changes by just adding caps in parallel without touching the original solder joints. I will give that a try next time I open it up. I'll also do a full analysis of the signals at different points in the circuit before and after the mod and document the differences, sharing them here. I have a fairly decent oscilloscope, and supporting test equipment. I can capture screen shots of signals in the time domain as well as spectral analysis in the frequency domain.

Steve

 

[LoudStroud]

I've been trying to locate my notes from the forum where this was discussed. Someone had Eddie's amp on the bench at one point and documented it. A friend sent it to me years ago. I have a '70 Bass 50 set up this way. it wasn't completely stock when I acquired it, so a perfect amp to experiment with.

The one other thing I can't recall is if the .0022 coupling cap on input one should be an .022. Think so but will confirm. Either way worth comparing anyway.

Being that yours looks pretty stock (and is a rare circuit) , I totally understand why you wouldn't want to get too crazy with it. Good idea adding caps in parallel to achieve same values.