harddriver
Well-known member
So I have had this 1972 JMP 50 watter (last of the PTP boards into 1973) for many years, I inherited the amp from my best friend who passed away tragically so it is a very sentimental amp to me. It has always been a pretty aggressive harsher more strident sounding Marshall as were alot of Metal panel amps compared to my 68 plexi and it was also kinda noisy and hissy for as long as I can remember. The amp has 20 year old plus LCR filter caps and it had the original 46 year old Erie 320uf electrolytic cap on V1A and Erie 10uf/150V electrolytic bias supply caps. I always just kept an eye on the bias caps for obvious leakage but I had the"if it ain't broke don't fix it" for originality sake. So since we're all amp-heads here I wanted to share the results of my experiment which may be helpful to others.
I finally decided it was time to see if I could address the hissy/noisey nature of this amp and see what effects some simple maintenance would do out of curiosity. I actually was considering changing out the aged LCR's with new ARS filter caps but I want to see what these changes would do first. I changed out the Erie bias supply caps (which also showed no signs of leakage or bulging) for new Sprague atom 10uf's and I found the V1A Erie 320uf cathode cap while not actively leaking electrolyte there were hints of a dried yellow substance on the end of the cap, I thought it might be leaking DC which contributes to noise and DC on the pots, so I changed it out for an NOS BC 330uf cap. This amp has a Lemco Mica 500pf cap on the tonestack but a factory installed 500pf small diameter ceramic disc bypass cap on the 470K mixer resistor for the bright channel. 1969 Superleads onward also could have the red Lemco dogbone ceramic caps in the tonestack and mixer bypass cap here as well as Lemco/RS micas and ceramic disc caps, it just depends on what Marshall had on hand when the amps were built. On my 69 Superlead build I was running an 560pf RS mica cap on the tonestack and a 560pf ceramic cap on the 470K mixer and the amp was more harsh than my 68 that has Lemco mica caps in both positions, after I changed out that ceramic for another RS 560pf mica cap the amp sounded like my 68 and was no longer harsh. So I thought that the ceramic cap here in the 72 probably was contributing to its harsh qualities as well. I changed out the ceramic 500pf mixer bypass cap for an old CDE 560pf silver mica cap. Old ceramic caps are also known to leak DC causing noise. So here is the complete list of items changed on the 72.
10uf bias supply caps
320uf V1A cathode bypass cap
500pf ceramic cap
The results were amazing, I was actually pretty shocked...The amp is no longer the hiss queen that it was, and it seems any gain related oscillations and preamp tube plate ring that were there before the changes are now dampened quite nicely plus the amp is no longer the harsh metal panel that it was , it sounds more pleasing and in line with my 68 plexi and 69SL. So if you have an old Metal panel or later Marshall that's harsh you might want to see what type of mixer and tonestack caps it has as well as the age of the V1A cathode and bias supply caps. I also won't delay changing out the bias supply caps or extremely aged electrolytic cathode caps in the future, I always knew they should be changed at least every 10-30 years and the results of the experiment seems to prove the wisdom to be true. I did not test to see if these caps were leaking DC because I was changing them regardless, but with the great reduction in noise I have to assume there was some DC leakage going on.
I finally decided it was time to see if I could address the hissy/noisey nature of this amp and see what effects some simple maintenance would do out of curiosity. I actually was considering changing out the aged LCR's with new ARS filter caps but I want to see what these changes would do first. I changed out the Erie bias supply caps (which also showed no signs of leakage or bulging) for new Sprague atom 10uf's and I found the V1A Erie 320uf cathode cap while not actively leaking electrolyte there were hints of a dried yellow substance on the end of the cap, I thought it might be leaking DC which contributes to noise and DC on the pots, so I changed it out for an NOS BC 330uf cap. This amp has a Lemco Mica 500pf cap on the tonestack but a factory installed 500pf small diameter ceramic disc bypass cap on the 470K mixer resistor for the bright channel. 1969 Superleads onward also could have the red Lemco dogbone ceramic caps in the tonestack and mixer bypass cap here as well as Lemco/RS micas and ceramic disc caps, it just depends on what Marshall had on hand when the amps were built. On my 69 Superlead build I was running an 560pf RS mica cap on the tonestack and a 560pf ceramic cap on the 470K mixer and the amp was more harsh than my 68 that has Lemco mica caps in both positions, after I changed out that ceramic for another RS 560pf mica cap the amp sounded like my 68 and was no longer harsh. So I thought that the ceramic cap here in the 72 probably was contributing to its harsh qualities as well. I changed out the ceramic 500pf mixer bypass cap for an old CDE 560pf silver mica cap. Old ceramic caps are also known to leak DC causing noise. So here is the complete list of items changed on the 72.
10uf bias supply caps
320uf V1A cathode bypass cap
500pf ceramic cap
The results were amazing, I was actually pretty shocked...The amp is no longer the hiss queen that it was, and it seems any gain related oscillations and preamp tube plate ring that were there before the changes are now dampened quite nicely plus the amp is no longer the harsh metal panel that it was , it sounds more pleasing and in line with my 68 plexi and 69SL. So if you have an old Metal panel or later Marshall that's harsh you might want to see what type of mixer and tonestack caps it has as well as the age of the V1A cathode and bias supply caps. I also won't delay changing out the bias supply caps or extremely aged electrolytic cathode caps in the future, I always knew they should be changed at least every 10-30 years and the results of the experiment seems to prove the wisdom to be true. I did not test to see if these caps were leaking DC because I was changing them regardless, but with the great reduction in noise I have to assume there was some DC leakage going on.
