NPMAD

ivan H

ivan H

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I've owned a few nice amps that I really regret getting rid of over the decades, but my "grail" amp has always been elusive,,, up until now.
I've just score a 1968 (Plexi) Marshall JMP50 head in good condition, factory applied red lacquer still on better than 90% of the solder joints, original transformers, filters have been replaced some time back with original types, fitted with a duet of 1965 Rohen Telefunken winged plate (heat sinks) 00 getter EL34's & 3 Blackburn Mullard CV4004 12AX7's. I have a good many NOS spare parts for this amp, should they ever be needed, but we'll get back to that later. Here's pics16111860603691888413939333094082.png16111859451616712289963397231470.png16111860897845913628991049130156.jpg16111861574541971568650364284240.png16111861431868661433138061612938.jpg16111861740737743417092215546224.png16111862072616541940578260286709.png1611186329041495901054049508901.png16111863038857483814635568879401.png
I'm almost at my pic posting limit so more to come in another post. Cheers
 
Obviously plexi amps don't come up for sale all that often (especially here in Oz) & when they do it's not uncommon to find that someone has been inside & performed some "non stock" mods to the circuit. This one has, at some time had a loop or out/in arrangement added, since removed & put back to stock, but the evidence of can be seen in the next two pics16111862337311530743626178420350.png16111862791614368280891527499405.png
The only part that I'm unhappy about is that SOME D_ _ _WART (that is obviously hopeless with a soldering iron) has melted into the insulating material on the 0.68uf (Philips Chicklet) V2a cathode bypass capacitor. We'll come back to this but in the meantime, here's a couple more pics16111863582217499488474885846012.png16111863836052475391727016398570.png
These valves test good for emission, but I do have NOS valves for it should they be needed. These consist a matching date code & factory matched duet of the now very hard to find 1962 Blackburn Mullard double halo getter xf2 EL34's, the output valves that would have been originally fitted to this amp by Marshall in 1968. I also have 163 series Blackburn Mullard 12AX7's for all 3 positions.
Back to the soldering iron marked V2a cathode bypass capacitor. I do have NOS Chicklets & also C296 mustards in the correct value & voltage rating, but as this one is still functioning properly, & I do not want to disturb the lacquer on the solder joints I'll leave it as is.
I also have a couple of complete sets of NOS Philips Mullard C296 mustard capacitors for this amp, should any of them ever go leaky. Cheers
 
Thanks for the kind words guys. Yes, the IEC AC connector is an addition. They either had a power cord or Bulgin connector from Marshall. I'll look for the correct Bulgin item.
Thanks for the input regarding year of manufacture @67plexi . As we know, the most consistent thing about Marshall around this time was their inconsistency, so properly dating amps can be a challenge. I know it's no indication, but serial No. is 11225.
This amp was purchased in England & then brought out to Australia by the previous owner. Since being in Australia it has been serviced & maintained by Marshall Australia. No, I haven't played through it yet, but the amp was given a clean bill of health by Marshall Australia at the time it was offered for sale just recently, & was sold through a dealership so I'm not really worried in that regard. It is being couriered to me at the moment. Yes, their is a return policy.
I tried to get a decent pic of the NOS duet of Valvo branded 1962 Mullard xf2 00 getter EL34's that I have to install if neededIMG_20210121_140039.jpg
The date code of B2J3 on both of these interprets as
B = manufactured at Blackburn plant
2 = 1962
J = October
3 = 3rd week of month
These emission test off the charts.
The 00 (double halo) getters can be seen up inside the silver "getter flash" part of the valves. I also have a few NOS duets of 70's era "single halo getter" xf2 Blackburn Mullard EL34's.
I'm sure many here know this, but 50 watt Marshalls from this era run the EL34's without the use of screen grid resistors or grid stopper resistors.
While "old production" EL34's like the Telefunkens that are in it or the xf2 Mullards that were original equipment in these amps (or many other quality old production EL34's) can easily cope with the stress of being run without screen grid resistors, new production types simply cannot & will fail in short order. I do have Iskra & Piher resistors suitable for use as grid stoppers & will chase up some (matched) 1k/5watt Ohmite screen grid resistors in case I ever decide to fit them. Cheers
 
syscokid said:
View attachment 56786

What is this cap for?
Click to expand...
These caps were fitted to the JTM45's, JTM50's & also are found in early JMP50's. If we look to a JTM45 schematic16112022990291999975283384733386.png
we see the 0.05uf cap from B+ (between rectifier valve & the standby switch) to ground.
If we look at a 67/68 JMP50 gutshot (Marstran technical info) 1611202471537162435805896459956.png
we see the same cap mounted between the HT fuse & ground, & again, this time from a 67 JMP50 gutshot (Marstran technical info)1611202737518655344233786967067.png
we the same 0.05uf cap. This cap helps with standby switch pops etc. It is also effectively paralleling the main filter to ground. Note the Bulgin AC socket in both these pics, I will chase a Bulgin socket & plug up.
Also from the Marstran technical info page (same '67 JMP50 amp as in the last pic)16112027833202377052304300274874.png
Note the voltage & impedance selectors, giving credence to @67plexi suggesting that my new purchase may be a 67 rather than a 68. Cheers
 
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