New Plexi JMP50 Day

ivan H

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I've always liked the '68 spec JMP50 amps, lusted after would probably be more correct. I've built clones of, the last with period correct iskra resistors, C296 mustards etc, but stayed on the lookout for the real deal. Finally got one in reasonably good condition, serial No. S/11225, not "pristine", but still has all original Iskra resistors, C296 mustards (Chicklet 0.68uf cathode bypass caps), RS silver mica's, factory applied lacquer/dye still on 90% of solder joints, original Drake iron (PT 1202-118, OT 784-139, Filter Choke 352-114).
Finding an amp of this type & vintage that has not been molested in some way isn't common. This one has at some time had some kind of out/in loop added, now removed & put back to stock. Also, some damn fool has scarred the V2a cathode bypass (Chicklet) cap with the soldering iron. I do have this cap in both the C296 mustard & Chicklet types that I could replace it with, however as it's still functional I'll leave as is so as not to disturb the lacquer/dye on the solder joints. The Bulgin AC socket has been replaced with an IEC type, but I've ordered the correct 3 prong Bulgin socket & plug to put back in. It has been re-capped at some time. Previous owner had it re-valved a while back with NOS Mullard CV4004 12AX7's & a matched duet of NOS 1965 Rohen Telefunken "00" getters winged plate EL34's. Here's pics
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I have NOS "spares" for this if ever needed that include a complete set of Philips Mullard C296 mustard caps & Philips Chicklet 0.68uf 250V cathode bypass caps & some Iskra resistors. NOS valves that I have include a matched duet of matching date code (B2J3, Blackburn plant, 1962, October, 3rd week) Blackburn Mullard "00" getters xf2 EL34's, (the EL34 type it would originally been fitted with), a few duets of the 70's era Blackburn Mullard single halo getter xf2 EL34's & a good few Blackburn Mullard 163 series 12AX7's. I'm working away a the moment so will have to wait for the weekend to give it a clean-up & once over & play through. Cheers
Edit:
I'll most likely be changing out the bias supply electrolytic capacitors as part of the clean-up/once over. Cheers
 
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ivan H

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Beautiful! You did well finding one that wasn't hacked up.
Thanks. I looked for a few years. Most I saw offered had some non stock components like PI coupling caps, resistors or the PT & most often they wanted more than I payed for this. Cheers
 

cooljuk

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That's a beautiful score! I know how it goes, waiting for the right one to come along at the right time and price. There's a couple 60's and 70's Marshalls I search for every day, like that. Gotta be just the right one to shell out like that.

So, now that you have your dream amp, want to sell any of your vintage parts for my NOS parts build, while I wait on my ship to come to harbor? :naughty::dunno:

If you get a chance to compare those CV4004s to your I63s, report back with your thoughts. I have some thoughts on how they compare in my Marshalls but I'm curious if those differences translate to other players and their amps.
 

Jon W

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So here's the $4500 question, since you've built a clone of this amp (or similar)... how do they compare?
 

J50315

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Congrats on the plexi! Enjoy it and play it loud.
 

el84ster

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I’d like to hear your comments on the comparison too. It’s one thing to try something out, but owning you can really get to understand the nuances.
 

cooljuk

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I’d guess that since comparing two originals is likely to give two different sounding amps that, if a clone sounded just like an amp bought later it would be extremely rare. If it sounded different is would also not say anything to the authenticity of the sound of the clone. ....but still, it would definitely be interesting.

I wonder, would I re-voice the clone to sound even more different, in that situation? Just for more variety. Cool spot to find yourself in, at least! What an amp!
 

el84ster

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In general, I find older amps to have more character than a modern recreation. I’m curious if the OP finds that’s true here.
 

ivan H

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Thanks guys, appreciated. My next thing to search out is a '68 era basketweave 1960A cab, & of course a matching numbers HRD Vincent Black Shadow,,, but I digress.
Comparing the clone to the real deal crossed my mind as soon as I got this amp. I have a Metropolous (Metroamp) Head Master switcher that will allow an easy "real time" side by side comparison. I just tried to upload a gutshot of the clone but it's telling me the file is too large. I'll have to figure a way around this (I'm "digitally challenged"). The clone is very close to this component wise, the only differences being C296 mustard V1b & V2a cathode bypass caps rather than Chicklets, RBS 560pf silver mica treble & treble peaking caps rather than RS 560pf silver mica's, Piher 1Meg resistors rather than Iskra, & of course Marstran iron rather than Drake.
Even though clones can get quite close, they often fall a few % short of the real thing. I have a few theories on why, but that's for another time. Still, a comparison will be interesting. Likewise, a comparison between the Mullard CV4004 & 163 series 12AX7's will also be interesting. Cheers
 
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bblooz

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What a cool find! I hope you have as much fun playing it as you did looking for it.......
 

Jon W

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My experience as well. Something about these old amps is special and more than the sum of the parts.
 

ivan H

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You need a '68 Custom to go along with that!
Funny you should say that, about 6 months or so back I had given though to a late '68 or early '69 Custom offered for sale here in Oz, seen here.
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Previously been owned by one of the founding members of Aussie rock band The Angels (Angel City in the USA). I was put off by the fact that he'd had the neck re-profiled to replicate the neck of his Epiphone Casino. Had I bought my available funds may not have yet recovered enough to buy this amp.

Regarding a comparison between the Mullard CV4004 & the 163 series 12AX7's, I should mention that the CV4004's in the amp are the ribbed plate types, not the later box plate types. Here's pics of one
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Cheers
 

ivan H

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Ok, on close inspection the main filter had started to swell the relief seal a little so I changed out the filter caps, including the two in the bias circuit. With the amp on standby I check for proper operation & full range of the bias supply. All good so next, condition filters. To do this I removed the HT fuse, then using two alligator clips, jumpered a 100k 1watt (current limiting) resistor across the two terminals of the fuse holder & powered up (no valves installed). Took 4 ~ 5 hours for the voltage drop across the resistor to get down to 4 volts. Done. Here's a pic with new filters installed & the 100k resistor jumpered across the HT fuse. Meter leads attached to the resistor are for monitoring the HT voltage (1 meter) & the voltage drop across the meter (another meter).
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This morning I'll perform a few more checks, then install the valves & bias it up. Cheers
Edit:
I'm still deciding whether or not to fit screen grid resistors & grid stopper resistors to the power tube sockets.
I know the old production EL34's can handle being run without screen grid resistors, but these valves are expensive & getting more rare all the time. Maybe take the precaution & make their working life easier?
As to the grid stoppers, I did an experiment with one of my clones once, first running it without grid stoppers, then with them (5k6) fitted. Measured plate current dropped by about 4mA per tube when the grid stoppers were fitted, showing that while not audible, there was some ultra-sonic oscillation happening. Cheers
 
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