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    JMP 50 circa 1973 Circuitry

    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...
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    JMP 50 circa 1973 Circuitry

    Thanks for the link. I already replaced my 250pf cap with a modern 500pf cap but I may pick up some of those.
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    JMP 50 circa 1973 Circuitry

    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...
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    JMP 50 circa 1973 Circuitry

    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.
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    JMP 50 circa 1973 Circuitry

    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...
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    JMP 50 circa 1973 Circuitry

    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...
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    JMP 50 circa 1973 Circuitry

    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...
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    Completed JohnH Attenuators?

    Thanks for your comment Gene. You are right, I should drill a few ventillation holes in it. Steve
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    Completed JohnH Attenuators?

    Yes, and it seems to be working properly. I have only used it a bit so far. I am going to check all of my connections one more time to make sure I didn't make an error or a poor solder joint. I'm contemplating attaching a large heat sink on the top as well. It got really warm after playing...
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    Completed JohnH Attenuators?

    There is only one box which has two separate internal compartments. It is a solid piece of cast aluminium with a divider shelf half way up. I suspect it was made this way to isolate different parts of the rf modulator circuitry that it contained. So my build has half of the circuitry stacked...
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    Completed JohnH Attenuators?

    Another M2 build. The case is a nice heavy aluminium box that housed aircraft electronics. I am going to use it with my 1987 JMP 50W (circa 1973), 18W clone, 5E3 clone, and a future 5881 homebuilt amp. Cosmetically, I am going to leave it as is. Cost: Resistors and heat sink pads cost about...
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    Western Electric considering adding more tubes to their current USA production

    From the Western Electric Website: "In 1938, Western Electric manufactured the first type 300B electron tube. It was a breakthrough power triode, but in those days it hummed away from behind a curtain in early Hollywood sound systems. The same things that served moviegoers back then—low...
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    Bad News Tubes: 3/11/2022

    I just shared by posting it in the 'Tube Audio' section over on the Audiokarma forum. I don't have enough posts here yet to share the link. For those who are interested the thread title that I started is: Western Electric Contemplating New Tube Production
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    WTB: 2045 Cabinet

    I have one of these which came with a JMP 50W head circa 1973. I purchased the set at a pawn shop in NE Quebec Canada about 15 years ago. Unfortunately I don't know their history. Given that one was identified in Australia, and I have one in Canada, it's possible that Marshall may have sold...

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