Recent content by Chappy

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    What do we have here? (Kitchen-Marshall PA)

    Perhaps an early variant of the PA-20. https://www.thefretboard.co.uk/discussion/156230/fs-72-kitchen-marshall-pa20 https://www.worthpoint.com/worthopedia/kitchen-marshall-pa20-guitar-537673284
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    RAD

    Gene, Thanks for the comments. The speakers are Korean made Sammis, model ME/300B50. I acquired them about 15 years ago. Guys were raving about them on the 18Watt forums. IIRC one of the 18Watt forum members, Greg, acquired USA distribution rights. I was able to acquire 3 of them from him...
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    RAD

    How I spent half of my "stay at home" summer vacation, see pics below. Originally I was planning on restoring the pair and selling them to fund other smaller amp projects. The head is just so damn loud for home use, which is all that I will ever use it for. Then I was directed to this forum and...
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    RAD

    Refurbished Amp Day! How my vintage 73 1987 head and 2045 cab looked early last week after hours of stripping and sanding. When I acquired it all of the tolex was stripped and both were sloppily covered with varnish:(
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    Simple Attenuators - Design And Testing

    John, Actually it did come from Ebay. It is a US military AN/URM-90 test set. Built using top of the line components back in the late 50s. I had to wait quite a while for one to show up that was still in good condition. Fortunately I was the only bidder on this one, and I got it for a steal...
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    Simple Attenuators - Design And Testing

    Effect of Screw (ferrous content) on Inductance I was curious as to how much a normal machine screw placed through the centre of the coil would affect the inductance in my attenuator build. Results from my spare 0.91mH coil from Solen Canada measured on an LRC bridge. Air core only: 0.94mH...
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    '77 JMP 100W MKII Red Plating

    Check out the following schematic: https://www.thetubestore.com/lib/thetubestore/schematics/Marshall/Marshall-JCM800-Superlead-100W-1959A-Schematic.pdf hard to read but the resistors are there labelled 5K6 on the output tube grids. They are between the two screen resistors labelled 1K 5W. As...
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    JMP 50 circa 1973 Circuitry

    Very nice. The serial number on yours is only 16 digits higher than mine!
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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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