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Marshall JMP 100w fuse questions

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  • #1

    Marshall JMP 100w fuse questions

    Hi guys!

    I have a 60's vintage Marshall JMP Super Tremolo 100 watt head on my bench that came in with a blown HT fuse and an EL 34 that is running away. trouble is, there is no indication of what the correct values are on the fuses, and I cannot find an accurate schematic. The closest I come to it is this 1959T drawing, but it's not exact.

    http://www.drtube.com/schematics/marshall/1959t-66.gif

    Three problems:

    1. There was a blown 1 amp in the HT socket when I got it. This schematic doesn't even show this fuse, which is wired in line with the standby switch.

    2. There was a 5 amp slo blo in the mains socket. This schematic shows a 2 amp, which I don't really trust, and other 100 watt EL34 Marshall schematics I've seen show a 4 amp.

    3. The 1 amp slo blo fuse shown in this schem between the PT secondary CT to ground is there in a little fuse block, but it is a 10 amp, and there is a little sticker on the chassis that says 10 amp/50V.

    I can't give this back to the studio until I am sure what is the correct values for these fuses. What to do?
    It's weird, because it WAS working fine.....
    Tags: None
  • #2
    http://bmamps.com/Schematics/marshal...100w_1959t.pdf

    Try that, a different version of the 1959t. It shows the fuses.

    You have a 100 watt Marshall. I see no reason the fuses would be different in value from all the other 100 watt Marshalls. 4A mains and 1A HT.

    10A 50v? Sounds more like a heater fuse.


    How many fuse holders you got? Two on the rear panel? ANy others?


    I suspect the 2A mains fuse on your schematic is for 240v operation.
    Education is what you're left with after you have forgotten what you have learned.

    Comment

    • #3
      Hi Enzo!

      Nope, it's not that version either. There's two fuses on the back labeled Mains and HT, and one fuse block type inside the chassis coming from the PT and going to ground. Heater fuse makes sense, but doesn't jive with the schematic. Lots of inconsistencies here.

      But, I am happy to go with your take on this Enzo, as always. Thanks!
      It's weird, because it WAS working fine.....

      Comment

      • #4
        Only the Canadian exports had the extra fuses for heaters, bias etc.
        Was it modded or does the extra fuse look stock?

        The original Marshall schematics are everywhere, I assume that's what Enzo posted so I would go with that.

        Comment

        • #5
          This looks very, very clean. So, I'd say it is stock. So far I have not found a schematic that fits this model exactly. It's an oddball. If you know of one drewl, I'd be much obliged!
          It's weird, because it WAS working fine.....

          Comment

          • #6
            Post some pictures, maybe we can identify any irregularities.

            These old amps are simple but great.
            There's a good chance that a 40yr old amp has been modded at some point.

            Comment

            • #7
              The thing is like drewl said, Canada requires all the transformer secondaries to be fused. The same amp in the USA will have wires across where those fuses went. The schematic won't show those Canadian market fuses.

              From your description, I didn;t think the schematic I linked was a match either, but it showed fuses and ratings. Minor variations in amp occur all the time, and often are not documented.

              You want to know what that 10A fuse does? Remove it and then see what stopped working. None of this is deep mystery.

              There are many amps out there with no schematic to match. In a case likie this, the amp circuits are close enough you could service them. The power supply, even if different is a basic circuit and simple enough to trace out if need be. And if they move the high voltage fuse from the center tap ground connection to a place after the standby switch, well that is easy enough to spot and understand. In your schematic, the HV fuse was in the center tap ground. Works out the same, interrupt the current path anywhere and the circuit is no longer complete.
              Education is what you're left with after you have forgotten what you have learned.

              Comment

              • #8
                Mains fuse is 4A slow, HT is 1A slow.
                Now the internal one, which CT is it connected to, HV winding or heater CT?
                How is the rear panel HT fuse wired, in series with B+?
                Looking at the various drawings, there are 2 methods for HT fuse. One method has a 1A fuse connecting HV CT to ground. This version also has a choke.
                The second method has 1A fuse in series with B+. In second type HV CT goes to the junction of the totem pole 1st filter node, not to ground.

                I would guess your 10A fuse is going to CT of heater winding, probably for CSA version. Does it have a CSA or Ontario hydro sticker on it?
                Originally posted by Enzo
                I have a sign in my shop that says, "Never think up reasons not to check something."


                ​

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