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Marshall JTM60 screen supply resistor

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  • #1

    Marshall JTM60 screen supply resistor

    I have a JTM60 in for a couple of things, and I find some screen resistors have been replaced. R109 has been replaced, and measures 10K, good. The 10K at R108, which supplies plate voltage for the preamp has some kluge soldered across it to measure 275 ohms, clipped that. And the screen supply R113 has been replaced with a 270 ohm that is a bit discolored in places, but doesn't look like from heat actually, but more of a stain from something. Anyway, I can't read the value on the schematic. Does 270 ohms seem like a reasonable value here?

    https://www.thetubestore.com/lib/the...-Schematic.pdf

    edit, oh, and R113 had one of it's legs not soldered, so I'm thinking it wasn't getting good or reliable screen voltages.
    It's weird, because it WAS working fine.....
    Tags: None
  • #2
    I found a clean JCM600 schematic that looks a lot like the JTM60, in the power section at least, and it shows 270R 5 watt at R113, so I'm going to go with that.
    Last edited by Randall; 10-23-2018, 01:14 AM.
    It's weird, because it WAS working fine.....

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    • #3
      The copy I have shows 1K ohms.
      Experience is something you get, just after you really needed it.

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      • #4
        1K 5W are the screen resistors, the one I was inquiring about is the one that feeds B+ to them.
        It's weird, because it WAS working fine.....

        Comment

        • #5
          Someone has fixed the drawing and shows 270R here: https://music-electronics-forum.com/...0&d=1461914616
          Also mentioned as 270R (in German) here: https://www.tube-town.de/ttforum/ind...?topic=12092.0
          Originally posted by Enzo
          I have a sign in my shop that says, "Never think up reasons not to check something."


          ​

          Comment

          • #6
            "Also mentioned as 270R (in German) here: https://www.tube-town.de/ttforum/ind...?topic=12092.0"

            Remind me to never speak German around you!
            It's weird, because it WAS working fine.....

            Comment

            • #7
              Originally posted by Randall View Post
              1K 5W are the screen resistors, the one I was inquiring about is the one that feeds B+ to them.
              Dang! Then 270 5W is right, but I see you already found that out.
              Experience is something you get, just after you really needed it.

              Comment

              • #8
                Originally posted by Randall View Post
                "Also mentioned as 270R (in German) here: https://www.tube-town.de/ttforum/ind...?topic=12092.0"

                Remind me to never speak German around you!
                I can only read German numbers, google translate does the rest.
                Originally posted by Enzo
                I have a sign in my shop that says, "Never think up reasons not to check something."


                ​

                Comment

                • #9
                  Originally posted by Randall View Post
                  "Also mentioned as 270R (in German) here: https://www.tube-town.de/ttforum/ind...?topic=12092.0"

                  Remind me to never speak German around you!
                  What's the problem?
                  - Own Opinions Only -

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