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Bugs in a scratch built JTM45

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  • #61
    To run EL34's you will probably need to go to the 15K. With the resistor you have now your range is from about 8watts to 20 watts. You shouldn't need to go that cold (8W) so see what kind of range you get with the 15K.
    Also note the B+ (pin3) is coming down as the bias is increased. The high voltage issue was partly due to the amp being biased so cold.

    Edit: Or you could leave the 23K in there as MWJB suggested, and then see what kind of range you get with EL34's.
    Last edited by g1; 03-16-2013, 08:05 PM.
    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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    • #62
      Originally posted by mort View Post
      The loud hum was cured by swapping the OT secondary wires around.
      That's what I thought when I first looked at this thread because the same thing happened to me when I fired up mine. Set the phasing on the transformer per the color coded wires and BRRRRRRRRR! lol Somebody wasn't testing their stuff...
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      • #63
        Originally posted by guitician View Post
        Set the phasing on the transformer per the color coded wires and BRRRRRRRRR! lol Somebody wasn't testing their stuff...
        There is a note about this on the Weber schematic. Sometimes the OT leads need to be reversed. It is a common issue that can come up whenever installing OT's in tube amps, so good to take note!
        Click image for larger version

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        Originally posted by Enzo
        I have a sign in my shop that says, "Never think up reasons not to check something."


        Comment


        • #64
          Originally posted by g-one View Post
          There is a note about this on the Weber schematic. Sometimes the OT leads need to be reversed. It is a common issue that can come up whenever installing OT's in tube amps, so good to take note!
          [ATTACH=CONFIG]22446[/ATTACH]
          I certainly got it now!

          I was noticing on that schematic [and on the weber layout], they don't show a cap on the diode in the bias supply. My bias was all wonky without it. Is that an error on the drawings?
          ~Semi-No0b Hobbyist~

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          • #65
            Yes, it appears to be an error in the drawings. Seems to be a fair amount of online discussion about bias problems with this particular kit. Yet no mention of the missing cap anywhere. You were lucky MWJB caught it.
            Originally posted by Enzo
            I have a sign in my shop that says, "Never think up reasons not to check something."


            Comment


            • #66
              Originally posted by g-one View Post
              There is a note about this on the Weber schematic. Sometimes the OT leads need to be reversed. It is a common issue that can come up whenever installing OT's in tube amps, so good to take note!
              [ATTACH=CONFIG]22446[/ATTACH]
              The issue is that color coding the Blue/Brown wires is worthless if they don't test for phasing primary-to-secondary when making the transformer.
              Now Trending: China has found a way to turn stupidity into money!

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