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1960s era ampeg jet j-12-a has tremolo problem

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  • #16
    I suggested a 1 meg pot, not resistor. Wire the wiper to the grid resistors. That way you can dial down th intensity . As you found out, just raising that resistor does nothing. If the trem signal is too hot, it can run your grids into the bias voltage. I mean the bias level acts like a wall the thing hits.

    Have you scoped this? How hot is the trem output? And what is the cathode bias on the tubes?

    Education is what you're left with after you have forgotten what you have learned.

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    • #17
      They can be matched tubes yet still not matched in the amp circuit. You need to check idle current for each power tube socket.
      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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      • #18
        A leaky 0.02µ coupling cap or a missing screen voltage could cause an imbalance.
        Last edited by Helmholtz; 04-14-2022, 07:16 PM.
        - Own Opinions Only -

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        • #19
          Checked the idle current on the 7591 tubes - got 154ma on one and 165ma on the other. measured using a 1 ohm resistor on pin 5. Checked the voltages on all the screens and they are all there. The trem sine wave is about 9.5 volts pk/pk at the junction of the 680k resistor.

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          • #20
            Originally posted by lectricmandan View Post
            Checked the idle current on the 7591 tubes - got 154ma on one and 165ma on the other. measured using a 1 ohm resistor on pin 5. Checked the voltages on all the screens and they are all there. The trem sine wave is about 9.5 volts pk/pk at the junction of the 680k resistor.
            That sounds like massive current to me. Is that with no signal and no trem ?
            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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            • #21
              Originally posted by lectricmandan View Post
              Checked the idle current on the 7591 tubes - got 154ma on one and 165ma on the other. measured using a 1 ohm resistor on pin 5.
              Sure you really measured the voltage drop across the individual 1R resistors and not to ground?

              Something like 160mA per tube would melt the plates - or blow the fuse.
              - Own Opinions Only -

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              • #22
                I don't know what was wrong but that was the numbers I got, I think there was a problem with the resistors I used. So today I did it the real way with the fluke 87 in series with each one . At V3 I got 42.4 ma and at V4 I got 40.4 ma these numbers look a little better. I've never done this using resistors before and probably not do it again as I will do what I normally do . : )

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