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  • #16
    Originally posted by dmartn149 View Post
    Okay, It sounds pretty good now, maybe still just a little bit over cooked with my guitar all the way up. I haven't decided yet. If I was going to bring the gain down a little more, would I need MORE resistance between the cathode and ground?
    It's a complex problem that has no simple answer. Because of the common cathode, options are limited.

    I need to know the cathode, plate and B+ voltages to see if the biasing is optimal. Simply changing the current won't change the gain that much, only about 20%. I'm thinking the problem is limited headroom because of poor biasing, not too much gain. The best option might be to chage to a different tube type.
    WARNING! Musical Instrument amplifiers contain lethal voltages and can retain them even when unplugged. Refer service to qualified personnel.
    REMEMBER: Everybody knows that smokin' ain't allowed in school !

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    • #17
      Okay, so I'd measure cathode v at pin 6 and plate v at pin 5. Would I measure B+ at the other end of that 220k resistor? (the end away from the tube)

      Here is part of the schem showing the changes so far. The grid stoppers are 47k and the cathode resistor and cap are 2.2k and 22uf . Not exactly the values I was looking for, but I was trying to make due with what they had at my local Cel Phone ShackClick image for larger version

Name:	schem 002 rev.JPG
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ID:	825540
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      • #18
        Originally posted by dmartn149 View Post
        Okay, so I'd measure cathode v at pin 6 and plate v at pin 5. Would I measure B+ at the other end of that 220k resistor? (the end away from the tube)
        Yes, it says 295V there now but that voltage may have changed.
        WARNING! Musical Instrument amplifiers contain lethal voltages and can retain them even when unplugged. Refer service to qualified personnel.
        REMEMBER: Everybody knows that smokin' ain't allowed in school !

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        • #19
          I'm measuring 2v at the cathode, 165v at the plate and B+ is 287v. Are these okay?
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          • #20
            I drew a load line on the curves in an old RCA data book. The bias looks about as good as it could be. Lowering the plate resistor will lower the gain but also reduce the headroom on the plate. That must be one hot guitar. Since the cathodes are tied internally, you can't add cathode degeneration because that will inject an out of phase signal into the other channel. So to improve headroom on the input you would have to re-wire for a different tube or add an attenuator on the input, in effect another volume control between the guitar and the tube.
            WARNING! Musical Instrument amplifiers contain lethal voltages and can retain them even when unplugged. Refer service to qualified personnel.
            REMEMBER: Everybody knows that smokin' ain't allowed in school !

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            • #21
              Well the amp isn't really distorted, just not super clean, and over all it sounds pretty darned good, and really loud too.
              I sure do appreciate all your help and advice. It's very unlikely that I would have ever gotten this amp working right on my own.
              Thank you very much Loudthud
              Vote like your future depends on it.

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