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KT66 in a Peavey 5150 combo

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  • #1

    KT66 in a Peavey 5150 combo

    I am having trouble biasing up a matched pair of JJ/Tesla KT66's in a Peavey 5150 60watt combo for a customer. Plate voltage is 475vdc. Current is running at 60mA's...way too hot! I have -53vdc bias voltage. I believe I am tapped out for this bias voltage doubler supply from this amplifier. I need to at least get to -60vdc bias voltage to tame the current. I Put the 6L6's back in and they are running at 38mA's with the - 53vdc. Reasonable, right? I have 20vdc on the postive side of C65. Been changing out R12 and R114 resistors but the best I can get is -53v bias
    Any ideas?



















    e grids
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  • #2
    get some real kt66's that aren't kt88's or 6550's with a kt66 label.

    Seriously though- unless they're the russian gold lion, chinese sino or some kind of original tube like a GEC they're probably not going to bias up like true KT66's. The screen voltage may well be too high to allow them to bias correctly.

    You could try lowering the voltage to the screens- add a dropping resistor.

    My other question would be why? The 5150 doesn't exactly strike me as an "output distortion" kind of amp. Why not stick some nice high power 6L6's in there and let it be? Save the KT66's for a jtm45 or other classic output distortion kind of amp.

    jamie

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    • #3
      Have you considered the additional demand on the heater supply of KT66, compared to 6L6GC?
      My band:- http://www.youtube.com/user/RedwingBand

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      • #4
        Doesn't sound like he has a low emission problem.
        Education is what you're left with after you have forgotten what you have learned.

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        • #5
          I soldered in a 50k drop resistor in series with the screen resistor and my bias current went down. My plate voltage is 500vdc and now my screen voltage came down from 500 down to 485vdc. Now I am in range to use my bias pot!. I have decided to not use the KT66's. My filament voltage was teetering around 5.8 - 6.0 volts and thought it wouldn't be reliable and possibly hurt my power xfmr. I did throw some EL 34's in there after jumping pins 1 & 8 and it sounded real nice and dynamic, but the 6L6's still had more beef and bottom end.

          Thanks "IMARADIOSTAR" for your advise and everyone else for your thoughts!
          You guys are great and am going to use this site more often.

          Have a Great Weekend!!
          KE7VHN

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          • #6
            EL34 have a higher heater demand than 6L6GC also.
            See
            electron Tube Data sheets - Search
            My band:- http://www.youtube.com/user/RedwingBand

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