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Cathode voltage 6L6gc query

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  • Cathode voltage 6L6gc query

    Hi folks

    With pairs of tubes, should the cathode voltages be equal?

    see the attached schematic section (Proamp viper 60/kmd gv60), I have different voltages at the output of T3 and T4 which goes to my cathodes of my two 6L6s. One is 4v greater than the other (63.5v and 59v).

    I am scratching my head over this and wondering whether it points to a failed component elsewhere (if not one of the TIP41c Ts).

    any help here?

    cheers

    Steve Click image for larger version

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  • #2
    The output stage uses transistors and tubes wired in a series arrangement as cascodes.

    The actual tube bias voltage (Vgk) is given as grid bias (+21V) minus cathode voltage.

    Different cathode voltages not necessarily mean different idle currents with this circuit, as the 2 tubes may need somewhat different bias voltages for same idle current.
    What happens to cathode voltages when you swap power tubes?

    You can find actual idle currents from the voltage drops across R69/70 divided by 3.9.

    Apart from that, a difference of less than 10% is rather insignificant.
    - Own Opinions Only -

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    • #3
      No, completely normal.
      Tubes are not matched within 1% by any means but show the normal spread; both are passing the exact same current, by definition, (drive transistors FORCE them to) and they develop needed grid to cathode voltage to accomplish that.

      In fact I would be surprised to find them the same.

      FWIW at that very low idle current (which is fine, thatīs the way it works); they are getting:

      * top one is getting +21V (grid) -63.5V (cathode)=-42V net bias

      * bottom one is getting +21V (grid) -59V (cathode)=-38V net bias

      You sure those are 6L6?
      I mean, maybe somebody used them, but those Bias voltages are definitely in the EL34 field.

      6L6 would demand around -52V .... unless those 6L6 are in coma and the Music Man style circuit is squeezing down to the last electron out of those worn cathodes.
      Juan Manuel Fahey

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      • #4
        BTW, it should be possible to balance power tubes via VR9. Adjust for minimum output hum. This typically means equal tube idle currents.
        - Own Opinions Only -

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        • #5
          Also check that R77 and R78 are both in spec.
          "Everything is better with a tube. I have a customer with an all-tube pacemaker. His heartbeat is steady, reassuring and dependable, not like a modern heartbeat. And if it goes wrong he can fix it himself. You can't do that with SMD." - Mick Bailey

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          • #6
            Originally posted by g1 View Post
            Also check that R77 and R78 are both in spec.
            yes, these are 470r as per spec. The tubes were new JJs.

            Unfortunately I have just taken a stupidity pill. As I was testing plate voltage, I forgot to put the insulated cover on my DMM probe and touched pin 4. The (new!!) tube red plated so I switched off.

            I think I have fried T4. Permission to say “cock”!

            Is there anything else I am likely to have killed?

            p.s, anyone want to buy a matched pair of 6L6 tubes - less than an hour’s use!

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            • #7
              “As I was testing plate voltage, I forgot to put the insulated cover on my DMM probe and touched pin 4. The (new!!) tube red plated so I switched off. ”

              Oh dear, that’s unfortunate, with most amps that wouldn’t be a problem. The associated cascode transistor may have taken a hit.
              I find it’s much better (less prone to issues and errors) to measure the HT, rather than the anode, voltage.
              My band:- http://www.youtube.com/user/RedwingBand

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              • #8
                What’s so stupid of me is that I had already tested it once!

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