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6G6G power tubes in a 5F2A ??

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  • 6G6G power tubes in a 5F2A ??

    I'm thinking that if I sub a pair of NOS RCA 6G6G's, dial down the bias, and use one of the other speaker taps (I have 2,4,and 8 ohm) to better match the impedance, I can get a lower volume with that nice crunch. And I probably should use a 5Y3 rectifier to lower the B+ some.

    My question is which output tap would be the best match, will the plates handle 420v, or is this unreasonable, and will I damage anything in my attempt to attenuate this beast??
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    Last edited by airweld; 06-09-2009, 05:09 AM.

  • #2
    Correction: 6G6G tubes in a 5F6A Bassman

    I was always told, " There's no such thing as a stupid question", however in this case we may have proven them wrong. I hope this makes more sense. I was wondering why no one responded.....maybe ??
    Last edited by airweld; 06-11-2009, 02:30 AM.

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    • #3
      Originally posted by airweld View Post
      will the plates handle 420v, or is this unreasonable?
      (Well the datasheet says 300, but that is only the vaguest of guides) the max recommended plate dissipation is 2.75W, so if you were going to try running them at 420V, the plate current would need to be 6.5mA to achieve this, which is stuff all, meaning you need a higher value cathode resistor for cathode bias or more negative grid voltage for fixed bias. Also it say 12k load resistance, so you could probably get away with 8k for PP (at a guess). As to whether the plates would take 420 V, well you could always hook them up and see if there was any flashes within the envelope (but be prepared to switch it off quick-smart) Was that any help?
      Building a better world (one tube amp at a time)

      "I have never had to invoke a formula to fight oscillation in a guitar amp."- Enzo

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      • #4
        Thanks tubeswell for your thoughtful response. I have a tube tester and when I get the new tubes (should have been here by now?) I will see if they can hold up to the higher voltages. The tester uses a variac to apply voltage to the plates. The heaters use a separate voltage source.

        My OT in the 5F6A is a Hammond 1750M 4200 Ohm input with 2, 4, and 8 Ohm out. I'm not clear on which output would best match (with 8 Ohm speaker) the 12k of the 6G6G RCA tubes. Do you think -50v grid will be enough to bias properly? I only have two of these tubes so I hope I don't ruin one in the process of getting them to work.

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        • #5
          Originally posted by airweld View Post
          TDo you think -50v grid will be enough to bias properly? I only have two of these tubes so I hope I don't ruin one in the process of getting them to work.
          You only need about -6V to -12V or so according to the datasheet
          Building a better world (one tube amp at a time)

          "I have never had to invoke a formula to fight oscillation in a guitar amp."- Enzo

          Comment


          • #6
            Finally got the tubes and they seem to handle 420v in the tester so I put in a 5Y3 rectifier and with output to 8ohm speakers, the resulting plate voltage was 460(!) with plate current as low as 1ma there was a red glow from inside the plate (screen, I assume) when a chord was struck. Sounded really nice but I guess I'll have to take more measures to lower B+ voltage. I have ordered some zener diodes and will try that. I was hoping it would be easier to put these tubes into the 5F6A but it sounds like it may be worth the effort.

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            • #7
              Sounds like they are biased way to hot. The datasheet rates them as fairly low-powered tubes
              Building a better world (one tube amp at a time)

              "I have never had to invoke a formula to fight oscillation in a guitar amp."- Enzo

              Comment


              • #8
                When I saw the 460v and red glow I yanked them out, but I could try to see how low I can bias them and maybe keep them from going out in a blaze of glory. I am thinking correctly aren't I? Lowering the plate current will bias them colder (but will raise the plate voltage also?).

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                • #9
                  Originally posted by airweld View Post
                  I am thinking correctly aren't I? Lowering the plate current will bias them colder (but will raise the plate voltage also?).
                  Yep
                  Building a better world (one tube amp at a time)

                  "I have never had to invoke a formula to fight oscillation in a guitar amp."- Enzo

                  Comment

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