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Biasing a '64 Vibroverb Custom

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  • #16
    To revive this thread: Can anyone tell me if the Bias spec for a Fender ‘64 Vibroverb AB763 circuit is the same as the ‘64 Vibroverb Custom (reissue) spec (38mA on the 6L6GC PT cathodes at 455 VDC on the plates)? I have the schematics for both, but my AB763 schematic doesn’t specify the bias requirement.

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    • #17
      Not trying to be flippant or dismissive, but check post #2! Personally, I think whatever sounds best that doesn't kill tubes or other parts IS best. But you have to do the math. I just leave a tiny bit of wiggle room for myself, frequently aiming on the hotter side. There's a maximum amount lf heat that a tube can dissipate, and anything below "self-destruct" is fair game. Also, both schematics specify +/-20%! That's a pretty wide margin of error.

      If you tell us all your measurements and how you're getting them, we CAN help you find out if you're in "safe" range. But whether it's "correct" or not? Anything goes.

      Justin
      "Wow it's red! That doesn't look like the standard Marshall red. It's more like hooker lipstick/clown nose/poodle pecker red." - Chuck H. -
      "Of course that means playing **LOUD** , best but useless solution to modern sissy snowflake players." - J.M. Fahey -
      "All I ever managed to do with that amp was... kill small rodents within a 50 yard radius of my practice building." - Tone Meister -

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      • #18
        Justin, I don't take your response as flippant or dismissive, but I did read post #2. I just was curious as to what the Fender spec was for the AB763 circuit, as a general starting point. By my understanding (and please correct me if I'm wrong), I was under the belief that 70% of max plate dissipation was the general biasing target (or at least one of the methods used for biasing). But with the '64 reissue, if my math is correct, 38 mA produces 17.3 watts at 455 VDC plate voltage, or 57.6% of the max 30 watt plate dissipation, meaning that the Fender spec is kinda cold. Again, if my math is correct, 70% would give you a bias setting of 46 mA. -Just looking to improve my overall understanding of biasing.

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        • #19
          Fender has always set there bias low.

          Just where the '70%' thing got started, I don't have a clue.

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          • #20
            Originally posted by ewd View Post
            Justin, I don't take your response as flippant or dismissive, but I did read post #2. I just was curious as to what the Fender spec was for the AB763 circuit, as a general starting point. By my understanding (and please correct me if I'm wrong), I was under the belief that 70% of max plate dissipation was the general biasing target (or at least one of the methods used for biasing). But with the '64 reissue, if my math is correct, 38 mA produces 17.3 watts at 455 VDC plate voltage, or 57.6% of the max 30 watt plate dissipation, meaning that the Fender spec is kinda cold. Again, if my math is correct, 70% would give you a bias setting of 46 mA. -Just looking to improve my overall understanding of biasing.
            Many 6L6 tubes available today struggle to meet the 30W dissipation spec. Safer to assume 25W, so 38mA sits perfectly with that at 455vdc. Anything over 25mA can't really be considered cold. You can hear the mA (say 20, or 30) much clearer than you can hear % dissipation, by that I mean that 30mA at 420vdc still sounds OK, as it does at 520vdc, despite being a very different % dissipation.

            You wouldn't typically bias a fixed bias Fender to 46mA per 6L6.

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            • #21
              Now that I think of it, somewhere between 30 & 40 seems to be the usual, for the reasons you describe - most 6L6s (I use JJ & SED) don't seem to handle the full 30W. Usually they'll start to redplate after half an hour at about 25W, and I dial them back a little bit more for drift and safety.

              Might be where the "70% Rule" came from - knowing the tubes can't handle the claimed dissipation.

              Justin
              "Wow it's red! That doesn't look like the standard Marshall red. It's more like hooker lipstick/clown nose/poodle pecker red." - Chuck H. -
              "Of course that means playing **LOUD** , best but useless solution to modern sissy snowflake players." - J.M. Fahey -
              "All I ever managed to do with that amp was... kill small rodents within a 50 yard radius of my practice building." - Tone Meister -

              Comment


              • #22
                Originally posted by Justin Thomas View Post
                Now that I think of it, somewhere between 30 & 40 seems to be the usual, for the reasons you describe - most 6L6s (I use JJ & SED) don't seem to handle the full 30W. Usually they'll start to redplate after half an hour at about 25W, and I dial them back a little bit more for drift and safety.

                Might be where the "70% Rule" came from - knowing the tubes can't handle the claimed dissipation.

                Justin
                I think the 70% suggestion, as a viable ceiling based on logic, traces back to Randall Aiken, following sine & square wave analysis. But fixed bias, A/B amps aren't normally biased anywhere near the max as the cooler idle & potentially larger signal swing is where the increased efficiency comes from over cathode bias.

                70% of 25W & 70% of 30W are still both "70%" ;-) But this is where there is potentially an issue, you have to know that the tube in question is realistically rated, rather than assume max dissipation.

                FWIW Fender only started listing "mA" with regards to bias in the last few years, previously BF P-P 6L6 amp schems just listed voltage and the C- (measured at the junction of the 220K grid loads) would typically be ~11% of the B+. I do believe that biasing by mA is more informative, but if you have a bias pot you may as well go by ear & use the figures to confirm you're not burning anything up. You bias to make the amp sound good principally, then fine tune to ensure realistic part life. Assuming a fixed parameter whether by voltage/%/mA somewhat defeats the object as some tubes in the same amp may have a different sweet spot & some applications require wildly different idle currents.

                As Jazz P Bass says, Fenders, especially the fixed bias blonde & brown tolex amps from the early 60's, were biased cooler than today's consensus, especially taking into account lower wall AC. Still go by ear.

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