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Switching bias on a bias-vary trem 6L6 Fender (e.g., 6G16, 6G11/A, etc.)

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  • Switching bias on a bias-vary trem 6L6 Fender (e.g., 6G16, 6G11/A, etc.)

    Hi all,

    I'm semi-interested in a 6G16 brown Vibroverb build. I've done a Super Reverb AB763 build about a dozen or so years ago so not a complete stranger to Fender builds.

    But, I am learning that one of the big issues with bias-vary trem for the 6L6 based amps (e.g., 6G16, 6G11/A, etc.) is that the amp is either biased to have a great tone (hotter bias) or great trem (colder bias).

    IOW, I can't "have my cake and eat it too", apparently... or can I?

    Stupid idea/question but I was thinking to have a switch (DPDT) to toggle between cold and hot bias settings. Then, I could have my cake and eat it too?

    I would most certainly be interested as to why this would be stupid/wouldn't work.

    Thank you in advance.​

  • #2
    I'm no expert by any means, so wait till one of the OG's chime in. But it doesn't seem like a good idea. There's that split second that there would be no bias when switching. Also, what If on the odd chance the switch fails, or is stuck in the middle. I think you could get away with it on a potentiometer though. Set the adjustment window narrow enough you can't swing it too far in either direction and mount it externally for on the fly adjustment. I'm willing to bet that there's a sweet spot in the bias where the trem is good and isn't so cold it's getting crossover distortion.


    • #3
      I was assuming I wouldn't be hot switching between the bias.

      Although, I wonder if using a shorting type switch would mean there's always a bias of some kind...?


      • #4
        I think the generalization that hotter bias sounds better is highly exaggerated if not a myth. Have you found it to be true with your own experimentation? Classic Fender amps (and many others) that are considered gold standard of tone were all biased quite cold. Many amps biased 50% or even lower do not have noticeable crossover distortion or objectionable tone degradation.
        Originally posted by Enzo
        I have a sign in my shop that says, "Never think up reasons not to check something."


        • #5
          seven, I'd still be leery. But I've seen several amps that have a switch that changes the bias for different tube types. The Carvin x100b for example, you could swap between 6l6 and el34s. I mean if you wired it so that there's always a safe minimum bias amount and then have your switch. The switch would have a failsafe, so in my mind seems reasonably safe.

          I'd experiment with it on a pot first. figure out where it sounds good on the trem and then for the optimal bias for your tube type. You can measure the resistance on the pot and it'll give you an idea what value resistors to use.

          I used that method to fix the bias on a few amps that were non-adjustable. The owner was a goofball and didn't want to "change his amp" by installing a pot on the pcb. So figured out the value, did some math and stuck one in parallel with the original bias resistor to get it cooled off. Blues Jr's love running el84 on thr ragged edge of meltdown hahaha.

          Edited, 100% what @g1 said. Run your tubes for what the data sheets safe minimum operation says...