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Vibroverb RI biasing

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  • Vibroverb RI biasing

    Got a Vibroverb RI in for a bad PT, among other things. I replaced it, and now it works, but someone has modded the bias circuit to run 51mA at 427v for 21.7 watts. That's just about 70% max. but seeing it is a bias wiggle circuit, current will swing up much higher than that, correct? Possibly being the cause of PT failure? It had the 33K R59 jumpered for -42v C-. The stock 33K R59 supplies -54v C- for 13mA at 450v = 5.8 watts. Seems more than a bit low. What is a good place to set these at? Or should I leave factory set good enough alone? It does sound good like this, what little I have strummed thru it.

    http://www.prowessamplifiers.com/sch...face_2x10.html
    It's weird, because it WAS working fine.....

  • #2
    IMO, if the factory/designer thought it was good enough, then that's good enough.
    "I took a photo of my ohm meter... It didn't help." Enzo 8/20/22

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    • #3
      A bias wiggler does make it run hotter and cooler several times a second, yes, but as far as the amp is concerned it averages out. SO the fact that there are a few momentary current peaks each second, it really doesn't strain anything.

      I'm with Dude here, the stock circuit works. If someone made it idle at 70%, well, that whole 70% thing is just internet lore.
      Education is what you're left with after you have forgotten what you have learned.

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      • #4
        If you think about it, the greatest amount of signal modulation by the trem should be available if biased at midpoint (50%). I'm pretty sure I've read here reports of the trem sounding poorer if not biased fairly cold.
        Originally posted by Enzo
        I have a sign in my shop that says, "Never think up reasons not to check something."


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        • #5
          Originally posted by g1 View Post
          If you think about it, the greatest amount of signal modulation by the trem should be available if biased at midpoint (50%). I'm pretty sure I've read here reports of the trem sounding poorer if not biased fairly cold.
          Hmmm.?. I built an amp using that trem. It does run a higher plate voltage and it uses el34's so there's less bias voltage (or, rather, higher wiggle to bias voltage ratio). It's currently biased at about 70% because there's a little garble sound in the trem that get's worse if it's biased colder. I'm not sure but I think it's a result of the bias being driven too far negative.
          "Take two placebos, works twice as well." Enzo

          "Now get off my lawn with your silicooties and boom-chucka speakers and computers masquerading as amplifiers" Justin Thomas

          "If you're not interested in opinions and the experience of others, why even start a thread?
          You can't just expect consent." Helmholtz

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          • #6
            It's factory biased at 20% now. I am still wondering why they did this.
            It's weird, because it WAS working fine.....

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            • #7
              Originally posted by Chuck H View Post

              Hmmm.?. I built an amp using that trem. It does run a higher plate voltage and it uses el34's so there's less bias voltage (or, rather, higher wiggle to bias voltage ratio). It's currently biased at about 70% because there's a little garble sound in the trem that get's worse if it's biased colder. I'm not sure but I think it's a result of the bias being driven too far negative.
              Yes, if I really think about it , I was mixing up mid-point of the linear section of transfer characteristic; and mid-point of plate dissipation. Not the same thing.
              Even if you have more room for wiggle, you will be introducing crossover distortion and might that be the garble you hear?
              Originally posted by Enzo
              I have a sign in my shop that says, "Never think up reasons not to check something."


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              • #8
                Originally posted by Randall View Post
                It's factory biased at 20% now. I am still wondering why they did this.
                Probably because it sounds good like this and cold biasing significantly extends tube life.

                I have this amp and while I also own Marshall, Vox and Fender BF amps, the Vibroverb RI is my most played amp.
                I think this is the best sounding Fender RI amp and the only thing one might want to change is the speakers.

                Don't ask me about the tremolo. Never even tried it because I generally don't care about tremolo.
                - Own Opinions Only -

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                • #9
                  Originally posted by Randall View Post
                  The stock 33K R59 supplies -54v C- for 13mA at 450v = 5.8 watts. Seems more than a bit low. What is a good place to set these at? Or should I leave factory set good enough alone? It does sound good like this
                  Originally posted by Randall View Post
                  It's factory biased at 20% now. I am still wondering why they did this.
                  You answered your own question (along with Helmholtz remark about tube life).
                  You've also given great proof of why the "70% rule" should not be taken as gospel.
                  Originally posted by Enzo
                  I have a sign in my shop that says, "Never think up reasons not to check something."


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                  • #10
                    Originally posted by g1 View Post

                    … I was mixing up mid-point of the linear section of transfer characteristic; and mid-point of plate dissipation. Not the same thing.
                    Thanks for putting that so well, it occurred to me too, but I got struck trying to explain it

                    My band:- http://www.youtube.com/user/RedwingBand

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                    • #11
                      Amen. Some of the best lessons I learned came from trying to explain something, only to find I was holding erroneous assumptions.
                      Education is what you're left with after you have forgotten what you have learned.

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