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Thread: Bias question

  1. #1
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    Bias question

    I have a late 60's twin that I have reverted to AB763. I have a TAD quad bias meter. I can not get the bias current (using adjust pot) any higher than about 25 mA. My rail voltage is 457VDC so charts say I can go as high as 47 mA. I have dbbl checked my conversion to AB763 and seems ok. Can anyone give assistance or guidance?
    Thank you for your time.

  2. #2
    Forum Member cdw2000's Avatar
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    Re: Bias question

    Check the voltage out of the middle terminal of your bias pot (referenced to ground). You should be able to adjust it into the ballpark of -52V according to the schematic. More negative reduces bias current, less negative increases bias current.

    Assuming the rest of the power section is wired correctly, my guess is that for whatever the reason your negative bias is too high (too negative). If the above voltage check confirms this, check the 27K resistor to ground that forms a voltage divider with the pot. Make sure it is the correct value and has a good solder connection to ground.

    Also check the voltage at the high side of the bias pot, it should be around -60V. If it is higher than this you may need to reduce the value of the 27K resistor to maybe 22K to compensate.

    Line voltages are higher than they were in the 60's and this could throw off the bias adjustment range.

    Hope this helps.
    "Time is an illusion, lunchtime doubly so" -- Douglas Adams
    "If something has a 1 in a million chance of occurring, 9 times out of 10 it will happen" -- Terry Pratchett

  3. #3
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    Re: Bias question

    hey cdw2000.
    Great advice. I was over -70VDC on the high side of bias pot. With the wiper all the way over I was getting -54VDC as the minimum VDC. I dropped the 27k resistor to 22k and things were better but still couldn't make even 30 mA bias current. Changed resistor to 15k. All is well & I biased at 45 mA. Chart says I can go to 47 mA. Thanx again, I would not have thought about the general voltages being higher in silver face. When considered, it sure makes sense.http://www.thefenderforum.com/forum/...milies/lol.gif

  4. #4

    Re: Bias question

    I converted my 72 Twin Reverb with AA270 circuit bias balance to AB763 bias adjust. I left the 15K resistor. I get 429 volts at the 6L6 on the volt meter. I can easily get 15ma-80ma. I set the bias at 36ma. I did not try a 27K or 22K resistor even though I have them. Yes this is an old old thread but this info is constantly sought after.

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