Super Reverb Bias volatge -49 to -60V..???

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by pfrischmann, Mar 19, 2006.


  1. pfrischmann

    pfrischmann Member

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    This can't be right.

    I just gor some of the shuguang 6l6's and couldn't get them to go any higher than 32.8 ma @ 458 V on the plates. The bias pot tops out there.

    So I pulled the tubes and checked the -DC voltage on the control grid...-49V to -60 volts. Could my resistor have drifted this much?

    I'm amazed the guy I bought this froms tech didn't catch it.

    Could it bee something else?
     
  2. Fuchsaudio

    Fuchsaudio Supporting Member

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    Some of the newer tubes need more range than the older bia pot's could provide. Just a simple resistor change. If you put those tubes in, how could the guys technician have known the pot wouldn't have enough range for your tubes ? I gotta admit, 49 to 60 is a little narrow. I'd like to see -35 to -60 or so myself.
     
  3. John Phillips

    John Phillips Member

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    Where's the problem? 32.8mA is dead on 15W @ 458V, or 68% of max dissipation. Perfect... you don't want it any higher.

    The voltage will read a little high with no tubes in most likely - try measuring it from inside the chassis with the tubes in. Fender's original factory spec setting is -52V, BTW.
     
  4. slider313

    slider313 Silver Supporting Member

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    If you want more range,change the bias pot resistor(should be a 27K) to a 15k.
     
  5. pfrischmann

    pfrischmann Member

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    Yeah ,
    I know, I'm almost at 70% of max wattage dissipation. It's more the fact that I'm completely out of range to get there.

    I guess,
    The reason I'm surprised is that I would have though someone would have looked at the negative voltage and figured the resistor in the bias supply needed to be changes. The 5881 WXTs were biased really cold.
     
  6. John Phillips

    John Phillips Member

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    Or, perhaps, someone looked at the negative voltages and concluded they were sufficient to achieve any bias point you want from too cold to hot enough, and so decided there was no point in fixing something that wasn't broken. I would have done exactly the same (ie, left it alone).

    Is this an old Super BTW? If so, remember that line voltages have risen since the amp was built and the bias supply voltage is directly proportional to that.

    Why modify the amp when it doesn't need to be? You don't need the part of the range you can't get to now, so why worry about it?

    Where were the 5881s biased? Below 50%? If not, that's not 'really cold'. 70% is NOT a target, its a maximum.


    Sorry for the slight rant, but I really can't see why people get so hung up on stuff like this. If you can bias the amp so the tubes run in the right range and it sounds good, that's all that's necessary.
     
  7. pfrischmann

    pfrischmann Member

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    That's fair,
    I don't look at it as a rant. I posted because I wanted to learm from someone more experienced than myself. Any information you or any of the other techs share increases my knowledge base.

    Including when you tell me not to do something stupid:D

    BTW, I thought 70 of SWD was a target. I guess it makes sense that it's the top end, gotta have room for the tube to go into overdrive.

    Now,
    onto the tremolo........


    Thanks,
    Paul
     

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