Biasing trouble with 1959 SLP

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by StratTone, Oct 2, 2006.

  1. StratTone

    StratTone Member

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    Ok, here is the deal. I have the 100 watt Plexi RI and bought some power tubes that should have biased easily at 38 ma in the low 400 volt range. Even with the pot adjusted all the way I can only get 30ma's........

    What do you think it is? Basically the way I understand it is that the SLP has 2 pots. VR1 and VR2. One is for the bias and one is for noise. Correct?
     
  2. Blue Strat

    Blue Strat Member

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    Not sure about which pot is which, but it's not uncommon for the bias pot not to have a wide enough range to cover every set of tubes.

    30s is fine and you probably won't be able to hear much difference.

    You either need a "hotter" quad of tubes or to change the bias "range" resistor in your amp.
     
  3. donnyjaguar

    donnyjaguar Member

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    It sounds to me that the power tubes have gone soft. Cathode stripping comes to mind here. I guess its possible that they just have low gain, but I think you'll find once you bias them up you won't net full output power. That said, I'm sure they sound fine.
     
  4. Blue Strat

    Blue Strat Member

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    He said he "bought" some power tubes, I'm assuming that he means they're new.
     
  5. Dave C

    Dave C Gold Supporting Member

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    VR1 is the bias adjust , VR2 is the hum balance adjust for noise from the heater circuit. VR1 is a 20k pot which doesn't give much range . You could either change the bias range resister or the pot. I'd change the resister, but you might have to change it back again with the next set of tubes. As Mike said .....30ma is fine.
    Dave C
     

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