Installing a tube bias pot

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by ryanverbena, Sep 16, 2008.

  1. ryanverbena

    ryanverbena Member

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    Just bought myself a 59 RI bassman and I was wanting to install a bias pot which only the ltd reissues have. I want to put in a tube rectifer and 5881's in the power amp. I'm not hugely technical minded, but competent in the basics and was after a simple step by step from someone who knows how to do it. A diagram would be perfect. If you can help me out it would be very much appreciated!
     
  2. phsyconoodler

    phsyconoodler Member

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    That is not an easy task for a beginner.First of all,the reissue has no 5v taps for a rectifier tube so you need to add a filament transformer.
    The bias pot is not particularly hard,but I don't have a diagram.
    For a rectifier 'tube like' feel,get a 'copper cap' from Weber VSt.He makes one that will just plug in to the socket and it will emulate a tube rectifier.No 5v tap needed.
     
  3. Dave C

    Dave C Gold Supporting Member

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    The bias pot should be reasonably easy for you to do. Replace resister R-41 with a 50k trim pot. it's in Gerald Weber's book "Tube amp talk for the guitarist and tech" where he has line drawings and step by step text.

    The tube rectifier is a lot more involved. It will require a 5v filament supply either from the power transformer (don't think yours has 5v filament tap) or a separate filament transformer. You will also have to have a hole punched in the chassis for another octal tube socket. I'd suggest looking into a "sag" resister or a zener diode B+ V dump from Ted Weber if you still feel the need to drop B+ voltage.
     
  4. Blue Strat

    Blue Strat Member

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    I thought Bassman 59 RIs were already wired for tube rectos. I've seen several that were.

    +1000 on the Weber book. It's worth having anyway.
     
  5. WaltC

    WaltC Member

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    All the '59 Bassman re-issues I've worked on (all 90's models or very early 20xx models admitedly) were wired for tube rectification and the 90's models all came with a plug-in solid state rectifier to replace the 5U4 rec. tube.

    +++ on Dave's info on the bias pot. It may be possible to just plug in the 5881s w/o out changing the bias and is worth checking if it will avoid work and pulling the pc-board IMHO.
     
  6. phsyconoodler

    phsyconoodler Member

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    I thought the reiisue I worked on recently was wired for a rectifier tube too,til I opened it up and couldn't find any 5v taps.Some may indeed have them but not all.Save your self the hassle and get a Copper cap.You can get all voltage drops to simulate the tube rectifier.
    As far as the bias pot,it is easy for a skilled tech,but it involves removing the circuit board to install properly.
     

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