Bias question - Holland Magma

Discussion in 'Amps and Cabs' started by strat68, May 26, 2008.

  1. strat68

    strat68 Member

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    Hi, I have basic (read- limited) electronic skills but have used a meter to bias my own amp before- a Fuchs Bassman mod with controls on the back of the amp. On the Fuchs ODS mod, there are two pot adjustment screws where the convenience outlet used to be on the Fender, so you can bring each tube to the desired value.

    On the holland there is only 1 adjustment pot, but two probe terminals. Well- I believe they are for the positives of the meter; RCA jacks on either side of the adj. pot... So one tube's current (on the left) was 35 and the other at 40, so I tried lowering it to bring the 40 down some, but the one on the left went down too. (They were a matched pair of Svet EL34s).

    I realize this may be a dumb question, but should there be a switch somewhere for the adjustment pot to change sides, since there is only 1 pot? I'm not even sure if .035 is right for the Holland, I just picked that value because that's what the p-touch label says which Andy put on the back of the Bassman mod.

    So, I'm not sure if Holland is still in business and am wondering if anyone has used this amp, or can point me in the right direction to get the power tubes working well together.

    Thanks,
    Steve
     
  2. SatelliteAmps

    SatelliteAmps Member

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    I am not familiar with the Holland, but most bias adjustment adjust both tubes at the same time, which is what that sounds like to me.
     
  3. Blue Strat

    Blue Strat Member

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    What Satellite said. 35 and 40 is fine.
     
  4. RvChevron

    RvChevron Member

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    Some amps offer individual bias adjusment for each power tube.

    Why can't this be standard on every amp that needs bias adjusment when changing tubes is beyond me......
     
  5. strat68

    strat68 Member

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    ok thanks, so it is not critical to get both tubes very close to the same number?

    On the fuchs amp with 2 adjustments, I try to keep them within 1/10 e.g. 35.X and 35.Y. I notice on this amp which has a 6L6/EL34 switch that I have to travel pretty far with the adjustments when going between tube types.

    So would I be correct in concluding that the adjustment is to compensate for change in tube types, and we depend on matched pairs to have them reasonably close to each other?

    Thanks,
    Steve
     
  6. Blue Strat

    Blue Strat Member

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    Not critical. Anything within 5 is fine.

    On the Fuchs, try adjusting the 2 tubes to a variety of settings. See if you can hear the difference between 10ths apart and 5mA apart. Oh, and have a friend make the adjustments so you don't know where they're set. Otherwise, it's not a valid test.
     

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