New power tubes and the bias is VERY high - normal?

Discussion in 'Marshall Amps' started by h45, Jan 9, 2020.

  1. h45

    h45 New Member

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    I just took out the old stock tubes put new Sylvania EL34 power tubes in my DSL40C (along with 3 new preamp tubes).

    When I went to bias, the initial reading said 65! Is this uncommon? I of course biased down to 38.

    I checked my multimeter and was on DCV 200m and all that. The amp is still on my bench, so before I put it all back together, just wanted to make sure about this.

    I do have another DSL40C with stock tubes, but don't want to take that apart to check right now.

    Thanks for all your help!
     
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  2. ken361

    ken361 Well-Known Member

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    should be set like 32 per tube, left trim pot works the right tube and vice versa. Try to have the pot really low and work up and go back and forth each tube.
     
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  3. ken361

    ken361 Well-Known Member

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    might be a bad tube and cant set it low it can happen
     
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  4. marshallmellowed

    marshallmellowed Well-Known Member

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    Unless you know what it was set at, prior to changing the tubes, it could have been high to start with (with the old tubes). As long as you're able to set it within spec. with the new tubes, I wouldn't worry about it.
     
  5. ampmadscientist

    ampmadscientist Well-Known Member

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    Start the reactor... Free Mars!
    It sounds like you did it right.
    Does it hold 38 after 1 hour?
    Different tubes require different bias settings and this is normal.
    Old and new tubes are probably going to be different for biasing.
     
  6. South Park

    South Park Well-Known Member

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    Are checking ma or vdc
     
  7. Jethro Rocker

    Jethro Rocker Well-Known Member Premium Member

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    Same diff with bias depending where you read from. As long as you can get it to the good range, its fine.
     
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  8. South Park

    South Park Well-Known Member

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    Reading grid voltage is not the same as grid ma . You can move the voltage up and down. It is the power the grid is going to ground that controls the plate power going to ground
     
  9. Matt_Krush

    Matt_Krush Well-Known Member

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    you can get 1 set of tubes be at one end of the extreme vs another set.
    not that uncommon.
     
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  10. marshallmellowed

    marshallmellowed Well-Known Member

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    Actually, it is the same. Bias circuits are typically setup to read the voltage across a 1 ohm resistor, the voltage and current are directly proportional. So, 1mv across a 1 ohm resister = 1ma of current through the resistor, 32mv = 32ma...
     
  11. h45

    h45 New Member

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    I did follow exactly the video here and double checked my meter an all:

    I put the amp back and played through it for half an hour or so. Should I recheck the bias? I didn't know about waiting an hour.

    I think tomorrow I will check the bias on my other DSL40 with the stock tubes, just to see what it's at.

    I like the sound I'm getting with the new tubes. Still has a bit of that harshness, so thought about the C19 mod too. This one has a Greenback (vs a Creamback with the stock tubes) so that may be part of it though.

    One last thing...
    The plate voltage measured at 438. If I'm reading the calculator right, could I then bias it around 40?
     
    Last edited: Jan 9, 2020
  12. EADGBE

    EADGBE Well-Known Member

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    Remember tubes can vary. I believe it's because they can have different transconductances. And like someone said as long as they can be biased at a safe setting you're okay.
     
  13. Jethro Rocker

    Jethro Rocker Well-Known Member Premium Member

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    Depends on dissipation. 36 - 38. Whatever. If its not real cold or redplating and sounds good, leave it there. I did the C19 mod when I had a DSl40. With a V30. Worked great
     
  14. mickeydg5

    mickeydg5 Well-Known Member VIP Member

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    It is always good to check, play, let idle and check again.
    40mA idle bias is fine if it works for you, the sound and the current situation.
     
  15. cspencer

    cspencer Active Member

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    This is a good thread.
     
  16. scozz

    scozz Well-Known Member

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    Wait,.....what?,,........a Dsl40 with a Greenback in it?!?!

    A 40 watt 1-12 combo with a 25 watt speaker?!?!
     
  17. ampmadscientist

    ampmadscientist Well-Known Member

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    Start the reactor... Free Mars!
    Because 1 volt across 1 ohm = 1 amp of current.
     
  18. EndGame00

    EndGame00 Well-Known Member

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    See if the bias reading jumps up when you strike a note v. on idle... If it is within tolerance, then it's fine (34-37)... If it jumps way up, then toss those tubes into the trash...
     
  19. cspencer

    cspencer Active Member

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    Sometimes it takes a few days for tubes to settledown.
     
  20. marshallmellowed

    marshallmellowed Well-Known Member

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    Um, yeah, basic Ohms Law... I = V/R.
     
    Last edited: Jan 28, 2020

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