Bassman 59 LTD:biasing 5881 tubes

Discussion in 'Amps and Cabs' started by Jerk64, Jan 29, 2015.

  1. Jerk64

    Jerk64 Member

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    Hi guys!
    Today i replaced the two 6L6 GT with two Tung Sol 5881 RI power tubes. Then i checked the bias with the multimeter through the bias test resistor. But, with the bias pot totally cranked, the multimeter said 63 mA....
    Too low as a maximum value....!
    What's wrong? Problems inside the amp? Or with the new tubes?
     
  2. Bluewail

    Bluewail Silver Supporting Member

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    First of all remember true 5881's are not 30 watt tubes like a 6L6GC but 25 watt tubes so they bias differently. The LTDs bias range does not go very high. I find with most 6L6's I can only get it to about 55% max of total power. It would require a change in the bias resistor value to get it up higher.
     
  3. Jerk64

    Jerk64 Member

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    25 X 0,7 (70% of dissipation)/ 490 (plate voltage) = 35,7 mA
    Too far my bias pot.... (63mA/2 = 31,5mA.....:thud)

    Which kind of change for the resistor?
     
  4. Jerk64

    Jerk64 Member

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    Anyone tried 5881 in Bassman 59 LTD?
     
  5. Blue Strat

    Blue Strat Member

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    First, 70% is too high. There's never a reason to go over 60% especially with modern tubes unless your goal is short tube life.

    Second, tube ratings vary. You would need a pair with a higher rating to get you where you want to go. The same thing can be accomplished by decreasing the value of the bias range resistor (not a user friendly change).
     
  6. dmagalhaes

    dmagalhaes Member

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    I've got RCA 5881's and a 5R4 rectifier in my Ltd. I did have to have the resistor replaced in order to bias the amp but it was worth it. It starts to get a bit nasty around 3-4 on the bright input.
     
  7. fancychords

    fancychords Member

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    May be a dumb question but where do you get the 490 from..Please tell me the formula to bias 6l6's in a 59 reissue bassman.. thanks..I notice that with the weber bias probe when you increase one number the voltage number changes..
     
  8. dmagalhaes

    dmagalhaes Member

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    Wasn't my post but 490 would be the plate voltage.
     
  9. Jerk64

    Jerk64 Member

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    What changes if i put a 5751 in V1? Lower plate voltage? More headroom or earlier breakup?
     
  10. Blue Strat

    Blue Strat Member

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    No voltage changes, definitely not earlier breakup, possibly a tiny bit more clean headroom in the preamp but not in the power amp.
     
  11. Jerk64

    Jerk64 Member

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    A 5Y3 rectifier could change something?
    Lookin' for earleir breakup!...
     
  12. Blue Strat

    Blue Strat Member

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    For earlier breakup, the magic formula is NOS RFT 12AX7 in V1 and 5R4GB rectifier. A 5Y3 is a little under powered for that amp.
     
  13. Jerk64

    Jerk64 Member

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    A 5R4GA might fit?
     
  14. Jerk64

    Jerk64 Member

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    Currently i have: Tung Sol 12AX7 in V1 and V2, Sovtek 12AX7 LPS in Phase Inverter,
    2 Tung Sol 5881 RI and Sovtek 5AR4.
     
  15. Jerk64

    Jerk64 Member

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    Just installed the Philips 5R4GA rectifier.
    Now the bias value (maximum) is 42mA/2 = 21mA
    Too low?
     

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