Tube Bias Help

Discussion in 'Amplifiers' started by Ben, Jul 7, 2018.

  1. Ben

    Ben New Member

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    Does anyone know what the plate voltage is for an Archon 50? I'm installing El34's. Does anyone know the recommended bias point for El34's?
     
  2. Ben

    Ben New Member

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    I figured it out. Looks like 400 volts.

    According to my Groove Tubes bias kit manual, that means 20-50 mV is the range, with a nominal set point of 37.5 mA for El34's.

    6l6's are 25 to 60, with a set point of 45 mA.

    If PRS recommends 30 mA for 6l6's, their recommendation for El34's must be around 25....
     
  3. shimmilou

    shimmilou New Member

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    I would like to see the schematics for an MT15, to see what might be different about this amp. For any other amp, considering 400VDC on the Plate, I would set EL34s to around 44 mA each tube, and 6L6s to around 45 mA each tube. Figuring that EL34s as 25 watt tubes and 6L6s as 30 watt tubes.

    It seems that the PRS recommended setting is quite cold.
     
  4. elvis

    elvis Hamfisted String Banger

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    At 400V and 10mA (1mV = 1mA on the measurement lugs), the amp will dissipate 4W idle power. At 40mA it will dissipate 16W idle. This is per tube.

    The big question is what the ratio is between idle and max power. You'd want to set for appropriate max power.
     
  5. elvis

    elvis Hamfisted String Banger

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    Also 400V seems low.
     
  6. shimmilou

    shimmilou New Member

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    IME, ideal idle dissipation, for no crossover distortion, is a minimum of 50%, ranging up to 70% of the tube max rated Plate dissipation. With a fixed bias amp, for 6L6 (30 watt), I like it around 60%, whereas for EL34 (25 watt), and 6V6 (14 watt), I like it around 70%. Many Cathode biased amps run the tubes at close to 100% idle.
     
  7. elvis

    elvis Hamfisted String Banger

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    Checked with PRS. It's 450V.
     
  8. shimmilou

    shimmilou New Member

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    In that case, about 40 mA for 6L6, or about 38 mA for EL34, would be my setting per tube.
     
  9. LSchefman

    LSchefman Hears Tones

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    Lots of manufacturers have different ideas about tube bias. For example, Mesa runs their bias pretty cold so that they can use a variety of tubes. Other makers run them hotter. A lot’s going to depend on the designer’s thinking.

    Why not check with PRS Customer Service, and get the skinny from the horse’s mouth?

    Incidentally, I run my EL34s on my HXDA right at PRS’ recommendation. I like the way it sounds. Someone evidently thought this through.
     
  10. Em7

    Em7 deus ex machina

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    That's correct. I usually bias class AB amps at 70% of maximum plate dissipation (i.e., 25W * 0.7 / 450V * 1,000 ~= 39ma). However, running tubes that hot sacrifices tube life for tone.
     
  11. HANGAR18

    HANGAR18 ...a TWISTED SISTER pin?!

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    I bought a 100w Archon head and immediately installed EL34's. Upon asking PRS, PRS provided me this specific answer which I will now copy & paste into this post.

    "We recommend a bias setting of 30mV, you can go + or - 5mV to your taste. The bias adjustment is the little flat head inside the jack above the contact points."

    Then there is this also...
    https://forums.prsguitars.com/threads/handy-tip-for-archon-100w-with-el34s-fuses.18268/
     

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