33mA-43mA for EL34's but WHAT type of 12ax7???

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by SLOhand, Jan 7, 2008.

  1. SLOhand

    SLOhand Member

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    Hey guys,

    Any help would be greatly appreciated here!!!

    ...............I read somewhere that the plate voltage on a T-verb can be between 460 - 475 and my EL34 power tubes should be around
    33Ma - 43Ma.

    SO my question is what should the "pre-amp" tubes be rated at? :jo

    I want to get some NOS tubes but don't have any idea of what ma to get


    Thanks
     
  2. ked

    ked Supporting Member

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    You won't need to do any adjusting on the preamp tubes. Just get whatever NOS tubes you want and stick them in, they will work fine.


    Ken
     
  3. rockon1

    rockon1 Member

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    Yep,like ked said, the 12AX7's are "plug and play". FWIW - I wouldnt go over 37 ma on the EL34's if the plate voltage is 470. 37 is actually pushing it at 70% dissapation. 35ma is a safer bet. 43ma would be way hot. Bob
     
  4. SLOhand

    SLOhand Member

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    Thank You very much guys..........I was sure someone could help me out here. This is great, now I don't have to worry :munch
     
  5. SLOhand

    SLOhand Member

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    Next question,

    is 33mA on the cold side or is 43mA?

    if that doesn't make sense - let me rephrase the question
    If I had a 33 mA power tube would that be considered a warm tube in a Trem-O-Verb which has a plate voltage of around 470




    Thanks for any help guys



    :horse
     
  6. rockon1

    rockon1 Member

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    The formula for calculating idle dissipation is

    max tube dissipation / plate voltage X .7(70%) or .6 for a somewhat cooler 60% or anyhere in between...



    25 / 470 = .053 X .7 = .037 or 37ma Thats as hot as you want to set it. Personally I dont see any advantages to running at 70% idling. If it sounds good slightly cooler all the better. The tubes will last longer.At 33 ma your idling at a more conservative 62% idle dissipation. Bob
     
  7. Blue Strat

    Blue Strat Member

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    If you're talking about the tube RATING as supplied by the tube dealer, that doesn't tell you much. You actually have to measure and adjust the bias of the amp for any new pair of tubes to get things in line.

    Since Mesa's don't have bias adjustments order from a dealer who knows what ratings are correct for Mesas and just plug and play.

    We can do that for you.
     
  8. rockon1

    rockon1 Member

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    Oops I didnt even catch that! Thought he was talking about the setting he was at with it.
    OP- My post above is meaningless. Bob
     
  9. Blue Strat

    Blue Strat Member

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    I wasn't sure that was what he meant, just kind reading between the lines.
     
  10. SLOhand

    SLOhand Member

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    Thank you gentlemen for responding to all my noob questions......


    First off - Mike K you are first on my list to order tubes from, i've been drooling all over your website......lol

    back to my question

    If I understand you correctly 33mA is good, 37mA is warm, and 43 is over the top HOT/too hot.

    Much appreciated guys thanks so much


    Lance
     
  11. rockon1

    rockon1 Member

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    If when actually measuring the bias in the amp you got those readings,basically,yes. I would consider a setting of 37 ma warmer than necessary though. Bob
     

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