Tubing/Biasing Nightmare...FOR THE LOVE OF ALL THINGS GOOD, HELP ME!!!!

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by zodiactone, Jul 17, 2008.

  1. zodiactone

    zodiactone Supporting Member

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    okay, basic info:

    120 Watt Peavey JSX head
    Came with Electro-Harmonix EL34's stock.

    The tubes needed to be changed so I bought JJ/Tesla 6L6's. Switched the small switch inside the amp to the 6L6 position, and installed my new JJ's.

    Problem #1

    One of the tubes is not seating properly....we will call this "tube a". Tube A, when looking at the BACK OF THE AMP is all the way to the left. There is a very noticeable gap, and I am afraid to press it down anymore because I'm afraid I will damage one of the pins.

    #2 Biasing

    I have never done this before in my life...and I'm not super technically inclined. I don't have a clue of what my Plate Voltage of my amp is. I don't know what the Cathode or plate voltages of the tubes are. And every sight I have looked at stress not to go by the bias points (place to insert your leads from volt meter) that are on the amp.

    SO DEAR LORD WHAT DO I DO????

    the tubes say their bias point is at 20. I have a simple volt meter, and I have it set to the 20K position on the ohm side...(i don't even know if that is where I should have it). Every site I visit is giving different info about what the amp should be biased at. A common message that I"m reading is that Peavey Bias the amps too cold. ****there is a small bias pot used to set the bias....so if my volt meter settings are right (meter set at 20K on the Ohms section)...what should the bias point be?

    And how does some one get a number like 42 ma or 50ma...please break this down in terms that a 2 year old will understand it.

    :confused::worried:bkw
     
  2. trey85stang

    trey85stang Member

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    got a pic of the tube thats not seating? Are you sure you have the guide pin lined up correctly?

    set your meter to dc voltage. I dont know how the specifics about the jsx but if the test points are no good you might want to invest in one of those weber bias rites. It will make the whole process a little easier for a newb :)
     
  3. rockon1

    rockon1 Member

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    In order to bias it without using the "test points" you will need to

    1) get a bias probe

    2) Know the plate voltage.

    Once you know the plate voltage you can calculate the setting you need to bias the amp to your "target " setting.

    The formula is as follows-

    Tube maximum wattage dissapation divided by the plate voltage times the "target" idle dissapation.

    Ezample- 30 watt tube(6L6) / 400v (plate voltage) = .075 ma.(MAX dissapation) X 60%(.6) = 45ma setting for 60% idle dissapation.

    You might choose to use a more conservative rating for your 6L6 of 25 watts as max dissapation which would give you a setting of 37 ma IF your plate voltage was 400volts. Obviously you need to know the actual plate voltage before you can calculate the bias setting. Bob
     
  4. gldtp99

    gldtp99 Member

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    I'm going to help you : Take Your Amp to a Qualified Tube Amp Tech Before You Hurt the Amp or Yourself !!!
    I'm not kidding--- there's a reason that the tube isn't seating and the amp may have to be taken apart to find out why and fix it---- no offense, but by your own admission, you are not technically inclined or do you have a clue about how to proceed.
    I believe that paying a tech to do this work would be money well spent.........gldtp99
     
  5. phsyconoodler

    phsyconoodler Member

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    Agreed.It would take far too long to explain it to you in laymans' terminology.
    Take it to a tech. and save yourself a potentially expensive learning curve.
     
  6. doctorx

    doctorx Member

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    1. Back away from the amp and keep your hands where we can see them.

    2. Get out the yellow pages and look up "Guitar Amp Repair".
     
  7. GearHeadFred

    GearHeadFred Member

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    I don't disagree with the above posters.. and they are not just being snobs. It's because you can kill yourself messing with a tube amp if you don't know what you are doing, and no one wants to be responsible in any way for contributing to that.

    That said, if you're still set on doing this, I would urge you to use the external test points as that offers the lowest chance of you harming yourself. Contact Peavey and get specific instructions on how to use them.. You will want the meter to measure volts (Mv), not resistence (ohms).

    Regarding the tube not seating - hard to help without a photo.
     
  8. zodiactone

    zodiactone Supporting Member

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    I appreciate the help...(and I am being sincere there). I did the next best thing I knew to do, I called Peavey.

    I talked to the tech about how the tube wasn't seating as it should and he gave me a few options to try. And I also got him to tell me how to bias it (much easier than I ever dreamed it was). Basically, the amp's bias is preset for 6L6's and EL34's, they just give you a bias trim which only gives you a small variable to play with and find what you like. I like running mine a little hotter so I set it at 53.8. It just seems to sound great there.

    I appreciate everyone's posts!

    Zodiac
     
  9. doctorx

    doctorx Member

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    :eek: :eek: :eek:


    If this is 53.8ma then that amp must have a plate voltage of about 395 volts, does that sound low to anyone?

    That is assuming it is biased properly.
     
  10. rockon1

    rockon1 Member

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    Thats not ma. Its at the test points on the back of the amp that actually read negative grid voltage. Not very accurate from my understanding since it assumes a tube will pull a specific currrent value . Bob
     

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