Biasing a Bassman LTD?

Discussion in 'Amps and Cabs' started by Twanger, Jun 13, 2008.

  1. Twanger

    Twanger Member

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    My friend and bandmate has a Fender Bassman LTD that he just put new power tubes into (6L6GCs). I know that these amps have a bias adjustment pot, but is there as bias test point on the PCB where I can use a simple multimeter to get a bias reading…like on the HR DeVilles? Does anyone know?

    If not, what’s the best way to adjust the bias? I hope it’s like the DeVille, that would be simplest…

    Thanks for your help!
     
  2. Twanger

    Twanger Member

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  3. VaughnC

    VaughnC Supporting Member

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    If I recall correctly, the LTD only has one, one ohm, resistor in the output tubes cathode circuit. So, if that's the case, you'll be able to a sum current of both tubes with a DVM but you won't be able to tell much about each one individually. Best bet would be to purchase an actual bias meter (they plug between the output tubes and their sockets), so:

    1)You'll be able to see what each tube is doing individually, and;
    2)You won't be exposing yourself to the lethal voltages inside the amp probing around with a DVM, and;
    3)The bias meter will work on any amp you purchase in the future so it's worth the investment if you plan on playing guitar into your golden years.
     
  4. Blues_N_Rock

    Blues_N_Rock Member

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    Twanger:

    I posted a close up photo to the backside of a 59 Bassman LTD bias point.

    http://s170.photobucket.com/albums/u278/twhyg_00/59%20Bassman%20RI%20LTD%20Bias%20Point/?action=view&current=BiasPotPoint.jpg

    Clip the ground from your meter to anywhere on the chassis. Clip the positive end of your meter to the bias point shown on the photo.

    Adjust the bias pot accordingly. Keep in mind as VaughnC indicated the read you get for current is a total current read that is flowing to both power tubes. It won't tell you what each tube is drawing in current. So make sure to divide the current by 2 then multiply the voltage to get the average power.

    As VaughnC indicated, you can buy a bias meter to get a more accurate and SAFER read.

    There are other ways of taking direct reads from each tube to find out the actual voltage and current going to the tubes, but unless you know what you are doing, I would not recommend it because you could hurt or kill yourself or damage your amp.
     
  5. VaughnC

    VaughnC Supporting Member

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    That assumes that both tubes are reasonably matched...but I wouldn't assume that, especially with modern tubes. If the bias test point reads, for example, 60 mv, that could mean 30 ma/tube or any two numbers that add up to 60. Too risky for me....with a bias meter no assumptions are made.

    However, once you determine what the safe maximum bias current is (with a bassman, about 40ma/tube), set the bias by ear for the best sound (not to exceed the calculated safe maximum) as there's no real magic bias setting number.

    A little trick I use in setting the bias for my amps is:
    I start with the bias current set to the amps minimum setting. Then I plug in my favorite guitar, set the tone to my liking, and set the volume to what I consider to be my normal gigging volume. Then, while repeatedly picking a choked open low E string (this gives me a good thumpy, real world test signal) while simultaneously slowly turning up the bias control, I listen for the amp to "bloom" (for lack of a better word;)). Once the amp blooms, I find that setting the bias any higher has little effect on the tone and only shortens the life of the tubes. Then I check my bias meter to make sure that I'm not exceeding the safe calculated idle bias current. Hey, it works for me...it takes most of the guess work out of the equation, and probably extends the life of my precious NOS tubes ;).
     
    Last edited: Jun 14, 2008
  6. jumpnblues

    jumpnblues Member

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    Buy a Weber Bias Rite. Relatively inexpensive and very, make that extremely, easy to use.


    Tom
     
  7. Guinness Lad

    Guinness Lad Silver Supporting Member

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    Huge +1 this is what I do as well. I ususally find with 6L6 Fenders they sound really great around 31.5 ma. If you run them hotter all you do is kill the tubes and make the midrange response harsh sounding.
     

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