A Little Bias Help Please

Discussion in 'Amps' started by CyFan4036, Apr 25, 2019.

  1. CyFan4036

    CyFan4036 Well-Known Member

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    Need some help from you amp gurus. Got my new power tubes installed but I want to check/adjust bias. I'm thinking since the original tubes were still in there that this hasn't been done for over two decades. I know this can be a dangerous thing so I will definitely observe the one hand in pocket rule. But I have to know how to do this! I've attached pictures and a schematic for reference. From what I've read, I need to check between R39 and R40. But where the hell is the trim pot? I was in there at the end if a long day so forgive me if it should be obvious. Any help will be appreciated! This isn't about saving the $30 on having a tech do it, I want to learn! Thanks guys!
     

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  2. mozz

    mozz Well-Known Member Supporting Member

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    Says AP1 bias trim. Junction of r39 and r40 should be -34volts. That actually doesn't tell you what the tubes are dissapating, you need a bias tester for that. Few other ways to measure it also.
     
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  3. backinit

    backinit Well-Known Member

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    I just did my Amelia. I used the 3rd pin method to find the plate voltage and a cathode adapter to monitor the tube current while I adjusted.
    What model is this?
     
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  4. CyFan4036

    CyFan4036 Well-Known Member

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    It's the R-12r
     
  5. cybermgk

    cybermgk Well-Known Member Supporting Member+

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    It is marked AP1, near R59, C38, D21 and D22. From your pictures, at a guess, it would be on the board near the output tubes, pic 4, but can't see everything in pic, or near there on the main board.

    Also, it COULD be on the other side of the board with the Otubes (pic4), accessed via a hole near those output tubes on the other side of the chassis?
     
    Last edited: Apr 25, 2019
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  6. CyFan4036

    CyFan4036 Well-Known Member

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    I'll check for that when I get home from work. Man, there a lot of stuff going on in there! Lol
     
  7. mozz

    mozz Well-Known Member Supporting Member

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    You can just pull the output tubes and measure from pin5 to ground, that would be your negative 34 volts bias. Closer to zero, such as negative 30 would run the tubes hotter. More negative, such as -40 would be colder.

    But....................setting the bias according to the directions tells you nothing. The tubes may be biased cold, or hot, depends on the tube. You need to measure the plate current, plate voltage and/or cathode current to figure out how much the tubes are dissipating.

    https://robrobinette.com/Tube_Bias_Calculator.htm for some info.
     
  8. CyFan4036

    CyFan4036 Well-Known Member

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    This is going to take me awhile because I must admit to being a bit intimidated right now. BUT, I AM going to do this!
     
  9. CyFan4036

    CyFan4036 Well-Known Member

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    So can I just use my multimeter on pin 3 and ground to chassis to get plate voltage?
     
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  10. mozz

    mozz Well-Known Member Supporting Member

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    Yes, but without the tube the voltage will be a lot higher. Just measure the bias voltage to start, it should be your negative 34 (pin 5)as per schematic.
     
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  11. backinit

    backinit Well-Known Member

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    Once you find your plate voltage write it on a sticker and place it near your tubes on the chassis.
     
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  12. backinit

    backinit Well-Known Member

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    A nice headlamp works wonders in these cases.
     
    Last edited: Apr 26, 2019
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  13. CyFan4036

    CyFan4036 Well-Known Member

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    And I have a Harbor Freight less than a mile away(y)
     
  14. jtcnj

    jtcnj Well-Known Member

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    I dont know how much time you have with hands in amp chassis so I will just remind you to follow the safety procedures, such as one hand stays in pocket - as this can kill you.

    https://robrobinette.com/Tube_Amp_Safety.htm



    This is not hard at all once you understand what you are doing.
     
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  15. CyFan4036

    CyFan4036 Well-Known Member

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    Thanks ! I've got the safety part down, I just don't want to jack up my amp. In going to go slow so I understand the procedures and my amo before I do it. My new tubes look exactly the same and have the same 6 rating. Amo sounds great with the new tubes so I'm really just wanting to have it right in case it can become even better ! I'm greedy like that! Lol! Plus, since it had the original 20+ year old tubes in there, I'm pretty sure it's never been biased.
     

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