How To Bias an Amplifier ??? and Why ???

Discussion in 'The Squawk Box' started by BOBBO, Mar 11, 2008.

  1. BOBBO

    BOBBO Banned

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    I'm not very good with electronics ... So maybe someone with knowledge with Amps can answer a question ? How do you set the bias adjustment ? and what exactly does the bias control do ? I assume it adjusts the plate voltage so you can run your amp either with the tubes running cooler or hotter ?? Is the only tool required a bias probe ??? I'm currently running a Marshall 50 watt JCM800 combo with EL34's .... 2 x EL34 .... 3 x ECC83 .......
     
  2. Mike60

    Mike60 V.I.P. Member

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    my understanding is you only bias an amp with four power tubes not two power tubes...however,unless you really know what you are doing take it to an amp guy if you are experiencing any problems
     
  3. loverocker

    loverocker Member

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    Wrong.

    Bias - in some ways - is like the 'idle' setting on a car's engine. It sets the amount of current running through the amp when there's no guitar signal (i.e. you're not pressing the accelerator pedal).

    The plate voltage stays the same, and it's a matter of either:

    1) adjusting a negative voltage applied to the control grid (the same electrode the guitar signal is connected to) - this is counter-intuitively called a 'fixed-bias' design, although it's almost always adjustable by one or more preset pots. This covers 50W and 100W Marshalls, most >25W Fenders, etc.

    2) using a resistor in the connection between the power valve's cathode(s) and ground to set a voltage difference between control grid and cathode. This arrangement is called cathode bias and is almost never adjustable. It covers Fender Champs, Epi Valve Junior, Marshall 18W.

    So if your amp is in Group 2, you don't adjust anything.

    If it's in Group 1 and it has an adjustment (a few don't), you need the tools to measure current through the valve at idle. In most cases, that requires a 'Bias Probe' and a multimeter and a screwdriver to adjust the preset pot. Plus a knowledge of basic in-amp safety and valve dissipation calcs. See Aiken Amplification and head for the technical section.
     
  4. dwagar

    dwagar V.I.P. Member

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    as Loverocker described above, Bobbo, when you change your EL34's, you have to set the bias on your JCM800.

    When I was going to change mine, I bought a Bias Probe (I already have a couple multimeters around), it's not hard to do, just follow the instructions. I bought the probe that has one setting to read plate voltage, and the other to read the bias.

    And be extremely careful, there's enough voltage in there to kill you. Even if it's unplugged.

    and when you pull the amp out of the combo, remember you have to have a speaker hooked up, don't run it without a speaker load.
     
  5. drewbertca

    drewbertca V.I.P. Member

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    voltage isn't what kills you.....it's amperage that does.....otherwise static shocks would claim far more lives than it does now....but in any case be careful or take the amp to a tech.....
    but in dwagar's defense the higher the voltage over a load (your body in the dangerous case) the greater the current through it...
     
  6. b-squared

    b-squared Banned

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    My JCM2000 had a nifty spot on the back panel for the probes, and an adjustment screw to set the bias...never easier. You're only biasing the power tubes, btw.

    Older amps generally have be done by a tech unless you know what you're doing. It never hurts to have a tech give your amp a general checkup.

    My Vox AC30 requires no biasing...load and go. :)

    BB
     
  7. PraXis

    PraXis V.I.P. Member

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    so that means you don't bias the 12AX7's, just the 6L6, EL34's, etc?
     
  8. Shawn Fate

    Shawn Fate Senior Member

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    Correct, you do not need to bias preamp tubes, only power tubes.
    BTW get a matched set.
     
  9. 5F6-A

    5F6-A Senior Member

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    and a matched-triode phase inverter!
     
  10. Mikeeeeh

    Mikeeeeh Senior Member

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  11. PraXis

    PraXis V.I.P. Member

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    I just got my amp in december and it's fairly light-medium use.. once I feel they're dying out, I think I will go for the high gain option from eurotubes.. I do have a small gig at the end of this month, but I'm not worried since I'm doing keyboard and guitar work.
     
  12. loverocker

    loverocker Member

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    Unfortunately, that'd be a real waste of money. Because nothing else in the two sides of the power amp is matched. Not even close. So any potential benefit to it (hi-fi?) would be wiped out by the other components.
     
  13. mrkenny

    mrkenny Senior Member

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    I'm not a wiz when it comes to amp stuff but I do understand Biasing an amp. I have an amp tech friend, I had him install a bias pot in all my amps. When I want to check/change power tubes I always use the meter (ordered from Allesandro). My Meter has tow sockets and corresponding VU meters. Remove the tubes, and plug the sockets in the amp then re insert the tubes in the sockets. You now have the bias meter in between the amp and tubes. Turn on the amp and the VU meters will rise to the appropriate "Ma" reading. A matched set of tubes will read within 1-2 Ma of each other. My Bassman's 5881's should be set between 31-35 Ma, I set them at 32. If the tubes you are installing read to hot or weak just take a small screwdriver and turn the adjustment screw on the bias pot clockwise or counterclockwise to bring up or reduce the current, very simple. Like stated in previous posts, biasing is like tuning up you car.
     
  14. gem12

    gem12 Senior Member

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    Watch this one from youtube:
    YouTube - How to Change Tubes on Vacuum Tube Amplifiers : What is Biasing in Tube Amps?
     
  15. guitarstan

    guitarstan Senior Member

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    There is some good advice in these posts.

    Check out this web site, these guys have done a ton of homework to put together a lot of info in one place all about guitar amps:

    index2
     
  16. PraXis

    PraXis V.I.P. Member

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    Ugh... the high gain option from eurotubes is 170... so with shipping that's closer to 200... I'm terrified of getting shocked (I don't like to open things up).. so that means I have to find an amp tech.. how much would someone charge to change my tubes if I provide the tubes? If this is going to cost like 300.. that's half of what the amp cost!
     
  17. tiedstick

    tiedstick V.I.P. Member V.I.P. Member

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    It makes things easier but isn't always necessary. :)
     
  18. tiedstick

    tiedstick V.I.P. Member V.I.P. Member

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    You just have to discharge the capacitors...the residual voltages or whatever. If you don't know how to do this with tools (neither do I :D) you can leave your amp unplugged for a couple of days. Just turning it off won't do anything.

    At this point, I'll let the pros correct me or take over, but I don't think its should be anywhere near over $100.
     
  19. PraXis

    PraXis V.I.P. Member

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    But don't you have to turn on the amp and have it plugged into a speaker WHILE you are biasing?
     
  20. dwagar

    dwagar V.I.P. Member

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    yes you do, you just have to be careful.

    I didn't feel comfortable using a multimeter to read the plate voltage, so I bought a Bias Tester that reads plate voltage on one setting, bias on the other setting.

    The only time you are really inside the amp and have to be very careful is when you are adjusting the bias pot.

    The other thing, if you don't think you're going to be doing this very often, you can probably pay a tech about the same or less than the cost of a bias tester.
     

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