Amp biasing questions

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by MikeVB, Feb 13, 2012.

  1. MikeVB

    MikeVB Supporting Member

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    Note: I am fully aware of the dangers in a tube amp, draining caps, etc. Please don't crowd up the thread with the ubiquitous warnings and declarations that if 'you don't already know how to do it you should take it to a qualified tech.' Not that I don't appreciate others concern for my safety.

    I'm trying to learn how to bias my own amps. I've researched a lot of the online sources and forums, but there's some conflicting info out there.

    I've seen and read about doing it with nothing more than a multimeter, but lots of guys, sources, etc. say you have to buy/build a bias probe/bias rite/etc. But I watched a Tube Depot video on YouTube where he used nothing but a mm and screwdriver to bias a Fender HR Deville.

    I've got a good mm, so can I just use it to do my DRRI safely?
     
  2. rdamato

    rdamato Supporting Member

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    I highly recommend a Amp Head or similar probe, which connect through your tube socket. Simply place the probe between you Output tube(s) and the socket, connect to meter and adjust bias screw to correct voltage. I'm not sure where the bias adjustment screw is on your amp, but if it is on the board, take it to a tech and have them install it nest to your output tubes. Then, you'll not have to open the chassis to make adjustments!! I did this to my Mesa F-50 and it cost me $50.00 total. Worth every penny.

    http://www.amp-head.com/
     
  3. MikeVB

    MikeVB Supporting Member

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    Thanks, I'll check out the Amp Head probe.

    The DRRI has a bias adjustment screw on the bottom of the chassis.
     
  4. phsyconoodler

    phsyconoodler Member

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    you have to remove the chassis,plug it into a speaker cabinet.Then you locate the output transformer center tap.Take your multi-meter and put in on ohms.Measure between the OT center tap and pin 3 of one power tube.Record that reading.Then do the other tube,pin 3 as well.Then fire the amp up and put your muti-meter to DC volts.Measure between the same two points you just did the ohms measurement on.Record that for each tube.
    Then get out your calculator and divide the volys into the ohms to get the current in milliamps.
    e.g. say you get 47 ohms on one leg and 49 ohms on the other and about 1.5v on each leg. 1.5v / 49 = .0306 or 30ma.

    Or.....you could remove the grounds to pin 8 and replace them with a 2 watt/1 ohm 1% resistor.then simply take the mv readings and they convert directly to ma.
     
  5. big mike

    big mike Moderator - EL34 Emeritas Staff Member

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    Makes life SOO much easier.
    Much less voltage going through the DMM too.
     
  6. phsyconoodler

    phsyconoodler Member

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    1.5v isn't a lot either.The disadvantage to the 1 ohm resistors is you need to put ports on the outside of the amp or it's no easier to bias than before.
     
  7. big mike

    big mike Moderator - EL34 Emeritas Staff Member

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    True..unfortunately that's where the bias pots are on my 2 builds too.
    Must upgrade to a 10 turn locking pot at some point.

    Isn't the method you described above teh 'shunt' method? I just assumed.
    I never bothered to investigate that method since I installed the 1 ohm resistors to ground right off.

    one of these days I''ll buy an amp head just to make life easy.
     
  8. pdf64

    pdf64 Member

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    1/2 watt is well over twice the max foreseeable power that a functioning 6L6GC could cause it to dissipate, nevermind a 6V6?
    2 watt would make it bomb proof. However there's an alternative school of thought that would have resistors fuse when passing fault currents, rather than wait for the line fuse to act.
     
  9. phsyconoodler

    phsyconoodler Member

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    Quote:"However there's an alternative school of thought that would have resistors fuse when passing fault currents, rather than wait for the line fuse to act."

    Or just install an HT fuse.Or just use the correct value mains fuse.Fender never needed it.I don't think I've ever seen enough current even with a blown tube to fry a cathode resistor.Never seen a burnt cathode wire.Not ever.
     
  10. MikeVB

    MikeVB Supporting Member

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    Which is pretty much what I think I've decided as well.
     
  11. RupertB

    RupertB Supporting Member

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  12. Blue Strat

    Blue Strat Member

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    Bias probe like Amp Head, Compu Bias, etc. You won't even have to pull the chassis no less install sensing resistors or test points. Bias pot is accessible from under the chassis.
     
    Last edited: Feb 14, 2012
  13. TweeDLX

    TweeDLX Member

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    Drill 3 small holes (two for the jacks, onr for the screw). the bias pot can be mounted in an ext. Speaker jack hole (if you don't use it) or the Ground switch hole on an old Fender. If those aren't available drill through the BOTTOM of the chassis so it won't be accidentally bumped or moved.

    [​IMG]
     
  14. big mike

    big mike Moderator - EL34 Emeritas Staff Member

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    Wouldn't it be better to use the 3 jack method, tying the center there, as a black test probe to ground, and each of the others testing the bias for the respective tube?

    If your screw in the chassis were a grounded test point, both other probes red test points, are how I've seen it done before. Never seen it like you have it.

    Am I making sense? LOL
     
  15. big mike

    big mike Moderator - EL34 Emeritas Staff Member

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    Wait I get it.

    Really, both test jacks in the above can be seen as 'positive'.
    You're just grounding to the chassis instead.
     
  16. TweeDLX

    TweeDLX Member

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    Hi Mike,
    You could do a 3 jack set-up, but to my mind, it's just extra work. Not necessarily "better". I just touch my black probe to the chassis when I check bias. With the red probe in the test jack, that still leaves me one free hand to adjust the bias pot. Of course, you may want an extra free hand to hold your beer... :)
     
  17. wyatt

    wyatt Member

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    You all beat me to it. I need to type faster.

    TweeDLX's pic needs an adjustment, both Test points should be red for reference (the color of the jack used obviously doesn't alter it's function, but it helps identify it for others).

    The adding a ground (black) test point is handy (especially on a painted/powder-coated chassis), but is superfluous. You insert the red probe in either tube's test point and touch the black probe to any point on the chassis where it's grounded, exposed screws,nuts are great places (including the screw used in TweeDLX's drawing).
     
  18. TweeDLX

    TweeDLX Member

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    I use the two colors to differentiate between tubes. That way, I know which tube is drawing less/more current, having a bad day, etc... Really it doesn't matter what color they are. Whatever works for you. I merely copied what Dave Allen does.
     
  19. big mike

    big mike Moderator - EL34 Emeritas Staff Member

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    Yeah the color is what threw me for a loop. LOL

    No beer while biasing. I get nuts and crank it up to watch the pretty red plates glow....

    ;)
     

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