biasing question

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by Greggy, Dec 31, 2004.


  1. Greggy

    Greggy Member

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    Got a question about biasing my Soul-O 45 after replacing the existing Mesa 6l6s with GT 6l6Bs. Here is a copy of an email I sent to GT which has not been answered yet. Any help will be appreciated. Here it is:

    <<Hi,

    I'll get straight to the point. I own a Soul-O 45. The 6l6s recently went out on me. I replaced with a slightly used set of GT 6l6Bs I had sitting around the house. I have a copy of a tech service bulletin (mid to late 90s apparently) that describes how to attach a DVM to the second speaker jack and measure current draw on both output tubes. I did this and was reading around 29 mv on both tubes. I goofed around with the bias pot and ended setting current draw around 35 mv.

    What I need to know is what are the current draw parameters for this amp and these output tubes? I don't want to under or over bias the amp and cause problems later. Also, any charts or other documents that could guide me with respect to other output tubes or amps would be appreciated.

    Thanks for your time.>>

    EDIT: the GT tubes are rated with a number 7 on a sticker attached to the tube base. "Test 7, Rate IBL".
     
  2. Greggy

    Greggy Member

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    OK did some basic research and noticed that 30 mv or so is the norm for fender circuits and 6l6s (current draw at idle). I rebiased and using my ears as best I could arrived at 31.5 mv as the best setting for dynamic cleans and smooth compressed hi gain with pedals (fdII, Keeley BD2, and Barber DDSS).

    Am I missing anything? I'm a newbie when it comes to biasing.

    PS-I noticed that most texts measure bias in terms of ma, not mv. But the GT materials are clear that using their bias probe method you are to set the DVM to mv and measure the voltages. What's up with that?
     
  3. hasserl

    hasserl Member

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    You set the meter for mv because the probe the instructions are for uses a 1ohm resistor in the cathode circuit. Using Ohm's Law 1mv across 1ohm = 1ma.

    When setting ther bias on an amp you need to do a little math to determine the target current draw. This means measuring the voltage at the plates of the tubes then mutilplying the current thru the tube to determine the watts dissipated by the plates. Each type of tube has a max rating for plate dissipation (6L6GC's are generally rated at 30 watts max). In class AB1 amps you will typically want to set the bias to no more than 70% of max rated dissipation. For 6L6GC's that means 21 watts plate disspation (30 * 70% = 21) So for the 31.5ma you set the bias at your plate voltage could be greater than 500vdc without reaching the 70% of max dissipation for 6L6GC's. I don't know what the plate voltage is on your amp, but I doubt that it is 500vdc. The point is you need to check it and do the math. On the other hand, if it sounds good to you, that's all that really counts, and with the somewhat cold bias you'll get longer tube life.

    One other point to make is that when reading cathode current using a probe like you mentioned, you really need to subtract the screen current to get the true plate dissipation. Many folks prefer to just skip this step and leave the screen current in the equation as a small safety factor. It's all up to you. Your the one paying for the tubes, and your the one playing the amp. You can run the tubes hot if you like the tone and sacrifice some tube life, or run 'em cold for longer tube life if you like the tone that way.

    BTW, I haven't the foggiest idea how you can measure the bias current thru the ext speaker jack! That's a new one on me. I can't see how that can possibly be true. Where did you get that service bulletin from?
     
  4. Greggy

    Greggy Member

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    Thanks for the info.

    I'm not certain either. What I can tell you is that the extension speaker output jack is a stereo jack, and the bias probe wires I attach to my DMV has a 1/4" phone stereo jack on the end that plugs into the amp. The DMV + wire is soldered to the ring and the - wire is soldered to the sleeve (tip is left empty). Hope that explains it, I might open the chassis and see how they wired it.
     
  5. Greggy

    Greggy Member

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    Let's get more concrete here so I can understand.

    Let's say I buy a Weber biasrite. I determine the plate voltage is 460 vdc on each tube. I also determine that the current draw at idle is 32 mv = 32 ma (given a 1 ohm resistor, or ohm's law).

    First, how do I determine the "current" dissapation wattage for each tube from this info?

    Where are the "maximum" dissapation wattages listed for all types of tubes (especially 6l6 and el34 types and makes)?

    What is the formula for deriving current dissapation based on plate voltage and current (and by current I assume we are talking about mv = ma per above, right?).

    So if I understand, after determining tube dissapation given current plate voltage and current, I can adjust dissapation upwards or downwards by altering current via the bias pot until the dissapation reaches approx. 70% of max dissapation for a given tube. Right?

    Any formulas will be appreciated. Mucho thanks, again.
     
  6. Greggy

    Greggy Member

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    Glad you mentioned that. I forgot to say above that each tube can be measured separately by switching the speaker ohms switch from 4 ohms (left tube) to 16 ohms (right tube). This works of course when the stereo plug is attached to the ext. speaker jack. It's a cool feature for sure.

    Anyway, I found this link to a tube comparison chart with much of the typical operating conditions data for tubes and amp circuits (push pull, class A, AB1, etc.).

    http://www.pmillett.addr.com/tubedata/HB-3/masterindex.html

    Gonna go study these.
     
  7. hasserl

    hasserl Member

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    So those are GT instructions for a GT tool meant to be used on that GT amp. Okay, that makes sense I guess. Sounds like a clever option. Especially that impeadance switch that doubles as the cathode circuit switch for the bias probe.

    Also, that was a cool link, but it is a little difficult to find the correct info. Another source that I use alot is Franks Tube pages Just click on the Search link then type in the name of the tube you want the data sheet for. Save them in a folder on your hard drive for future reference.

    [What is the formula for deriving current dissapation based on plate voltage and current (and by current I assume we are talking about mv = ma per above, right?).

    That depends on the probe you use. Some measure cathode current and some measure plate current. If the probe measure cathode current you set your meter to read mv, if it measures plate current you set your meter to read ma. (The Weber unit measures cathode current) You need to know which type of probe you are using. But, once you know that you can use the formula: I * E = P. Where I is the plate current, and E is the plate voltage. In a fixed bias amp this would be the voltage between the plate and ground (in a cathode biased amp it would be the voltage betweeen the plate and the cathode).

    Also, as I mentioned before, when you are measuring cathode current, that includes the screen current. If you want to get the actual plate current draw you need to subtract the screen current. You don't have to do this of course, as if you just go with the cathode current figure it leaves a bit of a safety margin in that the actual plate dissipation will be slightly less than what your figures show. But if you do want to measure the screen current and subtract it to get the actual plate current, you can do this by measuring the voltage drop across the screen grid resistors (if equipped) and divide that figure by the value of the resistor. That will give you the current across it (again Ohms Law: E / R = I) which will be the screen current draw of the tube.

    Hope that helps,

    Hasse
     
  8. Greggy

    Greggy Member

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    Excellent. Thanks again for the link and info.
     
  9. Greggy

    Greggy Member

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    So a concrete example:

    Let's say I have 44 ma/mv current and 360 plate voltage. Wattage dissipation is then: .044 x 360 = 15.84. Is that correct?
     
  10. hasserl

    hasserl Member

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    Yep, that's correct. As you can see, you're not pushing those 6L6's hard at all. They're loafing at that setting.

    After my last post I rembered this worjsheet available from Duncan Monroe: Duncan's Amps go to this link and click on the Anode Load Calculator link and download his Excel worksheet. Then just put the vlaues in the corrct cells and it will give you some usefull info on current draw.
     
  11. Greggy

    Greggy Member

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  12. hasserl

    hasserl Member

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    Greg, you've got to measure it, you can't go by some old Guitar Player review. One of Weber's bias probes will also let you measure the plate voltage, maybe that would be a good iinvestment.

    Question, how is the bias adjustment on that amp made? via an externally accessible pot? If so, then the Weber probe would be just what you need. Measure the plate voltage, the cathode current, make your calculations and adjust, recheck and readjust as neccessary. Play the amp while your doing this to listen for the changes in tone (though measurements are always made at idle), and finally make sure the tubes are not red-plating when you're done!
     
  13. Greggy

    Greggy Member

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    If I buy the biasrite, I will get the option for measuring plate voltage. And the bias pot is on the outside of the chassis. Unfortunately, not so on my Rivera M60. Thanks.
     
  14. TheAmpNerd

    TheAmpNerd Member

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    Only make sure your playing is red hot!

    Naught the tubes dude...

    r a w k o n

    :dude
     

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