gettin your amp biased: why and how?

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by iamthebruce, Jan 2, 2006.


  1. iamthebruce

    iamthebruce Member

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    what's up yall. a friend refered me to this forum. i have a 5150 II that i bought used a long while ago. Now, im thrilled with the sound i get from my cab (marshall 1960a) at low and high volumes. however, after going through this forum, i keep reading about biasing the amp. I've never really even thought about it. I see it has the test points and a little bias pot right behind the grill.

    First. why should i do this, and what should i look/listen for?

    Second. how do i do it?
     
  2. number9

    number9 Member

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    There are so many opinions on "how to bias correctly" or "which method is best" that I tend to stay away from the "this is how to do it" answer... I will, however, say the following:

    You need to at least check the bias every once in a while, and I strongly recommend that you do it every time you change the output tubes (if you are dealing with an amp that bas an adjustable bias).
    Of course, even opinions on that vary from tech to tech or person to person. I will just answer the question: What is bias? Every tube/transistor (FET, MOS, BJT, whatever) operates at a certain point along its characteristic curve for the given voltages "around" the device (plate/cathode/grid, or drain/souce/gate etc). This point is chosen by the engineer to yield a certain outcome (symmertrical or asymmertical output, a certain peak to peak voltage, a certain maximum current flow, etc) for a given input. The "bias" is just that... lets go to tubes, after choosing a tube, a plate voltage is chosen, as is the max current through the tube. To operate the tube in this region, a point has to be chosen along this "line" which is the load line drawn on the characteristic curves. To get the tube to operate in this region it needs to be biased there so that at idle the current draw is the chosen point, which will yield the aforementioned outcomes the designer was shooting for (we hope). Keep in mind, I left about a million things out here. No need to flame me for not "telling the whole truth" or calling me an idiot since I left out a million things, I am just being brief.

    Ok, now that that is out of the way, still with me? The bias on the output tubes is typically
    read by checking the current through the tube at idle, or the bias voltage applied at the grid. Or about 100 other methods really... I can say this: choose a method that works well (all of them will get you close with the notable exception of the "listen to it" or "turn it up until it glows red then back off" methods, which tell you nothing about the actual point that you have it at (OTOH, if you have a bias balance (aka old Fender BF, Heathkits, etc) then you DO actually listen, but in reality the bias voltage is not changing, just the amount that you have on each tube)...

    I suggest reading this long-explination: http://www.aikenamps.com/Biasing.html
    And then perhaps looking at this nifty chart if your amp did not come with some chart/page that tells you the optimum point to put the bias at:

    tube typical bias range, just a start, depends on plate voltage and mode really,
    this would be AB1 in push pull.
    6V6 22 - 27
    EL84/6BQ5 22 - 27
    6L6, 5881, etc 30 - 35
    EL34/6CA7 35 - 40
    6550 40 - 50

    As always, don't just take my word for it. I could be a madman behind a keyboard.
    Ask a reputable good tech, or read up in an EE book, or go to Aikens site, he has a
    good habit of putting the real deal out there and letting you draw your own conclusions.

    Just my $0.02.
     
  3. iamthebruce

    iamthebruce Member

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    thanks man. this definitely helps me out a little. i didn't wanna start turning knobs and stick **** in places trying to figure it out myself. im gonna start lookin in to this further.

    thanks bro.
     
  4. hasserl

    hasserl Member

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  5. scottosan

    scottosan Supporting Member

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    In the case of your amp, those test point are of very little help. All tubes draw variosly different amounts of current. You want to be able to measure the amount of current in relation to the plate voltage. The test points on the Peavey stuff measure the bias voltge, not the current. This is useless in my opinon unless every tube pulled the exact amount of current at a given voltage. But hey, I'm sure the marketing guys are high-fivng
     
  6. rockstrongo

    rockstrongo Member

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    Correct me if I'm wrong, but aren't these usually the other way around? I thought 6L6's required more voltage than EL34's.
     
  7. Swarty

    Swarty Member

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    His numbers must be refering to mA, however without a specified plate voltage I'm not sure of the reference.
     
  8. Guinness Lad

    Guinness Lad Silver Supporting Member

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    I believe you are incorrect here. You can still set your bias within the specifications called out by the manufacturer. If the company states between x and x voltage they know the proper voltage range which will keep the tubes in a safe operating range without either damaging the tubes or the amp. For example; if Peavey knows at this point in the circuit if the resistor measures x volts they will know the bias is at x current, the question really should be how close is the voltage reference in relation to the tubes. The reason this is probably done is to provide a safe way of setting the bias safely, even an unexperienced person could do it without worry.

    In regards to every tube pulling the exact same amount it is highly unlikely that a matched set of 4 tubes will measure exactly the same. I had a matched Quad in my Guytron and they ranged from 30 ma to 35 ma the amp sounded fine and there were no problems with the amp or the tubes. My replacement tubes which were matched within 2% came in between 31 and 33. I asked Guy Hedrick (Guytron amp builder) about this difference and he said, "These values are shared because two tubes are accounting for the total sign wave so realistically this difference is even less then measured and it would be impossible to hear any tonal difference." I hope I didn't basterdize he comments too much but that was the general tone of his statement. Tubes drift and unfortunately most amps have only one bias resistor which controls all the output tubes. I would only worry if one tube was running at 25 ma and the other was around 40, then I'd think I needed new tubes or had another problem with the amp.
     
  9. JerryP

    JerryP Member

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    The test points on the back of the 5150II are totally useless for telling you anything about how the tubes are biased. You'll need a bias probe of some type or a scope to properly adjust it. The way Peavey built the amp they have the bias range set cold so you can pop in a set of tubes and adjust the pot till it sounds best to you. They have the bias range cold enough so you usually won't run into trouble. I have some info about why to bias in my FAQ section http://www.fjamods.com
    Jerry
     
  10. scottosan

    scottosan Supporting Member

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    NO, I'm quite confident that I am correct. Plate/Screen voltage and current draw are what is required to correctly bias an amp. Test points that measure bias voltge is not part of the equation. See JerryP's post above this one.
     

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