Adjusting bias (particularly Fender BF)

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by mbratch, Mar 11, 2006.


  1. mbratch

    mbratch Member

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    I've been doing a lot of reading on biasing and I'm finding that there are lots of different statements regarding what to set as a bias point. In particular, for 6V6 power tubes (used in smaller Fender BF amps). I read in one place that a "safe point" for 6V6 tubes is about 27mA, and it can be raised from there by using your ears (sound) and eyes (watching for too much tube glow while playing) to taste. Another place I read a safe point for these tubes was 35mA.

    My understanding (so far anyway) is that the plate wattage absolutely should not exceed the spec for the tube. So a 6V6, being 12 watts max plate power dissipation (at least in most cases?) in a Fender BF (measured at approximately 420V plate voltage) to me means I would never want to exceed 12/420 = 0.0286A = 28.6mA bias current in this amp. Is this correct?
     
  2. RussB

    RussB low rent hobbyist Silver Supporting Member

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    Have a look at the Weber website. Go to the tubes forum, and check out the neat bias calculators he has there.
     
  3. Dave C

    Dave C Gold Supporting Member

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    Mark your numbers are correct but keep in mind that is MAX dissipation. You want to set idle at 60-70% of that figure. 70% would be about 20ma , 60% would be about 17ma. Hope this helps.
    Dave C
     
  4. phsyconoodler

    phsyconoodler Member

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    In Gerald Webers' book,he recommends bias for a 6V6 in a Deluxe Reverb at 30ma.The idea is the voltage in a DL is around 425v +.As the current draw goes up the voltage comes down to a more acceptable level for the 6V6.If the amp is biased too cold,the voltage goas up too high.
    I have tried several bias points with the Deluxe Reverb and they sound pretty weak and shrill biased at 17ma.
    It requires the use of a very robust 6V6 tube to handle the high voltage and the high current in a Deluxe Reverb.JJ's fit the bill nicely.I have biased as high as 35ma in some and there have been no tube failures to date.
    Remember,this is only an opinion based on an article in GW's book.I have had good luck with it,but only using JJ tubes.Some current production tubes may not be able to handle it.
     
  5. mbratch

    mbratch Member

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    Now you see my quandry. I've read different answeres in different places.
     
  6. Wakarusa

    Wakarusa Member

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    I'd argue that the advice attributed to Weber absolutely depends on a robust tube. In my experience tubes are much more tolerant of overvoltage than excessive dissipation, so the argument that a colder bias allows voltage to stay too high combined with a hotter bias that relies on power supply sag is, IMHO, bunk and a sure-fire way to find the bad tubes in a production run.

    Also, although I try to play nice, once and for all -- anyone who suggests a bias method that uses red/orange glowing plates as one of the measurement metrics is a fool. Period.

    This might also be a good place to quote the GE Essential Characteristics manual's discussion of Absolute-Maximum Rating System. Basically, Absolute Max (in GE's view of the world) is based on the "worst probable combination of conditions with respect to supply voltage, signal, temperature, component values, adjustment of controls, and other variables". That doesn't mean that you can ignore the rating, but it does mean that you can cheat if you know what you're doing and know something about the circuit, the tubes, and the expected operating conditions.

    So, all else being equal, the 60%-70% is a good rule of thumb for a maximum static dissipation. Note the term "maximum" is used and not "target". The target is wherever the amp sounds good at or below 60%-70% of max dissipation.

    About those JJ's -- and this doesn't apply to any other 6V6 tube -- I am amazed at the abuse these will take. Pretty sure I've posted elsewhere where I've run these things at ridiculous voltages and bias points (including some that would melt down some current production 6L6GC). They sound good too. So if you're going to beat up a 6V6, this is a good tube to choose. As long as you keep in mind that JJ may change production specs at some point in the future resulting in a tube that actually operates to 6V6 spec.
     
  7. mbratch

    mbratch Member

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    Good point. The set in my BFDR are modern Russian Tung-Sol 6V6GT. The spec sheet I found says 12W. The set in my BF Princeton are Raytheon 6V6GTA (made in England), but I haven't found a spec sheet for these.

    The responses to the thread represent why I posted my initial question. I've read a lot of different methods/ideas on biasing and was getting very confused regarding what is really the right thing to do.

    So if I'm reading all this right, I should adjust the bias current so that the plate power dissipation (calculated by Plate Voltage X plate current) WOULD NOT EXCEED 12W for a 6V6GT or 14W for a 6V6GTA. And the current measured at the cathode is an approximation to the plate current (because of screen current). With a plate voltage of about 420V in the BF amps I have, that puts me back at the roughly 17ma number, which I'm also understanding here to be "bad sound" for a Deluxe Reverb. But if I use JJ's I can go over the fairly common 12W plate dissipation max (?).
     
  8. Wakarusa

    Wakarusa Member

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    Not quite...

    Your DR is intended to be biased for class AB1 operation which means that maximum dissipation occurs with signal applied (as opposed to class A bias where maximum dissipation occurs with no signal applied). So, for your typical audio class AB1 amp, you'd bias for idle static disspation (how much power with no signal applied) not to exceed 70% of published maximum.

    So if your 6V6 has a maximum dissipation of 12W, your 70% not to exceed dissipation is 8.4W. 420V * 20mA = 8.4W so 20mA is your upper limit using the 70% rule for this tube under these conditions. For a 14W max dissipation and 420V, 23-1/3mA is your 70% upper limit.

    Some folks make a good case for a 64% limit instead of 70%, but I'm not sure those arguments take screen current into account, so I tend to use the 70% as a max.

    It is always a good idea to first check line voltage before biasing an amp. You should stick your multimeter in the wall socket every now and again to get a feel for what "standard" line voltage is in your area. If you happen to bias your amp at a time when line voltage is unusually low (say you're doing this in the kitchen with the amp plugged into the same circuit as the refrigerator and the frig's compressor is running), then you'll be above spec when the voltage comes back up. (FWIW, I use an autotransformer on the bench to guarantee line voltage).

    Whether it sounds crappy when you're done is highly dependent on your amp, your tubes, your ears, and your personal taste.

    If you install JJ's and intentionally bias them past published limits the warranty void light will come on. They will, more than likely, be fine. They may or may not sound good or better to you.

    I have no experience with the Russian Tung-Sol branded 6V6, but think the Sovtek smoked bottle 6V6 is junk.
     
  9. mbratch

    mbratch Member

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