Safe bias levels

Discussion in 'Amps and Cabs' started by cisspcism, Feb 24, 2009.

  1. cisspcism

    cisspcism Member

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    OK I have asked similiar questions and most of you say do not ever bias an amp where you have 70% plate disapation and not to even do 65% cause I will burn through tubes. Most of you guys say 55% is good.

    However on ted webers site they say 70% is safe, I had a very respected amp builder tell me that on the amp of his I own that 65 to 70% is ok but no more and that he recommends 65%

    this is from webers site
    Bias Settings For Safe Plate Dissipation - Class AB 70% Class A 90%
    Explain This Class
    ABA Tube Type
    EL846K6GT6V66V6GTA6L66L6GC6L6WGB5881EL34576371897591KT66KT8870276CA76BQ5 Plate Voltage
    mA

    who is right and who is wrong. I just want to know what is safe or recommended
     
  2. rockon1

    rockon1 Member

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    55 to 70% or so. Set them where they sound best. Often Ive noticed that cooler sounds better than hotter-or at least there seemed to be no benefit from running them at 70%. That said though if I found that a particular amp and/or set of tubes sounded significantly better at 70 or even 75% I'd set them there! Just means they will burn faster. Bob
     
  3. Blue Strat

    Blue Strat Member

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    There are no absolutes and NO ONE has run tests to check longevity at different settings.

    Cooler gives more tube life. The bias control is NOT and overdrive control. 50% is fine, I recommend keeping them at 65% or less. Use your ear within this range and see what you like the best.
     
  4. cisspcism

    cisspcism Member

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    Thats what I was thinking of doing was leaving everything around 65% and I have had people jump all over me on this forum saying that thats too hot, that I should start at 55% and slowly work up from there and not exceed 65%

    I have no desire to increase gain. I like my gain on my amps as they are and the gain knob is never really past noon on any of my amps.

    So what does a hotter bias get you anyway? I was also told that a hot bias will not show any benefit unless the amp is cranked up loud.

    Honestly with my amps I am just going to check how the bias levels are currently(from the factory) and leave them there, and when I change tubes I will bias them to be at same levels. I am not asking these question in order to do something or make and amp hotter, I just want to make sure I understand whats going on behind the scenes.
     
  5. radioboy950

    radioboy950 Member

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    Lots of people (myself included) love to tweak because they can.
    Some tubes, JJ 6L6 for example, seem to improve with a warmer bias, but may lose a tiny bit of detail. Biased colder, they sounded thin to me.

    You make a good point about factory setup. Presumably, you buy a particular amp because you dig the way it sounds. So, it makes sense that you might use the same settings.
    I bought a Quiana because I like the way the Rivera sounded from the factory. Rivera sets bias at 40-42mA, which is roughly 60%. After trying different tubes and different bias settings, I ALWAYS go back to that 60% because I like the way it sounds.
     
  6. rockon1

    rockon1 Member

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    Seriously -cooler longer tube life. Hotter shorter tube life. Its certainly not going to hurt anything to run them at 65%.
    My cathoded biased V32 runs the tubes at 110%! Yes it goes thru them quick but over two years later the amp is fine. Its a PITA to change the cathode resistor so I havent bothered. Bob
     
  7. Gary Brennan

    Gary Brennan Old cavorting member Gold Supporting Member

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    Can one tell the % dissapation given the plate voltage and bias setting for a particular tube type, or is more info needed. I.E., 6L6 GC, 460 plate v, 30 ma bias setting, does this info tell one the %?

    gb
     
  8. Rosewood

    Rosewood Member

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    Looks like 46%
     
  9. Gary Brennan

    Gary Brennan Old cavorting member Gold Supporting Member

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    What is the calculation proceedure?

    gb
     
  10. FrankieSixxxgun

    FrankieSixxxgun Member

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  11. cliffc8488

    cliffc8488 Member

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    Idle dissipation = V * I / P_max

    For the above P_idle = 460 * 0.03 / 30 = 0.46
     
  12. rockon1

    rockon1 Member

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    Yeah,about 46% idle diassapation. To calulate for a particular setting just divide the tube wattage(30 ) by the plate voltage(460) to get the tubes max dissapation figure(100%) at that voltage then multiply that by the percentage you desire.

    ie: 30/460=.065(65ma) x .46 (46%) =.030 or 30 ma Bob
     
  13. cisspcism

    cisspcism Member

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    tube watts /plate volate = total dissiaption and then you take that toatl multiply by desired dissipation level ie 65% to get mA

    6l6GC are 30 watts so

    30/475= .063 or 63mA is 100% dissipation so 63 * 65% would be 41 mA
     
  14. radioboy950

    radioboy950 Member

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    That is exactly my setup, and I shoot for about 60-65% plate dissipation. Another way to look at it:
    30/460=.065 X .60=39mA.
    30/460=.065 X .65=42mA.

    So, I know I'm good if I've got the tubes between 39-42mA.
     
  15. justonwo

    justonwo Supporting Member

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    Guys, there are no absolutes in plate dissipation. It totally depends on how the amp operates. The 70% rule is just a good general rule of thumb to stay out of the danger zone with a class A/B amp.

    My Rivera sounds best biased at 70%, which is what Paul recommends.

    Randall Aiken explains it best:

    http://www.aikenamps.com/Why70percent.html
     
  16. rockon1

    rockon1 Member

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    :agree My Supersonic sounded better at the factory recommended 33 ma which for it was a 52% idle dissapation setting than higher settings I tried. Bob
     
  17. pgissi

    pgissi Member

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    No one really answered your question-

    A hotter bias = earlier onset of OD for starters but its not that simple, there are other characteristics a hotter bias delivers and Aikens article outlines it nicely-

    The only thing missing in the description above is what colder or hotter bias renders (cold = lower_current/higher negative voltage and hotter = higher_current/lower negative volts) and the tail end of this sentence is whats often missing, is current or voltage being referenced, take note.

    If its current, the more current the hotter the bias. If its voltage, the more negative the voltage (higher negative volts) the colder the bias.

    Hotter Bias = increases power output and distortion (increasing noise also) while reducing headroom and causing the earlier onset of OD is somewhat of a contradiction and is something typically glossed over. Some say a hotter bias reduces headroom and call it a day but thatÂ’s ignoring the increased power output a hotter bias delivers, although slightly it can be audible and is more or less perceived as a tonal change rather than raw spl output unless you went from very cold to very hot, then the output difference may become more discernable given other factors.

    In general a hotter bias can fatten the tone with increased/earlier or
    even apparent onset of clip/od versus a colder setting where the amp may have stayed clean. It depends on the amp and how high of a input signal is used for comparison, for experimenting keep all conditions constant when judging settings.

    So what of this contradiction (with a hotter bias) of increasing power output, although slightly, while causing the earlier onset of OD aka decreasing headroom. All at the same time your reducing the threshold of clipping meaning, "the amount of signal that can be applied before the output clips" is reduced, but yet overall power is increased and an increase in power is technically an increase in headroom although it is offset by the earlier onset of clip/od.

    In essence your amp can be slightly louder and fatter and yet go into clip/od earlier, this is where your ears will need to make the decision.

    The best way to understand this practically is to use the guitar vol pot when comparing a colder bias setting to a hotter. The hotter the bias, the more likely the amp will go into OD with a twist of the guitat vol pot from low to high and with a colder bias, its more likely the amp will stay clean or only go semi-clean/slightly dirty.

    To counter this loss of headroom, increase the amps overall output, roll back your guitar vol pot for a cleaner tone, turn it up for OD. With a hotter bias, you will generally find the pot interacts with the amp more responsively, backing off for clean, turning it up full for OD. With a colder bias this effect is reduced or not apparent at all and from a practical perspective, the volume pot IMO is more useful, can be used to render more subtle nuances of tone and od.

    To really hear the difference of cold versus hotter bias, experiement with a low wattage fixed bias amp which will make the listening tests easier on the ears and since low watt amps have inherently less headroom, as you increase the bias current to make it hotter, it may be easier to hear the difference.

    The higher wattage amps require listening for this difference at loud volumes, so you are right, a 30 watt amp or less is a great output to work with. Once you can hear it there, hearing it at louder levels is eaiser.


    Have my Rivera also set for 70% or just under and I find the hotter bias on this amp preferable. Not all amps respond this way and there are other factors to consider, speakers, musical style, amp and plate volts and even speaker load etc.

    Its all interactive and subject to interpretation, just keep the plates from glowing and avoid setting too cold.

    http://www.webervst.com/tubes/calcbias.htm

    Generally my philosophy is an anemic amp (for OD) is probably biased cold and at least thats a good place to start looking and if it turns out thats not the case, I investigate further.

    As far as plate diss, its true what you say and you may find some 6l6s can handle more, some less and this is true for all tubes. What you want to do there is to make note of the extreme hot bias point where the plates glow cherry red, find it and make note, back it off and find your tone with your ears not a meter and be aware of the cold extreme also.

    In fact for me, I work my way down from hot using my ears, not the dvm and as long as I am not glowing the plates, I dont care if its set at 90%, its the tone that matters. Once I find that spot I make note for future reference.
     

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