Haze15 Bias Noob - Check My Math?

Discussion in 'The Workbench' started by fitz288, Jul 4, 2021.

  1. fitz288

    fitz288 Well-Known Yinzer Silver Supporting Member

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    Hey yinz guys, spent my day putting some new 6V6 tubes in my Haze15.
    Got one of those Eurotubes Pro One bias probes that reads plate voltage and mA.
    Also got the basic probe socket to hook into my multi-meter to read the mA on both tubes at the same time.

    Here's some pics of old & new tubes on the meters.
    old tubes.jpg new tubes.jpg

    Using an old tread from @Micky Marshall Haze 15 - Everything you wanted to know... | MarshallForum.com
    I put together some calculometry to adjust the bias on the new tubes.
    (cells with the border are calculated from the meter readings)

    upload_2021-7-4_17-42-23.png

    If my math is correct, the old tubes were running at about 68%, and the new tubes are running a little hot at 84%?
    Playing through the amp, I have noticed less headroom on the normal clean channel.
    (the JJ's are taller, so... jk)
    The new tubes seem fairly balanced, I don't really use the clean channel cranked, and have not made and adjustments yet.

    So - Does my math make sense, and should I back the bias down to around 75%, or a little under 15mA per side?

    Thanks for any suggestions.
     
  2. Pete Farrington

    Pete Farrington Well-Known Member

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    Where have you got 7 Watts from?
    6V6 have a design centre anode dissipation rating of 12W.
    So in AB fixed bias, idle at about a max of about 70% of that.
    I suggest to aim for a max idle anode dissipation of about 8.5W.
    So up to 8.5 / 350 (anode voltage) = 24mA (anode current).

    Regarding the exact readings you happen to get, note that everything will track mains voltage at your wall outlet. As that goes up and down, anode voltage and current will follow likewise.
     
    Last edited: Jul 4, 2021
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  3. fitz288

    fitz288 Well-Known Yinzer Silver Supporting Member

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    I got the 7 from the thread by Micky.
    I think the logic was the tubes are rated at 14w each, but the amp is only a total of 15w divided by 2 tubes?
    And the reading I got of the old tubes seemed to coincide with the reading from the thread, but that's why I ask.
    If I put your 8.5w / 350v in my calc, I get the new tubes at about 68%?

    upload_2021-7-4_18-36-46.png
     
  4. Pete Farrington

    Pete Farrington Well-Known Member

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    That rationale doesn’t make sense to me, sorry.
    If Micky is the designer of the amp then fine, do as he advises. But as a generic approach, nah, it seems topsy turvey.

    No, I’m suggesting that 8.5W should be your upper limit of idle anode dissipation.
    Not a percentage of 8.5W.
    ie the idle anode current should be somewhere between 0 and 24mA. Though realistically it may not sound happy below 10mA.
     
    Last edited: Jul 4, 2021
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  5. fitz288

    fitz288 Well-Known Yinzer Silver Supporting Member

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    I had a little trouble following everything in that thread, but I'm not discounting my own ineptitude.
    Aside from the math and all this voodoo electrical mumbo-jumbo, I've been tweaking it while playing and kinda like the headroom on the clean channel with the mA set around 13.
    Similar to readings from the old tubes, and it seems to give me good clean tone up to a little past noon, and then starts to break up like a classic Marshall cranked amp tone.
    I probably should have just used that as a baseline in the first place to set the new tubes.
    The overdrive channel still sounds fantastic with the gain about noon with plenty to spare, but I've always loved this amp's tone - my 1st tube Marshall.
    Thanks for the input Pete.
     
  6. Pete Farrington

    Pete Farrington Well-Known Member

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    If you’re happy with it, that’s what matters!
    And at a low idle dissipation, you should get a long service life from the 6V6.
     
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  7. Gutch220

    Gutch220 Well-Known Member

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    The JJ website says their 6V6 tubes have a max watt dissipation of 14 watts (push/pull)

    so 14/356 = 39mA max

    39mA x 0.7 = 27.3 mA idle current

    am I missing something here?
     
    Last edited: Jul 4, 2021
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  8. fitz288

    fitz288 Well-Known Yinzer Silver Supporting Member

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    I think the 2 x 14w tubes (28w total?) in a 15w amp (OT power?) has something to do with the mA reading being about half of what is a "normal" calculation.
    I was trying to do the math in an attempt to learn a little of this stuff, but I'm about as confused as when I started.
     
  9. thetragichero

    thetragichero Well-Known Member

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    6v6gt is a 12w maximum plate dissipation tube (have not measured or anything with jj 6v6gt but they appear larger and seem to have a reputation as being 'a bit more' than a regular 6v6gt). they still bias the same. the limiting factor(s) in your 15w amp are the power/output transformer(s) (likely both) and that is how you won't get 24w or whatever from it. you still bias it at 55% or 60% or 70% or 69% or whatever your magic number is (there is no 'right' answer: if it sounds good at 50% or 80% it IS good, although tube life may be effected). i don't know how these fancy bias tester doohickeys work (i could take measurements and probably figure it out but i'm much to frugal to spend the money on one). i throw 1ohm 1% resistors from cathode to ground in my fixed bias amps and measure the voltage drop, plug in all the relevant information in this handy dandy calculator: https://robrobinette.com/Tube_Bias_Calculator.htm
     
  10. Pete Farrington

    Pete Farrington Well-Known Member

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  11. Gutch220

    Gutch220 Well-Known Member

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    Use the single-tube value. For example, 25-watt for EL34's (100w amp with four power tubes), you'd still use 25w for a 50w amp with two EL34 power tubes.
    The JJ spec page says 14w max, so that's what I'd use. The reason you had much less headroom is likely because you were running the amp super cold, like 35% or something.

    *remember to turn all of your knobs down to zero when doing these measurements.
     
    Last edited: Jul 5, 2021
  12. Pete Farrington

    Pete Farrington Well-Known Member

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    ‘Headroom’ is one of those technical sounding words that often gets misused by guitarists. eg ‘more headroom’ can be used correctly to mean ‘higher power output’, or incorrectly to mean ‘lower gain’.
    How exactly has the clean channel changed, what did you mean to indicate by the headroom terminology?
    Certainly a valve’s gm goes up and down as its anode current goes up and down.
    So reducing the idle anode or cathode current of the output valves will reduce their gain; hence to achieve the same output level as before, the amp’s gain / vol controls will need advancing a bit.
    That doesn’t equate to the amp being able to achieve a higher power output though, so it’s not an actual increase in headroom per se.
     
    Last edited: Jul 5, 2021
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  13. fitz288

    fitz288 Well-Known Yinzer Silver Supporting Member

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    From what my limited mental capacity can comprehend, I go by definitions like the following:
    What Is Headroom?
    Headroom is simply a term used to denote and describe how much power your amp can provide before the sound starts to break up and distort.

    Taken from for an example:
    What is Headroom in a Guitar Amp? | Fender Guitars

    If there's some sort of electrical techno jargon that not part of my vocabulary, this is what I was referring to - more clean before distortion when I turn the knob thingy.
     
  14. Pete Farrington

    Pete Farrington Well-Known Member

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    Unfortunately the rating system used isn’t noted, so it’s unknown what the basis for it is, and how it compares to the 6V6 type standard of 12W design centre.
    To add insult to injury, they only publish its anode characteristic chart in triode mode, ie totally useless for drawing guitar amp loadlines.

    So how did you determine that the actual power output had increased?
    Lower gain (eg needing to turn the volume higher to reach clipping) does not equate to higher power output.
     
  15. fitz288

    fitz288 Well-Known Yinzer Silver Supporting Member

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    I quest for knowledge.
    This seems counterintuitive.
    When I turned the mA values down, I got more clean before breakup on the normal channel of the Haze15.
    Seems like I was making it "colder" and getting more headroom?

    Edit: at 17 mA, I was getting distortion on the normal channel around 9 o'clock, making it not very "clean".
    When I lowered the mA down around 13, I could go to about 1 o'clock before starting to get breakup when hitting the strings hard.
    Please explain.
     
    Last edited: Jul 5, 2021
  16. Pete Farrington

    Pete Farrington Well-Known Member

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    Your parameter for assessing headroom is incorrect.
    The misapprehension is widespread amongst guitarists.
     
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  17. fitz288

    fitz288 Well-Known Yinzer Silver Supporting Member

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    Edit: at 17 mA, I was getting distortion on the normal channel around 9 o'clock, making it not very "clean".
    When I lowered the mA down around 13, I could go to about 1 o'clock before starting to get breakup when hitting the strings hard.
    Please explain.
     
  18. Pete Farrington

    Pete Farrington Well-Known Member

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    If you’d left the bias alone but turned the guitar vol control (or any other control that affected signal level) down a bit, you’d have got a similar result.
     
  19. fitz288

    fitz288 Well-Known Yinzer Silver Supporting Member

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    mA readings as shown in pics and as stated in my posts are taken at zero volume.
    There was nothing wrong with the old tubes, and I've always liked the way the amp sounded.
    I just wanted to change them and attempt to learn something about my tube amps (some are cathode biased and not much of a technical challenge).
    The readings from the old tubes and the mA settings on the new tubes that I found "worked for me" are about the same.
    None of the formulas are working for this and it makes me think there is some piece of information missing.
    Everything I have read say the bias % should be somewhere in the 70% range +/- for personal preference.
    Lower bias % gives less gain and prolongs the life of the tubes because they are not working as hard.
    Why would the old tubes and the new tubes be working as what seems normal if the bias % is actually "...like 35% or something...".

    There's something missing here...
    This should make sense, and be explainable with a functional equation.
     
  20. Pete Farrington

    Pete Farrington Well-Known Member

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    As you set the current of the new valves to be pretty much the same as that which the old valves were idling at, you might expect similar performance, hence ‘normal’.
     

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