Best/Most Accurate Biasing Method

Discussion in 'Shock Brother's DIY Amps' started by itsGiusto, Jul 2, 2020.

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  1. itsGiusto

    itsGiusto Tele-Meister

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    In his site, Robrob recommends the Output Transformer Resistance Method as the best and most accurate biasing method. However, I've always had a lot of trouble with this method, particularly because the measurements I take to not stay constant. When I measure the voltage or resistance, they're in a constant state of flux, both for short time periods and long time periods.

    For example, when I measure the plate voltage, I can't easily say "this is 410.1v". Instead, it will be constantly moving between 409.5v and 410.3v, oscillating around. And to make things worse, if I go and check it again about 3 minutes later, it'll instead be oscillating around a different range, maybe between 412.2v and 413.5v. The same goes for measuring anything, like the center tap voltage, or the OT resistance - constant flux, changing both over short and long time periods.

    Given that this is the case, I can't imagine that finding the voltages, subtracting them, and dividing by the resistance will be accurate.. what if when I take the voltage on the plate, it was in a low range, and then when I take the voltage on the center tap, it's in the high range? Then the voltage when subtracted, and the current when divided by the OT resistance, will be significantly lower than it would have been if I had just measured the current directly, right?

    I've tended to use the shunt method instead, though I know it suffers from issues as well. Most notably, I think if the resistance of the multimeter is too high relative to the resistance of the tube, then not enough current will flow through the meter, and the reading will be off.

    So my question is, can anyone justify which one is better than the other? If the OT resistance method is better, how can I take measurements better so I don't have to worry about fluctuating inaccurate measurements?

    (For the purposes of this post, I'm referring to an amp that does not have 1 ohm resistors on the cathodes, so I can't use that method)


    Update: Consider this scenario I posed in a later post in this thread, illustrating my problem with the OT resistance method:

    I take a reading on the tube plate. It reads 410.1v.
    1 minute later I take a reading on the center tap. It reads 408.1v.
    I therefore find the difference is 2.0v, and divide by my OT resistance which is 155ohm, to get a current of 12.9ma.
    Therefore the output of the tube is 5.29, which I determine is 38% dissipation for a 6V6gt. I bias the tube hotter as a result.

    But wait! In between when I took the plate measurement and when I took the measurement on the center tap, the whole voltage of the system had actually increased without my knowing.
    The plate voltage had actually increased to 411.8v during that time (but I wasn't aware of it, I thought it was still 410.1v), which messes up all my calculations. In fact the real voltage difference would have been 3.8v (411.8v - 408.1v), meaning the current was 24.5ma, and the tube was actually at 70% dissipation.​

    That's a massive difference, from a small fluctuation in voltage! Now when we bias hotter, we'll be putting the tube into over 70% dissipation, which will shorten the lifespan of the tube.

    So I'm not sure how to reconcile the fact that the system seems to fluctuate so much, if even these small amounts can cause drastically different calculations.

    To be clear, I'm not suggesting that subtle variations in current will ACTUALLY cause massive changes in bias or sound. I'm suggesting that subtle variations in voltage will cause you to miscalculate the current, and think it's way different than it actually is, which is why I'm especially wary of the OT resistance method as being accurate.
     
    Last edited: Jul 2, 2020
  2. Bill Moore

    Bill Moore Tele-Afflicted Silver Supporter

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    I don't think there is an absolute number you have to consider. If you put another meter on the A/C receptacle, I suspect it will move relative to the reading you are getting.
    (Better techs say to adjust for the best sound after getting the bias close!)
     
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  3. sds1

    sds1 Tele-Afflicted

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    You could do all the same things with 2 different DMM's, get 2 different sets of results.

    While I appreciate precision as much as the next guy, I've come to be resigned that bias measurements are just not all that precise. And thankfully they don't need to be.

    So my favorite method is installing 1R resistors from K to ground, knowing that a few mA belong to the screens but not necessarily subtracting them...
     
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  4. mherrcat

    mherrcat Tele-Holic

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    This:

     
  5. Lowerleftcoast

    Lowerleftcoast Tele-Afflicted

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    I agree with Bill above.

    The temperature of the transformers changes the resistance readings, so they should be at operating temp.
    The wall voltage varies. In the summer, midday wall voltage may be 15VAC lower than in the evening. It is important to know the wall voltage when you are taking measurements. I try to adjust bias when the wall voltage is high.
    As you pointed out a reading will fluctuate as you are taking the reading. I use the high end of the fluctuation.

    The important part of biasing is to make sure the tubes do not red plate. Usually the amp will sound best before the tubes red plate. I think it is best to bias by ear and then check to make sure the bias is in a range that will not red plate.
     
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  6. itsGiusto

    itsGiusto Tele-Meister

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    I get that the voltage from the wall is going to be in flux, but that's kinda why I think that measuring current directly should yield a more accurate result. Even getting close seems to be a challenge with the OT resistance method. See if this scenario makes sense:

    I take a reading on the tube plate. It reads 410.1v.
    1 minute later I take a reading on the center tap. It reads 408.1v.
    I therefore find the difference is 2.0v, and divide by my OT resistance which is 155ohm, to get a current of 12.9ma.
    Therefore the output of the tube is 5.29, which I determine is 38% dissipation for a 6V6gt. I bias the tube hotter as a result.

    But wait! In between when I took the plate measurement and when I took the measurement on the center tap, the whole voltage of the system had actually increased without my knowing.
    The plate voltage had actually increased to 411.8v during that time (but I wasn't aware of it, I thought it was still 410.1v), which messes up all my calculations. In fact the real voltage difference would have been 3.8v (411.8v - 408.1v), meaning the current was 24.5ma, and the tube was actually at 70% dissipation.​

    That's a massive difference, from a small fluctuation in voltage! Now when we bias hotter, we'll be putting the tube into over 70% dissipation, which will shorten the lifespan of the tube.

    So I'm not sure how to reconcile the fact that the system seems to fluctuate so much, if even these small amounts can cause drastically different calculations.

    On the other side, I'm wondering how much I can trust my current reading on the multimeter. Does enough current flow through the meter to be accurate?
     
    Last edited: Jul 2, 2020
  7. bblumentritt

    bblumentritt Tele-Afflicted Platinum Supporter

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    "For example, when I measure the plate voltage, I can't easily say "this is 410.1v". Instead, it will be constantly moving between 409.5v and 410.3v, oscillating around. And to make things worse, if I go and check it again about 3 minutes later, it'll instead be oscillating around a different range, maybe between 412.2v and 413.5v. The same goes for measuring anything, like the center tap voltage, or the OT resistance - constant flux, changing both over short and long time periods."
    First of all, you're only talking <1% difference between your high and low readings.

    Secondly, as Bill Moore said, it could be the fluctuation in the line voltage.

    You don't use them, but even the 1Ω resistors have a 1% tolerance, so one may be 0.9Ω and the other 1.1Ω, and if they're 2% or 5%, then all bets are off anyway.

    Here's what I do...
    1. I use a VariAC to set the line voltage to 120VAC ±.03VAC.
    2.Make sure all volume knobs are at "0".
    3. Let the amp warm up at least twenty minutes.
    4. Check the line voltage again, and adjust/repeat if needed.
    5. Make sure all volume knobs are at "0".
    6. Check the high voltage. If it jumps a volt or two, I don't really care. Here's why; you're talking 0.4mA difference:
    6L6GC
    409VAC = 44.0mA for 60% dissipation
    413VAC = 43.6mA for 60% dissipation
    44mA = 60.6% bias
    43.6mA = 60% bias​

    You're not going to hear, or notice, the difference between 60% and 60.6% bias. The bias adjustment pots can drift, and tubes change over time anyway.
     
  8. itsGiusto

    itsGiusto Tele-Meister

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    I understand it's only <1% difference, but I THINK that can make a big difference when it comes to the OT resistance method, specifically because we're subtracting two voltages taken at different times in order to derive the current, instead of measuring it directly. If you measure the plate when voltage is high, and center tap when voltage is low, or vice versa, you'll massively mess up the difference in voltage you calculate. See my previous post, right above this one, and please correct me if my math is wrong.
     
  9. bblumentritt

    bblumentritt Tele-Afflicted Platinum Supporter

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    f6d060b26b75ec4362112f957fe20bc1.jpg
     
  10. itsGiusto

    itsGiusto Tele-Meister

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    To be clear, I'm not suggesting that subtle variations in current will ACTUALLY cause massive changes in bias or sound. I'm suggesting that subtle variations in voltage will cause you to miscalculate the current, and think it's way different than it actually is, which is why I'm especially wary of the OT resistance method as being accurate.
     
  11. VintageSG

    VintageSG Friend of Leo's Ad Free Member

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    I use the type of probe that sits between the valve and the socket. I measure the plate and plate-cathode voltage and plug the resultant plate-cathode voltage into the calculator that @robrob so graciously provides. One multimeter per valve/probe + one to measure voltages. My probes allow you to measure plate current via a multimeter.
    Before buying the probes, I'd measure everything and do lots of maths, but as I suffer from an advanced case of the dumbs, I sought an easier way.
    I learned that single ended amps can measure over 100% and sound glorious while doing so. I was quite worried at first, now?, meh, it sounds so good.
    I learned that amps aren't often set up well from the factory. Damn near every adjustable fixed bias amp has been set cold from the factory.
    I learned that balanced valves aren't balanced after 100+ hours of use, and not to care about it.
    I learned that spec sheets are guidance only. I've seen over 15mA difference between a pair, and set to the highest drawing valve, and it sounds great.
    I learned to stop worrying. Make it safe, make it sound good, make some noise.
    I learned to allow an hour of hard use after the initial setup for fixed bias for the final tweak. Looper + load box + crank it up while I drink coffee, then tweak.
    I learned that cathode bias amps also need that time as resistors can drift when warm. Not much, but they do. If it's close, who cares.
    Relax, love the bomb.

    To me, the in circuit probe is the best method.
     
  12. Lowerleftcoast

    Lowerleftcoast Tele-Afflicted

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    So why are you not measuring the voltage drop from the OT center tap to the tube plate? It is more accurate on most meters than subtracting two high voltages.
     
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  13. itsGiusto

    itsGiusto Tele-Meister

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    Yeah, that's a good point, I guess I could do that. Robrob cautions against this in his post, saying it's dangerous, but I guess it's no more dangerous than the shunt method, probably a little less.

    But this also brings up my other problem with the OT resistance method, which is getting a consistent resistance value for the OT halves. It seems to vary a bit with repeated measurements, so I'm not sure what to use. But I guess it's less likely to cause significant error as compared to the plate-center tap voltage issue described above,
     
  14. Kevin Wolfe

    Kevin Wolfe Tele-Meister

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    That’s it right there.
    Don’t try to Over think it and re-invent the wheel @itsGiusto. If it ain’t red platting and sounds good, it is indeed good. Center bias actually means “ nearly” centered. If it walks like a duck and talks like a duck... well, you know.
     
  15. Lowerleftcoast

    Lowerleftcoast Tele-Afflicted

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    You can take these measurements hands free or one handed with just one end clipped on. It is no more dangerous than taking other high voltage readings.

    I would not use a probe in each hand. We don't need a Darwin award.

    Also the meter only measures the ~3volt difference, but you know there is ~400volt on the terminals.
     
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  16. King Fan

    King Fan Poster Extraordinaire

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    @Bill Moore and @sds1 had this dead to rights in the first two replies.

    No, we don't need super precision to hit a dissipation target that, in fixed bias, spans from 50% to 70% (or actually 85%, as Merlin notes) -- and then adjust by ear inside that range.

    Even so, all the fiddling and measurement error/imprecision of the OT resistance method, not to mention the added risk and difficulty of the OT shunt method means for my money they're not the best/simplest/safest.

    What is? For cathode bias, I say cathode resistor voltage drop. No loss of precision, little measurement error, highly safe, all in a couple simple steps.

    For fixed bias, without question, it's 1% 1-ohm cathode resistors. In fact, 1% tolerance is not from 0.9 ohms to 1.1 ohms, it's 0.99 ohms to 1.01 ohms. Two easy safe measurements, and at worst your (let's say) measured 20mA current is actually somewhere between 19.8 and 20.2...

    Yes, a bias probe is safe and accurate, especially if you spend the dough for the kind that also measures plate voltage -- but those cost over $100 and you still gotta pull and replace each tube -- twice -- while shutting down and powering up the amp!!!
     
    Last edited: Jul 2, 2020
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  17. itsGiusto

    itsGiusto Tele-Meister

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    What should the wattage rating be for those 1 ohm resistors, if I wanted to add them to my Princeton Reverb? Shouldn't be much, right, like if it's maximum 50ma at 1ohm, then normal old 0.25 watt metal film resistors should do, right?

    I only ask cause in the past when I've built kits, they often come with big-ol' 1-2 watt resistors for these 1-ohm cathode resistors.
     
    Last edited: Jul 2, 2020
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  18. Bill Moore

    Bill Moore Tele-Afflicted Silver Supporter

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    I've only added bias adjustment test points to one amp, my DR clone, and used 2W 1% resistors. DR-19.JPG
     
  19. King Fan

    King Fan Poster Extraordinaire

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    You're right, standard ½ watt resistors work fine; the trick is finding 1% tolerance. To be honest, when I did the math recently for a guy who could only find 5% tolerance, he still didn't end up with much imprecision. Say 20/1.05 = 19.05...
     
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  20. tubegeek

    tubegeek Friend of Leo's

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    Let's say 100mA to get crazy. I^2*R = .1x.1x1=.01W.

    Any 1 ohm resistor you can find will be safe to use. Precision is more important than power rating here.

    The bulkier resistors are probably supplied for physical robustness sake, since they are mounted directly to the sockets, not a board.
     
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