bias methods: cathode current [w/ pin 8 bias probe] VS. CT-OT shunt volt drop (plate current)

Discussion in 'Shock Brother's DIY Amps' started by owlexifry, Sep 25, 2021.

  1. owlexifry

    owlexifry TDPRI Member

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    which is more accurate?

    getting conflicting results between the two methods (4x EL34 100w amp)

    measuring cathode current with a bias probe (pin 8):
    EL34, assuming a max dissipation 25W plate + 8W screen = 33watts, (plate+screen)
    33 x 0.66 = 21.78w (for 66% max)

    (is it agreed that the cathode current should be a combination of plate + screen currents?)

    so with 21.78w,
    plate voltage = 451v
    max cathode current should be 48.29ma (for 66% max)

    with this in mind, i set the bias, checking each tube, tweaking as required, and eventually, out of the x4 tubes, highest cathode current reading is 46.8ma (according to the multimeter with the bias probe - pin 8)


    so then i go to use the OT-CT shunt method.


    OT plate-CT resistances 31.2ohm; 26.0ohm (measured when amp is hot)

    volt drop: 2.04v; 1.67v
    (plate voltage 451v; 452v)

    which calculates to 65.4ma; 64.2ma
    and yes these values look way too high.

    with the resistances mentioned above, for a ‘safe’ plate current, say 38ma, i would need to see a volt drop of 1.186v; 0.988v ...

    but then if i tweak the bias to reduce the measured volt drop (to reduce the calculated plate current to a ‘better value’),
    the cathode current readings with the bias probe method end up too low.


    i’m not sure which method to trust.

    or am i not assessing the cathode current correctly?

    (i’ve never really had issues with the OT-CT shunt method before, im just finding with this particular amp im getting odd results, so then i decided to try the bias probe method to measure cathode current directly, and now i’m even more confused)
     
  2. 2L man

    2L man Tele-Holic

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    There are two parallel tubes consuming current thru OT primary halfs so it is a sum of two and you don't get individual tube current.

    Is it necessary or not I would say for guitar amp it is not?

    Bias measuring probes/meter which have four individual sockets are rare and expensive. If you use meter which has less than tube count you need to turn amp off and back on and if you adjust one tube it effects other tube bias so it needs quite a lot of work.

    Measuring cathode current over 1 ohm cathode resistors on each power tube is convenient and fast and result actual power what tube waste when cathode current is a sum of plate and screen currents. But you just have to accept that getting four tubes close each other is a surprice :) and few hours later you have to adjust again. I have read from HiFi forum that sometimes tube biases stop wondering after 100 hours of use and sometimes never :(
     
    Last edited: Sep 25, 2021
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  3. King Fan

    King Fan Poster Extraordinaire Ad Free Member

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    +1 for the 1-ohm cathode 'measuring' resistors. Smarter folks than me will analyze your analysis and assumptions. I got lost early on when you were figuring out the sum of watts… Instead, lemme ask a few dumb questions to see if I understand.

    Are you using the OT resistance method (the safe one) or the OT shunt method? Whose bias probe are you using? I thought some measured plate current directly? is yours also measuring the plate voltage for you? Do you re-measure that when you change bias?

    Are you doing the math yourself or using an online calculator? No slam on your abilities; the online calculators sometimes add in (or leave out) helpful steps.
     
    Last edited: Sep 25, 2021
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  4. Lowerleftcoast

    Lowerleftcoast Friend of Leo's

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    Measuring at the cathode with one Ohm resistors will result in measuring the plate and screen current.

    Measuring at the resistance of the OT (resistance method) will result in measuring the plate current.

    Using a bias probe on the tube socket... one would need to know if the bias probe is measuring plate current or cathode current or if the probe is estimating plate current. The bias probe you are using may be adding to your confusion because it is not clear what it is measuring.

    Ime, the OT resistance method should be your most accurate reading.
     
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  5. NSB_Chris

    NSB_Chris Tele-Holic

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    So then would you use both? Use the 1ohm resistor to determine if the two tubes on the same OT primary are balanced relative to each other, and then use the OT resistance method to determine the plate current for setting the dissipation level you want?
    I am asking because I will soon be trying to bias a quad EF80 power section and I am anticipating that it will be a bear. I would eventually be asking similar questions. I have fixed bias with level and balance and implemented the 1ohm resistors. Helpful thread!
     
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  6. King Fan

    King Fan Poster Extraordinaire Ad Free Member

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    All good info. LLC and I have this little dialog frequently, just cuz we're buds. For me, the 1-ohm resistors including screen current is no big deal; heck, it may even be a feature. Why? Aiken: " If you install a 1 ohm resistor in the cathode lead of each output tube, you can measure the voltage drop across it to get the cathode current, as described above in the plate resistor measurement. The advantage of this method is that there are no high voltages involved, since there will only be a few millivolts difference between ground and the other side of the 1 ohm resistor. The disadvantage is that you must subtract the screen current in order to accurately determine the plate current. However, since the screen current is only a few mA, it can usually be ignored, and the error will be in the conservative direction, i.e., less plate current than expected, which is good for tube life. This method of biasing is the most highly recommended."

    **But** my point may not be germane here:
    1. You don't have 1-ohm resistors now, and you may not want or even be able to install them, depending on the amp.
    2. EL34 and similar pentodes balance screen and plate currents differently from the (6V6, 6L6) tubes I'm used to, and I'm not sure the proportion of screen to plate current is as small, or as stable. See this thread from (where else) EL34world.
     
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  7. owlexifry

    owlexifry TDPRI Member

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    as mentioned, the bias probe i am using inserts between pin 8 of the socket and pin 8 of the tube, hence it is measuring cathode current directly (with the multimeter)

    the amp is all PCB and there’s no room for slapping in 1ohm resistors for the mv - ma conversion method.

    and then for the other method is the classic OT resistance method to calculate plate current, as everyone has described above.
    - measure resistances between plate and CT
    - measure volt drop across plate and CT
    i’ve given the values above and the math isn’t that complicated.

    plate current = volt drop / resistance

    and yes, been rechecking and recalculating everything, everytime the bias control is adjusted.

    im finding with this amp, following the OT shunt/resistance method (for max 66% dissipation), it seems to be quite conservative and setting it quite cold, and the preamp plate voltages end up 15v higher than i’d like them to be.
    when following the bias probe for measuring cathode current (max 66% dissipation), it seems to be getting it right, preamp plate voltages are spot on, and it sounds great (but then the OT-resistance method would suggest those settings determined by the bias probe are too high)

    given the comments above, i do have doubts about the calculations/assumptions with the cathode current.
    maybe the screen current draw is a little overstated?
    is this assumption incorrect, that for an EL34, 66% max dissipation measured at cathode = (25W+8W) x 0.66 = 21.78w
    ?? this seems too simplified.
    maybe the cathode current needs to be lower than what my assumptions are suggesting....
    with the assumption that 66% max dissipation = 21.78w, i’ve been aiming for max 47.9ma cathode current.
    perhaps i shouldn’t be aiming this high.

    maybe i should measure the actual screen current with the volt drop across the screen resistors and factor this in instead?
     
    Last edited: Sep 25, 2021
  8. Lowerleftcoast

    Lowerleftcoast Friend of Leo's

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    It is info I want to know so I can hopefully duplicate it in the near future. I have no hope of having a different tube or even the same tubes, as they are worn with time, give a same/same result. I would say my *want to know* is academic rather than tone-fully useful because different tubes will give different results.

    For setting bias, you have installed a control feature to adjust many variables. It is said the two halves of the waveform sound better when they are not equal. The only way to set them is by ear. I suppose you can set them visually with an o-scope, or set them a certain percentage apart, but when you want something to sound different shouldn't you be listening rather than watching or figuring?

    Same with setting the bias. How do you know what dissipation level you want? Ime, it should be set by ear and then checked to see if it is in a safe zone for the tube. It really doesn't matter what the number measures as long as the tube will survive for a reasonable time.
     
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  9. Lowerleftcoast

    Lowerleftcoast Friend of Leo's

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    I am not an expert on the many different bias probes. From what I assume, some bias probes measure at the cathode but may use a different resistance than one Ohm to *estimate* the plate current.
     
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  10. Nickfl

    Nickfl Friend of Leo's

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    Are you sure your measurements are accurate? Specifically the measurements of the voltage drop across the OT primary. Sometimes poeple measure center tap to ground and plate to ground and then subtract, but that's not very accurate, much better to measure that voltage directly with one probe on the CT and the other on the plate.

    Also, how are you coming to that 8W screen dissipation number? Do you have screen grid resistors? If so, measure the voltage drop across them and figure out what your actual screen dissipation is and then you can determine a more accurate plate dissipation.
     
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  11. Ten Over

    Ten Over Tele-Holic

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    Those are the maximum dissipations for the plate and screen, but it doesn't mean that the screen is dissipating 8W when the plate is dissipating 25W.

    The screen grid current clusters around 15% as much as the plate current. The cathode current contains both screen grid and plate current, so Ik = 1.15 x Ia where Ik is the cathode current and Ia is the plate current.

    For 66% plate dissipation:
    25W x 0.66 = 16.5W
    16.5W / 451V = 36.6mA
    36.6mA x 1.15 = 42.1mA

    So you would set the cathode current at 42.1mA with 451V on the plate in order to get 66% plate dissipation.
     
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  12. owlexifry

    owlexifry TDPRI Member

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    ahhh this is what i needed to see!
    this makes better sense.
    i’m gonna do this tomorrow.
     
  13. owlexifry

    owlexifry TDPRI Member

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    yep i only measure directly from CT to plate, because doing the method where you measure each to ground just isn’t precise enough (not enough significant figures).
    i do wonder how accurately the multimeter is measuring that minuscule ~1v drop over 450+volts...

    i think ive been misled with that 8w screen dissipation number/assumption. gonna forget that nonsense, and go with the values @Ten Over has suggested.

    also gonna measure the volt drop across screen resistors as you have suggested.
     
  14. 2L man

    2L man Tele-Holic

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    First you wrote amp has 4×EL34 and to me it still looks you have not divided OT primary half current for two tubes on your calculations.
     
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  15. owlexifry

    owlexifry TDPRI Member

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    i’m not sure what you mean.
    can you please explain this?
    are you talking about averaging the currents of power tube pairs / quads?
     
    Last edited: Sep 25, 2021
  16. 2L man

    2L man Tele-Holic

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    This 65mA split to two tubes and 64mA split to other two. Kirchhoff current-law ;)
     
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  17. dan40

    dan40 Friend of Leo's

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    I believe that 2L man is asking whether or not you accounted for the fact that each half of the OT is connected to two EL34's. Using your OT/centertap resistance method, you will need to divide whatever number you get by 2 because each half of the OT is connected to two tubes.

    Edit... Sorry, 2L man was posting at the same time I was.
     
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  18. owlexifry

    owlexifry TDPRI Member

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    no i did not. whoops.
    many thanks!

    this would explain so much, if not all, of my confusion with biasing this amp (every other amp i’ve worked with only has x2 power tubes)


    thanks for pushing on!
    and ive just realized now, with the OT-CT resistance/shunt method, i’ve actually been measuring the volt drop from CT to each primary lead (before it splits off to x2 tube plate pins) instead of right at each tube plate pin, because it’s not possible to safely clip the meter probe on the back of the PCB octal sockets (much easier/safer to clip on where the primary lead meets the PCB).

    it almost seems like you knew i was doing it this way....

    im assuming i would expect to see half the amount of volt drop if i was measuring directly at the tube socket plate pins? (compared to OT primary lead)
     
    Last edited: Sep 25, 2021
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  19. Ten Over

    Ten Over Tele-Holic

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    No. The end of the primary lead is connected to both plate pins on a side, so the voltage is the same anywhere you check it on that side.
     
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  20. owlexifry

    owlexifry TDPRI Member

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    oh god.
    i think i get it now.
    thank you
     
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