How accurate is the 1r on pin 8 biasing method?

Discussion in 'Shock Brother's DIY Amps' started by Uncle Daddy, Jun 13, 2021.

  1. Uncle Daddy

    Uncle Daddy Tele-Afflicted

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    On my last build I'd fitted 1ohm/5% resistors between pin 8 and ground on both 6v6 tubes, along with a bias pot. I get 420v between pin 3 and ground with a 5u4- does that sound about right? The PT is rated at 330v/138mA. Multiplying the plate voltage by the voltage drop across the resistor gives me 8.1w, well below the 70% threshold on the JJs.

    I can imaging variations in the 1r resistor potentially affecting the reading, but is it still good enough or do I need a more accurate method.

    Sounds very "brownface" at the moment, which I like!
     
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  2. sds1

    sds1 Tele-Afflicted

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    1) it's only as accurate as the 1R resistor (in your case >= 95%)
    2) screen current flows thru there as well, this must be subtracted before you math the bias

    Ideally you get precision 1R resistors, <= 1% tolerance

    But regardless, inside 5-10% is surely "good enough" in tube amp world.
     
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  3. Uncle Daddy

    Uncle Daddy Tele-Afflicted

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    That's what I thought. What's the best way to get the screen current value?
     
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  4. sds1

    sds1 Tele-Afflicted

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    It's just a guesstimate, on Rob's site for cathode bias method he uses a factor of 5.5%

    If you look at the JJ 6V6S datasheet you will calculate anywhere from 11-18%. Those operating conditions are at significantly lower plate voltages though...

    I Googled and found a paper that says 10% is a good rule of thumb.
     
  5. King Fan

    King Fan Poster Extraordinaire Ad Free Member

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    ^^^ what he said!

    IME a ~ 5-10% screen current contribution is not a big deal in practice. Experts note it works in the 'safe' direction — against biasing too hot — and that after all we’re supposed to bias by ear inside broad limits. Merlin also notes that the high-end limit of 70% in fixed bias is not hard and fast.

    That said, I do use 1% resistors to lessen that source of *added* imprecision, and I guess if my ears wanted to go above 70% measured by plate current, I’d start to subtract a 5 or 10% current correction factor so I could bias that much hotter.
     
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  6. sds1

    sds1 Tele-Afflicted

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    Yeah excellent way to put that @King Fan, it works in our favor. You can bias right at 70% and let your unknown screen current be a safety factor.

    I typically go for more like 50-60%, so I bias right at around 60% and don't fuss over the screen current.
     
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  7. Lowerleftcoast

    Lowerleftcoast Friend of Leo's

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    This is not the best way to find *screen current* but there are more accurate ways to find plate current and that is what you originally wanted to know. Subtracting the plate current from the plate and screen current of the 1 Ohm cathode resistor method will result in screen current.

    These methods involve additional/more metering on high voltage sources than the 1 Ohm cathode resistor method so continued attention to safety protocols is prudent.

    The meter can be used to find the plate current by measuring current directly. A safer way is to measure the voltage drop across the primaries of the warm output transformer and dividing by the resistance of those same primaries. Rob describes this method on his bias site.

    I suppose you could use either of these methods to find the percent of screen current for the particular set of tubes in the amp, you could then use the 1 Ohm cathode resistor method and subtract the screen percentage to arrive at a more accurate picture of plate current.

    Ime, the ability to measure current without having to open the chassis is the real advantage of using the 1 Ohm cathode resistor method. Seconded by the *'safe' direction — against biasing too hot* argument that @King Fan mentioned.

    Bottom line... I agree with the other replies that the bias should be adjusted by ear and then checked to find if the bias is in a safe range.
    So you have found a setting that you like and the bias is within range.
    We have a winner!;)
     
    Last edited: Jun 13, 2021
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  8. mrriggs

    mrriggs Tele-Meister

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    If your amp has screen resistors then measure the voltage drop across the resistor and divide by R to find the screen current.

    Why not just put the 1R resistor on the plate so you don't have to subtract anything? You already have to stick a probe on the plate to get plate voltage, touch the other probe to ground to get plate voltage, touch the other side of the 1R to get current, then remove first probe from plate.
     
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  9. robrob

    robrob Poster Extraordinaire Ad Free Member

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    I have this layout on my "generic tube amp mods" page. There are probe terminals just large enough to get a meter probe into to keep it safe.
     
  10. King Fan

    King Fan Poster Extraordinaire Ad Free Member

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    I'm sure everyone has left the party, but I wanted to come back and clean up a bit. On this topic, I like Mr. Aiken:

    "The plate current can be indirectly measured by determining the cathode current in the tube. Since no appreciable current flows in the control grid of the tube, all of the plate current must also flow in the cathode. The cathode current, however, will also have all of the screen grid current flowing in it as well, since the cathode current is the sum of the plate and screen currents. Typically, the screen current in most commonly used pentodes is around 5mA (this varies, of course, depending upon the class of operation, the bias point, and the type of tube used). The screen current can be accurately measured by determining the voltage drop across the screen resistor and dividing by it's value (for example, 5V across a 1K screen resistor indicates 5mA of screen current). 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."
     
  11. Paul G.

    Paul G. Friend of Leo's

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    Amp bias isn't like tuning a carburetor, there is no "perfect" bias. 1. Bias will vary with wall voltage, wall voltage will vary at different times of the day. In addition, if you use your amp outside the house, all bets are off.

    The "you must bias at 70%MPD" crowd is pretty much full of it. I like to be conservative, and bias in the 60% range (+/-) and leave some room for safe operation and preserving the toobz. So, a couple percent either way due to estimated screen current or resistor tolerance is insignificant.

    Or at least that's what I think.
     
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  12. King Fan

    King Fan Poster Extraordinaire Ad Free Member

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    Yeah, I have yet to like 70% 'by ear' on any of my amps.

    I see people give a lot of reasons they don't want to use 1% 1R cathode resistors, but with an external-adjust (slotted actuator) pot and tip jacks (plus a tip jack for plate voltage and one for ground) I can bias my PR in less than a minute. And the ease and speed makes 'bias by ear' a lot easier, too.
     
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