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Bias Probe Help

Discussion in 'Amp Tech Center' started by Askwhy, Oct 29, 2020.

  1. Askwhy

    Askwhy Tele-Meister

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    Ok, so i have had one of the old Quadpros forever. I asked my tech to confirm bias and he measured 28ma while i was seeing 20. No doubt he was correct so after reading a thread i measured the resistance of the probe with no tube attached and it was 1.5ohms. Thinking it had worn out, i ordered another brand and it too measured 20ma at when used on the amp and so i measured the ohms again and got 1.5. Is this a coincidence and they both SHOULD measure 1.0 ohms? Is there something i'm missing in the amp like a shunt or something where it would give a different reading? Can i use it and just multiply by 1.5 and use that result? Thanks for any advice, well other than a different and better measuring method, i'm just looking for help on the bias probe as i like measuring without pulling the chassis. Thanks for any advice!!
     
  2. Lowerleftcoast

    Lowerleftcoast Friend of Leo's

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    Most meters have a hard time measuring small resistances. I wouldn't be concerned with the 1.5 ohm reading.

    Need more info on the 28mA vs 20mA readings. Sometimes the screen is figured into the reading and sometimes it is not. The answer could be as simple as that.

    Also the voltage and resistances may not have been read to the same extent. Such as 4.657 volts vs rounded off to 5 volts. The math can throw the answer off quite a bit when the numbers are not accurate.
     
  3. VintageSG

    VintageSG Friend of Leo's Ad Free Member

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    What does your meter read when you short the normal probes on a low resistance scale?. I don't know what your bias probe looks like, or how it sits in the grand scheme of things. Mine sit between the valve and chassis and breakout one of the legs which is then connected to an ammeter.

    You could try measuring the bias using your probe, then use direct measurements and/or transformer shunt measurement if applicable to compare the results.

    Confirm your ammeter is accurate, and confirm the voltage you're using too. If a schematic says 328V, but that's with a wall voltage of V and your wall voltage is V+/-12, things'll get messy. To get to the voltage check points on mine, I have to pull the chassis. It isn't a job I need to do often, and I take the opportunity to have a look around for signs of scorching or blebbing capacitors.
     
  4. slider313

    slider313 Tele-Afflicted Silver Supporter

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    What amp? How was the tech measuring tube current? What was the AC voltage at the tech's shop compared to the AC voltage at your place? Is the battery in your meter fresh?
     
  5. Askwhy

    Askwhy Tele-Meister

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    Thanks guys! More info- the multimeter is accurate as i tried 2 different meters on both probes and got the same reading. You insert the power tube into the probe and then into the amp and the ground and red leads into the multimeter (with fresh battery).
    Can you tell me more about this? Nothing mentioned in the probe instructions.


    Wall voltage may be a suspect, but my techs verification has always been close to my readings in the past with the bias probe/meter.
     
  6. Paul G.

    Paul G. Friend of Leo's

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    Too many variable to diagnose remotely.

    I wouldn't worry about it, it is what it is. Have your amp biased properly by your tech. When you take it home, use your Prob-O-Matic(R) widgit and write down what it is reading. The exact number doesn't matter, your amp is already biased, you're just getting a reference.

    When you retube, make sure you get the same number, even if it is totally imaginary. It's just a reference.
     
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  7. Askwhy

    Askwhy Tele-Meister

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    My place wall voltage 127
     
    Last edited: Oct 30, 2020
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  8. Lowerleftcoast

    Lowerleftcoast Friend of Leo's

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    FYI in most locals the wall voltage varies. Some places it varies a lot. Utah doesn't have many people so it may vary less. You may find the voltage is higher at night or on the weekend. During the day, especially when everyone has an air conditioner *on*, the voltage will be less.
    So, it is always a good idea to measure the wall voltage whenever you measure the voltages in the amp.
     
  9. Lowerleftcoast

    Lowerleftcoast Friend of Leo's

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    A quick search result for bias probe revealed several that measure plate current only. Different methods can be used to determine bias current of a tube.

    When measuring cathode current with 1ohm resistors on the cathode, all of the current from the plate and the screen is measured. The screen current is usually subtracted from the total to have a *more accurate* plate current reading. The tube datasheet will give a nominal screen current listing. Sometimes the datasheet will list it as grid #2. This mA figure is subtracted.

    If you are *getting* this you will surmise measuring the bias is not a super accurate measurement. The accuracy relies on many variables that are usually not controlled. Combine that with the knowledge that every tube is different... well... close enough is good enough.

    It seems to me your probe functions. Follow Paul G's advice and you will be in the ballpark.

    If you are on speaking terms with the tech, ask how he finds the bias current.

    For more info on figuring bias, look at the several methods mentioned on the *Robinette bias calculator* site.
     
  10. Askwhy

    Askwhy Tele-Meister

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    Interesting, thanks!
    So for EL34
    Screen Voltage (max) 425. V. Screen Current (max) 50.
    Plate Voltage (max): 800
    Plate Dissipation (max): 25
    Screen Dissipation (max): 8

    How would the calculation look to get the bias probe measurement to "match up" to a cathode current measurment in terms of the resulting bias ma reading?
     
  11. Lowerleftcoast

    Lowerleftcoast Friend of Leo's

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    You would add the approximately 8mA.

    Generally you would want to use plate current though. It is best to talk apples to apples.

    When I saw the difference in your 20mA and your tech's 28mA, I figured he may not have subtracted the screen current. Many factors come into play though. Wall voltage, meter/probe accuracy, math, etc. Could be one or... a lot of reasons.

    If you can, adjust the amp to stage volume and adjust the bias by ear. If you can find a sweet spot, use your bias probe to find the numbers at that bias setting. Many of us find that to be around 60% plate dissipation. The high gain amps may or may not be higher than that. Each amp, tube and ear is different so definitely YMMV.

    The main point is to have the tubes run in a safe zone so they do not red plate. Some want tubes to last longer so they choose low percentages. The vintage Fender products mostly ran on the cold side. The point is, don't let the numbers drive your choice of bias setting.
     
  12. Ten Over

    Ten Over Tele-Holic

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    Those are max numbers. You want typical zero signal operating conditions because that is where you are when you set the bias. A case can be made that screen current is 0.133 times the plate current in a typical guitar amp. So if you measure 34mA across a one ohm resistor, the plate current is 34 divided by 1.133 which equals a plate current of 30mA.
     
  13. Ten Over

    Ten Over Tele-Holic

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    Huh?
     
  14. Askwhy

    Askwhy Tele-Meister

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    Gotcha and the 8ma makes perfect sense based on what the difference was. And yes, I agree with the rest of your post and have also found I prefer 60%. I frankly can't hear much difference though in 50 and 70% (no golden ears here) so I use 60 for a good compromise of tube life and staying away from noticeable crossover distortion.
    So to be clear I would use (25 (el34)/475 (pv) *6) - 8 to set using my bias probe?
    Thanks for all the help, I like to be able to save tons of $ when switching power tubes to not have to go to the tech every time and while I understand just using the current number for this amp when retubing, I would like to be able to use the probe for other amps as well with no tech trip.
    Thx again!!
     
  15. Ten Over

    Ten Over Tele-Holic

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    That doesn't prove anything except that both meters can't accurately measure very small resistances. See post #2.

    The resistors in those probes are almost certainly 1r 1%. You just gotta have faith.
     
  16. Ten Over

    Ten Over Tele-Holic

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    Where are you guys getting 8mA?
     
  17. Lowerleftcoast

    Lowerleftcoast Friend of Leo's

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    Ten Over I was writing about the difference between metering the current at the cathode vs metering current at the plate. Current at the plate is just that. Current at the cathode is plate current and screen current. Screen current would have to be subtracted to arrive at plate current. On the robinette bias calc site 5.5% is subtracted from the cathode current to arrive at the plate current. The 8mA is from the tube chart. The actual may only be ~2mA. the point was to explain the difference in two methods of checking the bias.
     
  18. Ten Over

    Ten Over Tele-Holic

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    What tube chart? Can you post it?
     
  19. Lowerleftcoast

    Lowerleftcoast Friend of Leo's

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    @Ten Over .
    Askwhy posted some of the numbers from a tube chart in post #10. No actual chart (yet).
     
  20. Ten Over

    Ten Over Tele-Holic

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    No 8mA in that post.
     
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