Which of these bias probes is more effective / safer for biasing an amp?

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by Janus Alfador, Jul 12, 2019.

  1. Janus Alfador

    Janus Alfador Member

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    I'm looking at doing my own biasing at home, and notice some probes read different metrics.

    This probe measures plate voltage and plate current:

    https://www.ebay.com/itm/2-in-1-Tub...Voltage-test-probe-6l6-el34-kt88/282672928476

    It looks more convenient in that it has a switch to flip between its modes. I wonder if that can be switched between modes without powering down the amp or putting it into standby.


    This probe measures plate voltage and cathode current:

    https://www.tubedepot.com/products/tubedepot-bias-scout-kit

    It seems less convenient because the amp needs to be turned off to change the connectors for each metric.


    What it more comes down to, though, is whether plate current or cathode current is a more useful metric for biasing. If using the following formula to bias an amp, is the final mA reading of the tubes the plate current or the cathode current?:

    70% max dissipation of 25 watts divided by tube plate voltage = 0.00whatever mA you should try.
     
  2. Humble Texan Fan

    Humble Texan Fan Member

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    The top one seems similar to the Weber one and does allow you to switch on the fly. Great to tell the current at a specific voltage, which varies quite a bit - and shutting down the amp doesn't help there.
     
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  3. Janus Alfador

    Janus Alfador Member

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    Cool. I just don't know whether I want to be setting the plate current or the cathode current to 70% dissipation of plate voltage.

    If it matters, the amps I have to bias are a JCM 2203 and an Orange OR-80.
     
  4. Tron Pesto

    Tron Pesto Member

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    The most "correct" method is setting the plate current based on the plate voltage (assuming cathodes are connected to 0V reference (typically chassis) in a fixed biased amp). It is PLATE dissipation after all, right?

    Nevertheless, many people (and many probes), measure the bias based on the cathode current. And many people set that at 70% (we're talking fixed bias, AB1 P-P). Measuring this way is a little misleading because the cathode current includes the screen grid dissipation (assuming we're talking about pentode/beam tetrodes).

    Typically, screens shouldn't be dissipating more than 1W (but they could - EL34 max is 3.3W), so it's arguably ignorable when setting bias. Maybe better put, it provides a "safety" factor when setting the bias. Here's an example:

    Let's use EL34s with a max plate dissipation of 25W:

    70% plate dissipation target: 17.5W
    Plate Voltage: 400V
    Screen Voltage: 400V
    Cathode Current: 43ma (as measured on a bias tool)

    The cathode current times the voltage gets you to 17.2W, which is close to 69% of the max plate dissipation. However, the cathode current measurement includes the current drawn from the screen. Let's say the screen is drawing 0.8W (a possible real-world figure). Subtract that from the 17.2W to get 16.4W for the actual plate dissipation - now you're at 65.6% plate dissipation. Not a huge difference, arguably "ignorable" while at the same time it provides a little extra safety margin for when in the real world, components drift, or you plug into significantly different house voltage, or the amp warms up as it is played for extended periods.

    Nevertheless for me, I'd rather have a direct plate voltage and dissipation - I can dial in my own safety margin.

    I have a bias tool that I got a long time ago (a Bias King) that measures cathode current. I rarely use it - typically, there are other (better) ways to get measurements manually. And it seems on those rare occasions when I would break it out to check something, half of the time, the probe doesn't sit well in the socket (especially if there are bear claw retainers) or there isn't enough clearance for the tube when you add the extra height of the probe. I find they end up being more of a pain in the arse than they are worth, and I end up taking "manual" measurements anyway because I want to double check them anyway.
     
    Last edited: Jul 12, 2019
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  5. pdf64

    pdf64 Member

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    Adjusting the bias will change both plate voltage and plate / cathode current. Plate dissipation is the product of plate voltage x plate plate current.

    Such models may have a somewhat inappropriately high HT voltage for their OT's primary impedance. Aiken shows an extreme example of how that affects plate dissipation. Hence you may wish to consider idling EL34 somewhat below 17.5 watts to help accommodate that.
    Generally, for fixed bias AB guitar amps with big octals, ~30mA idle cathode current is plenty to eliminate the obnoxious sounding zero crossing crossing distortion. More than that just tends to reduce the distortion inherent in class AB operation, and increase gain (which, given a negative feedback loop, reduces distortion still further).
    Then just check the HT / plate voltage and verify the idle dissipation is under the 70% limit.
    http://www.aikenamps.com/index.php/idle-current-biasing-why-70-percent

    Note that Mullard's suggested EL34 fixed bias AB operating conditions have an idle dissipation of 12W - Brrrrr
    See bottom of p2 http://frank.yueksel.org/sheets/129/e/EL34.pdf
     
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  6. VICOwner

    VICOwner Supporting Member

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    What I like to do is to measure the DCR of the output transformer. Measure resistance from 1 plate to the center tap ( with the amp unplugged and caps drained ). Write this down. Now measure the other plate to center tap and write it down. Turn the amp on and measure the voltage drop across the plate to center tap and divide that by the resistance. That’s plate current. When you adjust the bias you will have to also measure plate voltage to ground to to get accurate calculations but once you get the hang of it it goes pretty quickly. Just write down the figures so you don’t get confused.
     
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  7. Janus Alfador

    Janus Alfador Member

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    I ordered the one that measures plate current.

    I was a bit leery about it being more dangerous because it reads the plate voltage in volts rather than millivolts like the TubeDepot probe, and it reads plate current instead of cathode current. However, it's more straight-forward, and I should be using it properly and not in ways that could invite danger, anyways.


    Since changing the bias will change both the plate voltage and current, when is the appropriate time to measure the voltage to calculate the % of dissipation - is it an incremental adjustment / calculation cycle that's repeated until the plate current is below 70% of the plate voltage?

    What would you say is a minimum, and a good moderate safety margin to the 70% maximum dissipation?
     
  8. pdf64

    pdf64 Member

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    Agree, but just to note that EL34 g2 rating (design centre :)) should be 8W.
     
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  9. Tron Pesto

    Tron Pesto Member

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    Ooops - I stand corrected! - the 3.3W is for the 7591s that were in an Ampeg Gemini I just worked on! Another great example that one should never do this stuff off the top or their head without double checking datasheets twice at minimum!
     
  10. HH1978

    HH1978 Member

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    I use that one. It's easy to use and gives pretty accurate readings (I controled once using other methods for biasing). I put the amp into standby before switching modes, though I forgot once and it doesn't seem to have hurt my meter. You are correct that it reads plate current, not cathode current.
     
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  11. Janus Alfador

    Janus Alfador Member

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    I have a question about what about using a probe that measures plate current is dangerous. The instructions say to never remove the probe connectors from the DMM while an amp is on. Is danger specifically in handling one of the probes while the amp is live, or is the danger specifically having the probe connectors touch each other while the amp is live?

    Obviously, I'm to turn the amp on only while the connectors are properly seated in the DMM, but I want to understand what is actually the danger.

    Also, is there a chance that a DMM failing mechanically / electronically could result in a shock or risk from handling the DMM while the probe is connected to it and the amp is turned on?



    One more question: The eBay listing says, "IF THE CATHODE ON YOUR AMP IS CONNECTED TO CHASSIS ( PROBABLY IS ) YOU WILL HAVE THE FULL B+ POTENTIAL BETWEEN EACH OF THE BANANA PLUGS AND THE CHASSIS OF YOUR AMP".

    Does that apply to a JCM 2203 and an Orange OR-80?
     

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