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?
     
  12. Tron Pesto

    Tron Pesto Member

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    The danger is when the probe is connected to the socket, the exposed leads carry the full plate voltage! If you don't plug in the leads into a DMM before you start up the amp, they are exposed and ready to bite you hard. Essentially, you have a live wire connected to the plate. Im sure they probably tell you to connect the probe to the DMM FIRST before seating it in the tube socket.

    A good DMM should be able to handle the plate voltage, and also it should have a fuse that will go first before it goes up in flames should there be some "failure". I don't see a "hazard" there.

    I think what they are saying with that statement is that with any Ammeter connection, you are shunting the current through the DMM which means that "ground" side of the probe is actually hot. It's what is carrying the plate current to ground - internally, the DMM (Ammeter) is internally shunting a portion for measurement purposes, the rest is flowing through the leads.

    The right approach mitigates the hazard - connect both leads to probe first, then probe to socket, then tube in probe, then turn on amp. Then after shutting off amp, remove probe/tube first. Then move on to next tube and repeat the process in the correct order again.
     
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  13. TD_Madden

    TD_Madden Gold Supporting Member

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    Try to find an Aiken WOMBAT...works great and no switches to flip
     
  14. Janus Alfador

    Janus Alfador Member

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    Good to be aware of that. So, there isn't a danger that a failed but properly-connected DMM will become a conductor of anything that will shock a person just by touching the DMM.

    They don't, actually. They suggest installing the probe before putting the leads into the DMM:

    "1. Turn off your amp.
    2. Remove tube
    3. Insert probe
    4. Insert black / blue banana plug in the COM jack of your multimeter
    5. Insert red banana plug in the V,Om jack of your multimeter
    6. Insert tube in the socket of the probe
    7. Turn on your amp"


    Can inserting the probe into the socket without the leads connected to the DMM cause a shock from stuff that's build up in the capacitors?

    Mine might be the best that $10 or $15 can buy. It does have a fuse. Think it'll hold up?

    [​IMG] [​IMG]

    Great, thanks!
     
  15. Tron Pesto

    Tron Pesto Member

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    I just took a look at the model and instructions, and a couple of interesting things jump out to me. Here are the instructions:

    1. Turn off your amp.
    2. Remove tube
    3. Insert probe
    4. Insert black / blue banana plug in the COM jack of your multimeter
    5. Insert red banana plug in the V,Om jack of your multimeter
    6. Insert tube in the socket of the probe
    7. Turn on your amp
    8. Allow tube to warm up
    9. For testing plate / anode current select
    " plate current " on the function switch of the probe
    10. Set your multimeter to the mV range ( most common is 200mV )
    You will get a direct reading in mA
    11. For testing plate / anode voltage select
    " plate voltage " on the function selector switch
    12. Set multimeter to the highest DC voltage available
    When done testing repeat in reverse from step No.8
    If using a autorange multimeter steps No.10 and No.12 are not used.​

    In steps 9 and 10, it says to test plate current, you flip the switch and set your probe to "mV". If that's the case, it's actually not testing plate current directly - I'm guessing they have a 1-ohm resistor internally between the pin 3 of the amp socket and the exiting pin 3 of the probe socket and you are measuring mV across that resistor. Using Ohm's law, that mV is directly equal to the mA current draw.

    When you flip the switch, one probe is connected to pin 3 (plate) and pin 8 (cathode) and you can read the actual voltage.

    Nevertheless, I disagree with their sequence. I would never connect the probe to the socket without the leads "buried" in a meter - even if I am diligent in draining caps, if I'm testing back and forth, there is no reason to take the risk of grabbing a lead that is directly connected to a plate. There is high voltage on at least one of those probe leads that is dangerous.

    Regarding your multimeter, while the brand of "Power Fist" leaves me a little disconcerted, if the meter is rated to handle 600V, you should have no reason to be concerned in connecting it first before you drop the probe in the socket, even if the caps are not drained.

    I have to admit that I have trepidation suggesting you do something different than the manufacturer's instructions. I'm just telling you what I would do. I would perform steps 4 and 5 before step 3.
     
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  16. Janus Alfador

    Janus Alfador Member

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    That makes sense. I'll do it your way.

    It looks like it says 500v MAX on the COM line, not 600v. Still good?

    Thanks for the great help!
     
  17. Tron Pesto

    Tron Pesto Member

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    You're welcome - but of course, caution is the better part of valor! Be methodical, consistent, and safe.

    Regarding the 500v MAX on the COM line - that's probably a max reverse voltage on the common ground. I see 750VAC and 100VDC ratings on the VΩmA terminal, so you should be good.

    And just to clarify their instructions, do NOT leave the amp running when switching from plate current to plate voltage testing mode. You'll want to shut everything down, then switch not only the probe, but arguably more importantly your meter range, before you fire the amp up again. The reason is that your meter won't be very happy if you have it set on the 200m volt range, and slam it with a few hundred volts from the plate.

    So to triple clarify, let's say you check the plate voltage first. Connect everything and set your meter to the 1000V range and the probe to Plate Voltage. Then fire up the amp and take the reading. After you shut down the amp (and drain caps would be a good choice), then switch the probe to Plate Current, then switch the meter to the 200m voltage range, then fire the amp up.

    Make sense?
     
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  18. Tron Pesto

    Tron Pesto Member

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    On another note, I fired up an Ampeg V-4 for the first time today after literally months of pecking away at in - nearly a total strip down and rebuild. It is a 4 output tube push-pull circuit. The logistics in the amp make it very difficult to install 1-ohm testing resistors.

    The issue with using the transformer resistance voltage drop method to check the current draw is that each side of the output transformer sees the combined current of two tubes wired in parallel. This makes it difficult to directly asses the current draw in each individual independently. So this is one of those interesting cases that could be helped with the use of a bias probe.

    I have an old Bias King dual probe - it measures cathode current pretty accurately. But alas, poor Ampeg! I knew him Horatio! Verily the Ampeg hath BEAR CLAW retainers. (Sorry for the Hamlet allusion.) I couldn't seat the probe well because it is larger than the base of a tube and the bear claws fight to eject it (and the sockets are riveted so it is not feasible to remove the retainers). It's seems that your probe might be sleeker and hopefully will be more easily seated in similar situations.
     

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