question on bias probes and differnces....

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by bryan k, Feb 9, 2006.


  1. bryan k

    bryan k Member

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    so far, i know of 2 types of bias probes, one that measures mV, and one that measures mA. both are availible at www.srs-webstore.com

    Most people are aware of the mV type, and set thier DMM to DCV 200m, and use the volt/ohms input (red) on the DMM.

    for those who have the mA type, would this mean using the DCA 200m setting, and pluging into the 10A (red) input on the DMM?
     
  2. Dave C

    Dave C Gold Supporting Member

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    From what the description at SRS says , I would have to say you are correct. Most of the probes I've seen are of the mv type and generally work pretty well. Hope this helps.
    Dave C
     
  3. bryan k

    bryan k Member

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    but does one probe work better, or is safer to use than the other? or it doesnt really matter.....?
     
  4. Dave C

    Dave C Gold Supporting Member

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    I've only used the mv types when I've used a probe at all , there are other methods for checking and setting bias too, but I have found the bias probe to work well and be generally safer than some of the other methods.
    Dave C
     
  5. bryan k

    bryan k Member

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    well, im back to square one.

    i tried pluging in the probe to the DMM using the 10A input , set the dial to DCA 200m, got no readings. SEt it to DCV 200m with same input, still no readings.

    so i switched to the regular volt/ohm input on the DMM, set it to DCA 200m, got readings, but VERY small values and numbers ( 30 max to 15 minimum after turning bias pot full turns to the max both ways).

    i set the dial to DCV 200m, same thing...very small values. what the heck is going on? is my DMM bunk, crappin out on me?
     
  6. bryan k

    bryan k Member

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    keep in mind i have a SRS bias probe (model mA).....................

    whats going on here!?:confused:
     
  7. Dave C

    Dave C Gold Supporting Member

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    Sorry you're having so much trouble ....what amp and tubes do you have ?Is this a Fender with a bias balance setup ? Just a guess on my part, but that might explain it. Your probe (if ma type) should be plugged into the "A"(or"ma"if your meter has it)and common connections. The numbers will be small if plugged into "A" .......like .015 -.030 because you're reading ma on the full amps scale....it would most likely read 15-30 on the "ma" scale , if you only have "A"(amps)on your meter .015-.030 would be read as 15-30ma disregarding the decimal places.Hope I haven't confused you even more.
    Dave C
     
  8. bryan k

    bryan k Member

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    no, you not confusing me dave. I have a Carvin legacy 100 watt head, with el34 (mullard re-issue) tubes. with these tubes, i should be getting a reading inbetween 20-60, but im betting lower values. and when i had the bias pot maxed out, The DMM read only 30, and the tubes plates were glowing RED hot!

    i have a funny feeling its the DMM, and this type of bias probe will work ONLY if the DMM has a specific input for mA, not using the 10A input.

    OR.......my DMM is a piece of junk, and is wacked!
     
  9. Dave C

    Dave C Gold Supporting Member

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    Yikes , I think you should try a different meter or something else for sure cuz that ain't right at all.
    I think I'd suspect the DMM as well. Let me know what you find.
    Dave C
     
  10. bryan k

    bryan k Member

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    yeah, i have a new DMM coming in the mial in the next day or 2. This one supprts a higher voltage, and has 4 inputs. a volt/ohm input, a 10A input,a mA input, and the common ground.

    i think this will work..............................
     
  11. fullerplast

    fullerplast Senior Member

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    Bryan- I'm assuming your bias probe puts the ammeter in the path between the cathode of the tube and ground. If you want to try something, you can simply connect a 1 (or even 10) ohm wirewound resistor where you would normally connect your meter. Make sure it's a good solid connection. Then simply measure the voltage across that resistor and you'll have your reading in mV. (Divide by 10 if you use the 10 ohm). That's the only difference between the two types of bias probes AFAIK.
     

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