Which digital multimeter for biasing amps?

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by jpervin, Feb 14, 2006.

  1. jpervin

    jpervin Member

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    Any recommendations on an inexpensive digital multimeter for amp biasing? I was at Home Depot last night looking at them, and they had 3 or 4 different models, which varied in price from $20 on up. Wasn't sure if I should look for one with a certain current value rating or if they're all pretty much the same in that regard.
     
  2. John Phillips

    John Phillips Member

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    If you're using a bias kit which plugs in between the tube and the socket, any multimeter - literally, even those little ones that sell for about $10 in Radio Shack-type shops - will do. All you're measuring is a low voltage (a few volts at most, so safety is not an issue, which is why it's done this way), to no more accuracy than 1% (the usual resistor tolerance, and the accuracy of almost any meter no matter how cheap) - and possibly a lot less so because the measurement includes the screen current, which can be up to 10% of the plate current and is not known precisely. In fact, biasing tubes isn't that critical in any case. As long as you get a reading which lets you calculate dissipation to about +/- 5% you'll be fine.

    A good multimeter is worthwhile for more involved tech work, but it isn't necessary for biasing.
     
  3. jpervin

    jpervin Member

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    Yes, my plan was to buy a Bias-Rite adapter head from Weber and use it along with the multimeter. Just wasn't sure about which multimeter to use.
    https://weberspeakerscom.secure.powweb.com/biasrite/br_page.htm
     
  4. SteveStrat

    SteveStrat Member

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    I paid 35 bucks or so for mine at Radio Shack. Works like a champ.

    Steve
     
  5. countandduke

    countandduke Member

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    +1 on the Radio Shack one. They sucker worked PERFECTLY!!!!!! in my THD Flexi.......

    Chris
     
  6. RL in Fla

    RL in Fla Member

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    Ditto the "low-end" meters . A $20 digital for readings , and a cheap analog is nice to have for finding intermittent connections in cords/jacks/etc because you can see the needle swinging when you're wiggling the connection . 1% is plenty good enuff . If you really feel the need to spend 100 bucks , buy a Tektronix scope on EBay . ;)
     
  7. bryan k

    bryan k Member

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    actually, the probe used depends on which multimeter to use. SRS web-store sells bias probes in 2 versions. One is the "bias probe mA" and the other is a "bias probe mV". The mV version is common and almost all probes are of this type ( have a 1 ohm resistor in the probe), while on the other hand the mA version measures the current directly with no resistor in the probe.

    if you have the mV probe, any DMM mutimeter will work. If you have a mA probe, you need a specific DMM meter that has mA inputs. Youll see mutilmeters usually with either 3 or 4 inputs. The common meters have a 10A (red) input, a volt/ohm (red) input, and the (black) common ground input. other meters which usually are more expensive have 4 inputs.... a 10A input, a volt/ohm input, a mA input, and the common/ground input.

    if you have the typical bias probe that measures current using the 1 ohm resitor in the probe, youll want any cheap meter with the 3 inputs. if you have a probe that measures mA directly, youll need the 4 input DMM meter
     
  8. John Phillips

    John Phillips Member

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    Bias probes that meter the current directly, by breaking the circuit and feeding the current out and through the meter are a very bad idea and should be avoided - this can cause the amp to become unstable and at the very least give spurious readings; at the worst you could blow something. The long wires to and from the meter can 'couple' electromagnetically to the transformers and form a feedback loop.

    The 'voltage reading' type that uses a resistor in the plug-in adaptor to 'sample' the current so it doesn't go outside the immediate circuit to the tube are the right ones to use.
     
  9. bryan k

    bryan k Member

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    very good point,......yet i have the mA version probe....and had good results with it.

    but your point is still true, and i would take a mV version, before the mA version.

    unfortunatly....i only own a mA version probe..........:BEER
     
  10. jpervin

    jpervin Member

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    Good to know. Thanks.
     
  11. GuitslingerTim

    GuitslingerTim Member

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    The Sperry model 4100-A can be purchased for $40. It has four inputs and measures everything one can expect from a basic multimeter.
     
  12. Matt H

    Matt H Guest

    for what you're asking- anything that'll read millivolts.

    i picked up a pair of el cheapo craftsman dMM's for 10 bucks a piece- and i'm in love with the things... if i blow one up/ well... whatev... hahah
     
  13. VintageJon

    VintageJon Member

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    They will all work. I note that the lower the voltage the less accurate the cheaper meters are.
    I'm just a nut for accuracy so it's Fluke all the way here, but it ain't cheap. (Oh course i do this for a living and document the measurements so it's gotta be right.)
    Rat Shack is good for the money, catch them on sale. I went with Rat Shack and AES before I could bring myself to buy a Fluke.

    -Jon
     
  14. donnyjaguar

    donnyjaguar Member

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    Add another one for Fluke instruments. I've been using them for years. They are deadly accurate, the AC volts are RMS measured, the diode checker shows actual voltage drop across the diode and many have peak hold features for capturing transient conditions. I have an older unit too that measures capactance. And the batteries last forever in them too.
     
  15. bryan k

    bryan k Member

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    yeah, i have the sperry 4100A.........it works well. i got it used for 15 bucks!
     
  16. donnyjaguar

    donnyjaguar Member

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    Just wanted to add that using an analogue meter for measuring low voltages and high impedances is not a good idea. These old meters are rated something like "20k Ohms per volt". That's wonderful, but if you are measuring 0.1V you're adding 2k to the circuit which can upset it. It will also load down plate and Cgrid circuits enough to leave you scratching your head sometimes.
     

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