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Is mV and mA the same thing?

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by TubeStack, Oct 13, 2010.

  1. TubeStack

    TubeStack Supporting Member

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    I'm trying to check the bias on my Marshall Vintage Modern 2466.

    All the Marshall info gives the recommended bias in terms of mV, ie. a reading between 72-80 mV on each bias test point is desired.

    However, my DMM doesn't have a setting for that range, and only has an mA setting. Using that setting, I'm getting a reading in the 30-35 mA range on each test point, which is much lower than 72-80, but I don't know if mA/mV is the same thing in this context? Some forums, including threads here, are using mA/mV interchangeably?
     
  2. whiteop

    whiteop Senior Member

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    No. mA = milliamperes and mV = millivolts
     
  3. embot

    embot Member

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    No. mV stands for millivolts - a voltage measurement. mA stands for milliamps - a current measurement. If your amp does not have a mV setting just use the V - voltage setting. 72-80 mV should read as 0.072 - 0.08 V.
     
  4. Blue Strat

    Blue Strat Member

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    Not the same but when taking a bias current measurement, you measure it as voltage across a 1 ohm resistor...hence 1mA becomes 1 mV.
     
  5. jchan

    jchan Member

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    There should be a mV setting. If not, use the V setting and it should be able to read mV (millivolts).
     
  6. TubeStack

    TubeStack Supporting Member

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    Okay, thanks guys, think I got it. I'm using the V setting and getting readings of .085 and .080, so 85 and 80 mV, which is higher than recommended. I tweaked the trimpot and am now getting 80 and 76 mV, which is in the desired range.

    Thanks, guys!

    (Still getting a mean hum from this brand new amp though, going to have the store's tech guy look at it...)
     
  7. teemuk

    teemuk Member

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    They are not interchangeable. You better learn how stuff works because due to how the meter works in current or voltage measurement modes you have a severe risk of destroying stuff if you measure from wrong places in a wrong mode:

    When measuring voltage the meter's internal impedance (affecting between the probes) is extremely high so that the meter won't load the measured circuit. The meter's effect in parallel with the measured circuit is almost comparable to open circuit. This is basically a "safe" mode because the meter's effect to the circuit is negligible - but you can only measure current flow mathematically, measuring voltage, known resistance and then using Ohm's law to calculate the current. It's not as fast, and often not too accurate either.

    When measuring current the meter's internal impedance is very low, so the meter is almost comparable to short circuit in parallel with the measured circuit. Thus you need to be overly careful what you are actually short circuiting inside an amp (or whatever you are measuring) and you need to take care that you don't accidentally forget the meter to "current" mode when you want to do other measurements. Luckily, in many meters you actually need to change one of the probes to a different terminal before current mode even works which prevents most misfortunes. Also, the meter will in many cases also carry very high currents in the current measuring mode and the cheapest meters may fail if you exceed their ratings, which are usually in the range of max. 10 amperes.
     
  8. phsyconoodler

    phsyconoodler Member

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    And....when there are test ports on the back of an amp they will always be across a 1 ohm resistor.So in your case Mv becomes Ma.
    You cannot measure the milliamps current at the cathode so that's why you need the 1 ohm resistor.
     
  9. jchan

    jchan Member

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    Good idea. If its brand new, there shouldn't be any hum, nor the need to adjust bias.
     

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