i need a new multimeter... help????

bandmaster188

Ok, so I got my new multimeter today and it appears that the old one might not have been broken, it might be me that is.

i'm trying to learn amp stuff and was watching a video of gerald weber biasing the amp using the shunt method (i think)

so with my old meter i was measuring in mA from the red lead of the transformer where it connects to the board to pin 3 of the output toooobs and it was not giving any reading. i have just tried it with the new meter and its the same. nothing.

what am i doing wrong? 

jpfamps

bandmaster188 said:

Ok, so I got my new multimeter today and it appears that the old one might not have been broken, it might be me that is.

i'm trying to learn amp stuff and was watching a video of gerald weber biasing the amp using the shunt method (i think)

so with my old meter i was measuring in mA from the red lead of the transformer where it connects to the board to pin 3 of the output toooobs and it was not giving any reading. i have just tried it with the new meter and its the same. nothing.

what am i doing wrong? 

Firstly you are employing a very dangerous method of measuring the anode current, and I do not recommend this.

Essentially you are shorting out the transformer with the multimeter and floating the multimeter up to the HT. Not a good idea. Slip with the probe and there will be sparks.

Notwithstanding this, my guess is that you are plugging the red lead into the wrong socket for measuring current; most DVMs have a voltage/ohms etc socket and a separate socket for measuring current.

There is a very good reason for this: voltage settings need to have a very high impedance so as not to load down the circuit under test and skew the reading. To measure current you need a very low impedance. If you could switch between the two simply by turning the dial you could inadvertently cause a short which may damage your equipment.

Again I would NOT recommend this method.

A safer method that doesn't involve a bias probe is to drain completely the filters caps (this is very important for getting the correct measurements) and then measure the DC resistance on each half of the OT primary. 

Fire up the amp and measure the DC voltage drop on each half of the primary (the voltage measurement is high impedance so you have less of a risk of shorting anything out). You can calculate the anode current using Ohm's law.

I should add that both these methods require poking around in the vicinity of the HT so great care should be taken. It's best us insulted crocs to clip the probes to the relevant part of the circuit, leaving you hands free.

jpfamps

Incidentally I can't think of the last time I had a need for measuring current with my DVM.

bandmaster188

yes i know, i'm poking around with stuff i don't fully understand, but this is the way to learn isn't it. hands on!!

does this mean i should move the common test lead to the 10A socket?    < silly boy!

[-O<

ICBM

What jpfamps said - do NOT ever use the 'transformer shunt' method. It's by far the worst for several reasons.

Firstly as jpfamps said it's extremely dangerous, and secondly it's very inaccurate - it relies on the resistance of the meter being effectively zero, and it isn't. Worse, the error is in the unsafe direction - the true current is *higher* than the reading. Finally it can often make the amp unstable and self-oscillate. (Although even the primary voltage method can, sometimes.) There really is no good aspect to it, even as a quick rough check.

bandmaster188

ok. warning heeded. but what was i doing wrong with my meter? and i promise not to try it again!

scrub that. i 've read jpmamps reply properly

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ecc83

Glad you have taken the advice of our two top techs on board!

So you now have TWO meters? Excellent because you can make a better measurement of anode current using the transformer resistance method since you can have a meter across both windings and read the two resistances and voltages.

But first make up some 1/2mtr 4mm plug to insulated croc leads. Then you can set the whole shooting match up "cold" and unconnected. Recheck connections and ranges then fire it up and hands off.

Like Jpf, I hardly ever measured current directly. In fact there is rarely a need to know it with any degree of accuracy and there is almost always a resistor you can clip across to infer the value. If you want better accuracy, measure the exact value of the resistor as well..Power off and dumped of course!

Dave.

ICBM

The only direct current measurements I normally make are mains current and HT current - usually by pulling the fuses, if the amp has them, and clipping across the holders - with my vintage analogue AVO meters. Accuracy is not necessary - even though those old meters are very good - it's just to get a ballpark figure which will let you know if something is wrong. I like analogue meters for that because the movement of the needle gives a good idea of the speed of any change, which can be important.

bandmaster188

Got a couple of sets of alligator clips ( that's what Americans say!) ready for a silverface bias party. But seriously my amp tech is gonna be showing me some bits and pieces and safe working practices very soon. So hopefully I should have a bit more of a clue what I'm doing with my multiple multimeters! Always grateful of the wisdom of you guys. Cheers!