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Using a Digital Multimeter for the first time

After 15 years of service I replaced my old voltmeter with a Radio Shack Digital Multimeter.The instructions read that the amps should be turned off when inserting the leads. This would mean that after playing the amp for 2 hours I would have to turn them off insert the leads and then turn the amp back on.I doubt if I could get an accurate reading this way but Radio Shack says I could.So,I ask for your opinions.
Thanks all
oem

Showing 3 responses by herman

herman
2,277 posts
 
what are you wanting to measure?
herman
2,277 posts
 
There is no reason to shut the amp off prior to measuring. You are actually measuring the voltage drop across a resistor under the cathode that is in series with the tube to give you the plate current.

V = I x R

You do want to wait until the amp is fully warmed up before adjusting the bias.

The meter is parallel to the resistor and so much higher in value that it effectively doesn't change or affect the reading.

The big difference in digital vs. analog meters (other than the convenience if it is auto ranging) is the digital meter has a much higher input impedance. This means it will have less effect on the circuit and give more accurate voltage readings.
herman
2,277 posts
 
Sorry you had trouble but the advice was correct.

I have made 10's of thousands of voltage measurements on live circuits with all types of analog and digital meters and never smoked anything. Properly used there is no way a digital meter set to volts could cause smoke. Impossible. The most logical explanation is you accidentally set it to measure current as you turned on the meter

You should have the meter on and then insert the leads.

You really should open it to inspect for damage and replace the part that smoked.
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