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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
oem OP
91 posts
 
Tube plate current
rodman99999
6,206 posts
 
Give the amp at least 30 minutes(after turning back on) to stabilize before taking your readings. How do your leads connect to the amp?
oem OP
91 posts
 
Rodman>The amp has jacks that accept the leads of the meter.
Correct me if i'm wrong.Your suggestion is to insert the leads of the volt meter into the jacks of the amp.Then turn the amp on and wait at least 30 minutes before taking a reading.
If this is correct can I play music while the meter is attached to the amp?
In the past,using an analog meter I inserted the leads while the amp was on to get the reading.Do digital meters operate different then analog meters?
Thank you for your help
herman
2,274 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.
jea48
4,605 posts
 
Correct me if i'm wrong.Your suggestion is to insert the leads of the volt meter into the jacks of the amp.Then turn the amp on and wait at least 30 minutes before taking a reading.

If this is correct can I play music while the meter is attached to the amp?
No..... Without any signal applied to the Amp's inputs. The Amp needs to stabilize. It cannot do this if you are playing music.
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