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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
16 responses Add your response
herman
2,253 posts
 
what are you wanting to measure?
rodman99999
6,130 posts
 
You'll have to wait for the voltages to settle back down, if you are trying to measure your bias adjustments(regardless of how they are read). I use a Fluke digital and just push the 1/4" plug(Cary provided) into the jack with the meter turned on(no problem), but my Carys read out the bias as current(in mA).
oem OP
91 posts
 
Tube plate current
rodman99999
6,130 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
2,253 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
4,425 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.
oem OP
91 posts
 
Jea48> Thank you for your advise.
Herman> This morning I warmed my amp for 30 minutes and with the amp on I inserted the leads from my voltmeter.I turned the meter on and the amp started to smoke.I shut everything off, removed the leads and let everything cool down.
I powered the amps back up and everything was fine.
I then followed the instructions of inserting the leads turning the meter on and then powering up the amps and left it that way for 30 minutes and then took the reading.This worked perfectly.
I agree with your method and have always biased that way.I have know idea why my new Radio Shack voltmeter operates differently.
rodman99999
6,130 posts
 
Mr O- In my previous post I was stating the method that works for my Cary amps, not suggesting you try the same. There are too many variations on the bias adjusting theme to make generalities. Glad you found a method that works without the smoke issue!
w_barton
16 posts
 
when you smoked your amp, where you in voltage or current mode with your digital voltmeter?Did it have a bad smell? if it did, you should have your resistors checked out, if you find any smoked, replace it for it has increased in resistance which will effect that circuit in the long run.
oem OP
91 posts
 
W.>I was measuring bias using a DC VOLTS scale.
The resister is still reading the same as is noted in the owners manual.I thank you for this information.
herman
2,253 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.
oem OP
91 posts
 
Herman> your advise was absolutely correct.I may have done something inadvertently that would cause the problem.
I still dont understand why the instruction manual would read that the amp should be off when attaching the leads of the Digital Meter.Wouldn't this defeat the purpose of biasing an already heated tube.

w_barton
16 posts
 
they put that blurb in there for safety reasons. It would be more correct if you would measure it like you suggested.
The reason you would want to change the smoked component is that it will have latent failure associated with the smoke and eventually derate its performance. Over a period of time the heat and increased resistance would eventually breake it down until it will no longer function and you would be wondering what happened.
Post removed 
minorl
1,016 posts
 

Voltage on a digital multimeter looks like an open circuit to the circuitry you are trying to measure.  Therefore, it can't hurt the measure while the circuitry is "hot" as long as you are in the correct setting.

If, however, you inserted the leads with the circuitry on, but the multi meter off, you ran the very real risk of a short circuit when turning your meter on.

it looks like the OP made the simple but easy to make mistake of shorting the circuit by having the multimeter on the wrong setting.  Either Ohms, or current, and not voltage. 

I can't think of one Engineer or Technician that I know, including myself that has no fried a circuit by not paying attention to the setting on the multimeter.

I would open the amp (unplug it first) and do a very close inspection of the circuit boards to see if there are burned traces or components.  Look closely.

Something was going bad and fried, or partially fried.

take a look.

enjoy

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