Multimeter Tutorial

Knowing how to use a multimeter is a very useful skill to have when building an amplifier. This tutorial will teach you how to use it.

Safety/General Information

This is very important information, please do not skip over this.

Always make sure that your test leads are in the correct jacks and that your rotary switch is in the right position. You might blow your fuse if you don't do this!
Only use one hand to touch leads onto a circuit which is turned on! If I am measuring voltage I always either tape my ground lead to the chassis or use an alligator clip to clip it to the chassis. If I am measuring current I use alligator clips on one lead, make sure it is securely in place and then use either an alligator clip or point with one hand. The reason two hands are dangerous is because the electric shock can go right through your heart if it comes in one hand and exits out the other.
Turn power on for voltage and amps. Turn power off for resistance measurement.

Measuring Volts

This is a very useful measurement since a lot of schematics on the web include voltages on wires and tubes. This measurement does not need to be a part of the circuit, unlike current measurement. This is useful because that you means you do not have to change any circuitry to get a useful measurement. If your amplifier is not matching up with the schematic and you made the amplifier to be the same as the schematic then you should go to the troubleshooting section.

Select highest maximum voltage on rotary switch
Make sure your positive lead is in the volts jack and your negative lead is in the ground/common jack.
Connect negative lead to chassis securely. Be sure that it will stay one without you touching it
Turn amplifier on. If you are measuring on a tube amplifier then you need to wait about 20 seconds for the tubes to warm up.
Take Reading
If reading is too low, then select the next lowest maximum voltage , take reading and repeat this step until you have a good reading.
To know what units are being used just look at the units on the maximum voltage and append those to the end of the reading. Examples are below.

Reading: 20.2	Max Voltage: 200MV	Units: MV	Actual Voltage:20.1MV
Reading: 1.10	Max Voltage: 60V	Units: V	Actual Voltage: 1.10V

Measuring Resistance

This is another useful measurement, especially if you are not sure that you correctly interpreted a resistor value! Although if you think you did you can also download the Reading Resistors (Excel) and use the examples section in the spreadsheet.

Select highest maximum resistance on rotary switch
Make sure your positive lead is in the resistance jack and your negative lead is in the ground/common jack. Power should be off in your amplifier if you are testing a resistor inside of there.
Put leads on the wires coming out of the resistor, the only thing between them should be the resistor
Take Reading
If reading is too low, then select the next lowest maximum resistance , take reading and repeat this step until you have a good reading.
To know what units are being used just look at the units on the maximum resistance and append those to the end of the reading. Examples are below.

Reading: 122	Max Resistance: 2M Ohms	Units: M Ohms	Actual Resistance: 122M Ohms
Reading: 3.56	Max Resistance: 200 Ohms	Units: Ohms	Actual Resistance: 3.56 Ohms

Measuring Current

There are two ways of measuring current. I recommend the first method since it is the least intrusive on a tube amp circuit. The second method should only be used if it is impossible to use the first, indirect method.

Measuring Current Indirectly (Recommended for tube amps)

Set your multimeter to measure volts, if you don't know please read the Multimeter Tutorial
Put the red lead on one side of the resistor and the black lead on the other side of the resistor. The only thing between the leads should be the resistor
Measure the voltage, lets say you measured 2V
Read the resistor value. If you don't know how then go to Reading Resistor Values.
I=V/R, so current=voltage/resistance. When doing this math, if you have prefixes on your units make sure you convert you unit to a normal unit without a prefix (22k Ohms, use 22,000 in your calculator).
If R=100 then I=2/100=.2amps

Measuring Current Directly (Not recommended for tube amps)

Select highest maximum current on rotary switch
Make sure your positive lead is in the current jack and your negative lead is in the ground/common jack.
Pretend the multimeter is the wire itself and the leads are the ends of the wire and hooks them up that way. The multimeter has to be a part of the circuit in order for this measurement to work, that is why the indirect method is recommended.
Take Reading
If reading is too low, then select the next lowest maximum current , take reading and repeat this step until you have a good reading.
To know what units are being used just look at the units on the maximum current and append those to the end of the reading. Examples are below.

Reading: 820	Max Resistance: 500mA	Units: mA	Actual Current: 820mA
Reading: 67.3	Max Resistance: 200mA	Units: mA	Actual Current: 67.3mA

If you have any comments or suggestions please let me know. Thank You.