Featured: Klein Tools MM720 Multimeter at Amazon.
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Heat is the enemy of every power amplifier. Transistors, tubes, resistors and transformers all run hotter as power goes up, and many amplifier failures start with something quietly overheating. A multimeter with a temperature input gives a builder a simple way to check heatsink and component temperatures alongside the usual voltage and continuity work. The Klein Tools MM720 is an auto-ranging, true-rms meter that includes a thermocouple.
What the MM720 measures
Klein lists the MM720 as measuring 1000V AC/DC, 10A AC/DC current and 60MΩ resistance, plus capacitance to 6000 µF, frequency from 1 Hz - 500 KHz, duty cycle, diodes, continuity and temperature. The temperature range is listed as -40° to 1832°F (-40° to 1000°C) using the included thermocouple. It uses a 6000 Count LCD and offers a low impedance (LoZ) mode, min/max and relative readings, and data hold.
Using the temperature input on an amplifier
A thermocouple lets you put numbers on what you would otherwise judge by touch, which is both safer and more useful. On the bench it can help you:
- Compare heatsink temperature before and after a cooling change, such as adding a fan or improving thermal compound.
- Check that paired devices run evenly. In a push-pull or paralleled stage, one side running noticeably hotter can point to a mismatch or a bias problem.
- Watch a test run into a dummy load and see how quickly temperature climbs.
Attach the thermocouple securely and keep it away from anything carrying RF or high voltage. Measure on the heatsink or case as close as you safely can to the part you care about, and follow the device datasheet for its temperature limits.
Know the limits
The frequency function tops out at 500 KHz, so the MM720 cannot measure RF frequencies in the HF or 27 MHz range. Its current function is listed at 10A, which is well below what a solid-state linear amplifier can draw from its supply, so never place it in series with an amplifier's main supply lead. Use the supply's own meter or a suitable shunt or clamp meter for that.
Klein rates the meter CAT IV 600V and CAT III 1000V, with IP42 ingress protection, and says it is built to withstand a 6.6-Foot (2 m) drop. The instructions also say to verify meter operation by measuring a known voltage or current before each use, which is a good habit with any meter.
What is included
Klein lists the multimeter, a carrying case, test leads with CAT III/CAT IV safety caps, the thermocouple, instructions and the batteries it needs. For a builder who wants one meter that covers the basics plus temperature, it is a practical combination.
A reminder that applies throughout amplifier work: external linear amplifiers are not legal for use on CB radio. Keeping a legal station's amplifier within its thermal limits is exactly where a meter like this earns its keep.
Where to get it
Check current price and availability: Klein Tools MM720 Multimeter at Amazon.
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