Checking frequency with MM

[dude]

I have a Klein MM700 meter, l’m trying to check if hum coming from volume knob is 60 or 120 cycle. I set meter to Hz % with amp on and humming, put leads across the speaker jack with speaker connected, meter reads 0.0. Hum is present, goes away with first preamp tube pulled, tried several, all hum. Power supply filters read no a/c on them. Amp sounds good just the hum. Wasn’t there before. An Allen Old Flame, topology Fender super reverb. Hum still there with reverb disconnected. It’s either 60 cycle or 120..? How do I check

[uki]

A high resolution picture of the whole circuit would help a lot to spot something. schematic also helps.

[dude]

Seems to be the guitar, singe coils, l should have suspected this from the beginning, my bad. You can’t believe all crazy fixes on the net, gets you going down the crazy rabbit hole, ha.
But l would like to know how to use my meter read to read frequency.

[uki]

Check your meter specs to see how much voltage it can handle in the frequency setting, then check the voltage in points of the circuit to read. To see if it doesnt excede the meter limits.
But when talking about amp hum, it may be tricky to find. Maybe some HV wire crossing a signal only wire or wires in parallel, transformer positions.

check this out: 

[sluckey]

But l would like to know how to use my meter read to read frequency.

Turn the dial to Hz. Connect the probes the same as you would to measure AC volts.

[PRR]

It may need 8 Volts to sense a Frequency. That will be your loudspeaker output at room-shaking volume. Hard on the amp and hard on your family. (The Maximum seems to be 600V.)
https://data.kleintools.com/sites/all/product_assets/documents/instructions/klein/MM700_1390112ART_WEB.pdf

[passaloutre]

Can’t help with the meter, but you can get free phone app that will monitor the frequency spectra from the microphone. I use one called Audio/Spectrum Analyzer on my iPhone