Measuring Cathode voltage on a DHT amp - why doesn't Klein MM400 work? Klein CL390 does...

Greetings Friends. I'm curious why one meter, the Klein CL390, can make a measurement when a similar product from the same maker, Klein MM400 can't.

I built a single-ended directly-heated-triode amp using 6B4G tubes. Each output tube has its own, simple DC rectifier fed by a dedicated transformer Like this one. When I was checking the filament voltages, I noticed that the MM400 could not find a voltage pins 2 & 7, while the CL390 quickly showed the voltage. Why is that?

The filament of these tubes has 2 voltages across them - the heater voltage, 6.3v here, and the cathode voltage, 45v. Did the MM400 get confused?

thanks!

w