However it won't really do you much good unless a variable bias mod is installed. Since you only are concerned with the one amp, it might make sense to have 1 Ohm bias sense resistors installed. I bought set of bias probes rather than installing resistors in various amps. Some have built in bias sense resistors and others have the cathodes wired directly to ground like your amp. I have a fairly large collection of amps. If R = 10 ohms, I = E/10 or E*0.1.so 180mV measure across the bias sense resistor indicates 18mA of cathode current. If R = 1 ohm, I = E.so 18mV measured across the bias sense resistor indicates 18mA of cathode current. Ohms Law E=IR where E is voltage, I is current, and R is resistance. With the milliammeter you measure the current directly, and with the resistor you measure you the voltage drop across the resistor and then use ohms low to derive the current. Some probes insert a milliammeter in the cathode circuit, and some insert a one or ten ohm precision resistor. The cathode circuit is broken between the pins. All of the pins are typically wired pin to pin between the top and bottom of the adapter, except the cathode. These work by inserting an adapter between the tube and tube socket. The easiest and least destructive way is to buy a bias probe. Here is a page that provides lot's of info: The Last Word On Biasing I too recommend you avoid the plate methods as they exposes you to high voltage and more risk of electrocution.

Variable bias vs fixed bias guitar amp