Measuring HV in an electrostatic set, tips?

[Jon1967us]

I’m a little new to electrostatic TVs and I have a Hallicrafters T54 and I want to measure for HV on the 1B3 inside the HV cage, however when I put the tube extender in and put the 1B3 in the extender, in order to test, the plate cap is too short and won’t reach the terminal on the tube.

Any recommendations? I will be using a pomona hv probe.

Also, should I be hearing a high frequency horizontal oscillator sound or a motorboating vertical sound on a working electrostatic set?