Now that I've checked it out, if you got supply and grid/cathode drive
and no output, it's bad.

Forget the 100:1 scope probe.

At first I didn't know this was a transmitting tube.

I was talking about telling if it's bad in circuit, which is good to
do. It's better than having a tester for each part.

Sorry that I wasn't directly responsive to your question. The only help
I can give you is; you need to find a way to see if the proper voltage
and drive is being applied to the tube. Luckily this is only a triode.
When cathode is too positive with respect to grid, tube does not
conduct. When the negativity of the grid with respect to the cathode is
not sufficient, the tube conducts.

Some of these circuits ground the plate of the tube at the heatsink
which means they will be using a negative supply, that is, a positive
ground. Instead of the 100:1 scope probe you might be better off with a
self contained HV probe, but you rewire the meter movement so it reads
negative voltage.

Alternately if you get a multiplier for your DVM, it shouldn't have a
problem with negative voltage.

Whatever this job is I think you can expect some high negative
voltages, if they only had one of these things they would call in a
contractor.

Best of luck, and I hope you got something out of my ramblings.

JURB