Tube Tester

Budget?

I would only consider a mutual conductance tester. TV7 is plentiful, and probably the entry level as far as cost is concerned. Built for military use, they are very durable. From there you can go up, up, up in cost.

It does sound like you had an output tube that shorted. To check for shorts in a tube, a tube tester is not required, a multi-meter would be sufficient. Good luck with the hunt.

How to check tubes with a multimeter.

https://youtu.be/_M_lCwQgCMA?feature=shared

Agree with Viridian that you want a tester that measures transconductance. And that usually means Hickok or B&K (often made by Hickok.) Transconductance of a given tube varies with plate voltage and current. Unfortunately very few vintage testers are capable of providing adequate voltage and current for power tubes. Hence you may get a misleading reading for transconductance. Best to test power tubes in the amplifier where you can know plate V and current by direct calculation. And measure grid bias as a guide to tube wear. For big bucks you can get a modern tester that does a proper job, but IMO it’s not needed.

Searching for a bad tube is pretty easy. I always have a spare set. You just substitute one at a time until you find the bad tube... or eliminate the problem from being a tube. Most companies that supply matched tubes write down their electrical test results on the box or tube, so you can order a single tube matched replacement.