I had some tubes and built a tester on my workbench, before I sold the
tubes. The buyer wanted to know how I tested them, then bought the tubes
and asked me to build a tester for him.

If you look at many of the old radio books from the 1930's, it will tell you
how to build such a tester. If you have a lot of jumper wires, you can
easily build one on your bench.

Simplest to start with a socket for the 211 and filament transformer. Then,
you need a plate voltage and a control grid voltage. In my test case, I
just grabbed stuff that caught my eye, and used 12 volts on the plate and a
variable 12 volts on the control grid. Using a microammeter on the plate, I
varied the grid voltage and measured the change in plate current with a
change in control grid voltage, and calculated the mu of the tube, using the
formula in the book.

That simple test will tell you a lot about the tube. It will not test the
tube at higher voltage levels, but you can use a hi-pot tester.

The tester I built for the 211 ended up using 1000 volts for the plate and
had a variable grid voltage and protection circuits. I recall I had 3
meters, measuring plate current and voltage (using a variac) and grid
voltage. I recall I then made a life tester by reducing the filament
voltage.

It was all in an old surplus wooden suitcase box that was polished up.
Looked like something important, with laminated charts, neon lights and a
leather handle.

73, Colin K7FM