In
http://www.tubeaudiostore.com/tubmatdem.html is explained the difference between mu and gm.
In same article it is mentioned that pentodes should be tested as gm and triodes as mu.
In order to measure the mu we need to have a constant current source. At the moment there is a current limiter in the plate supply chain however if we make this adjustable then we get constant current source.
We then measure AC current instead of AC voltage and we've changed the setup into a mu meter.
Plate voltage constant => measure at the plate the AC current variation = gm
Plate current constant => measure at the plate the AC voltage variation = mu
What does everyone think?