A Simple, Valve Curve Plotter

[SurreyNick]

The other option is to go for monitoring cathode currant with a separate grid bias supply and not the common supply used for transistors in the case of my late 1970s home design/build when I was a teenager and relied on bags of unknown transistors for all my experiments.
It can also be seen with a bench power supply as a range extender allowing power transistors to be tested in anger....

I'm a little embarrassed to confess I didn't understand any of this!
Nick