A Simple, Valve Curve Plotter

[AlanBeckett]

If I've got it right S2 and S3 appear to short out the Supplies when they're 'Off'. You're going to need a lot of Fuses, Transformers and Meters.
In general a high value Resistor across the Smoothing Electrolytic isn't a bad idea so that they get discharged when you switch the Mains off, but you've got the Pots for that.

Alan