[DoubleWound]

sven_swe's analysis has been of tremendous help to me.

Since then I have now tested a brand new ECC83 and got the correct Gm. Up until then I had only tested old valves so it was not possible to know if the 45C was accurate until his report.

I suppose minor variations between instruments are inevitable and, at the end of the day, will have no effect for valve matching or the ability to weed out a valve that is on its last legs.

Yes, I think they are great little testers although I would advise users to add more earthing bonds to various parts of the cabinet.