[Station X]

I must admit that I've been lead up the garden path more than once when taking voltage readings in valve circuits using a DMM having a high input impedance. The readings were high when compared with those in the service sheet.

The reason is that an analogue meter with a low sensitivity (ohms per voltage) can simulate say a leaking a leaking screen grid decoupling capacitor and effectively modify the circuit. This pulls down the reading to that quoted in the service sheet.

This brings me to another point. A low screen grid voltage can be down to the screen dropper resistor going high in value, or the screen grid decoupling capacitor being electrically leaky.