Tube Tester Meter Driver Circuit
 

Here's a meter driver circuit that can be used to drive a replacement
meter with a different full scale current. I figured the shunt resistor
value using an original meter full scale current and voltage drop across
it. The Hickok 600A 500ua meter figures out to be 236 ohms, and the ckt
is driving a 5ma meter. Values may vary depending on replacement mtr
used. The linearity is right on, but I haven't actually used this in a
600A. If anyone tries it, post some feedback. Ken