Depends on the model. I purchased to Lo460 which can use up to 460volts
on the plates. (Close enough to 500V for Randall)
You can email Paolo directly and discuss the models and pricing. I'll be a little less than on EBay. He communicates best in Italian. If needed, I'd be more than happy to help in answering any questions. Right up front, I'm not
financially involved in this. I just want to spread the word about a great product. If you do contact him please mention that I referred you, Pat Lozito.
The tester is controlled by a micro computer. All voltages are continuously
variable and accurate. These include, heater, plate, and bias with direct
digital readouts for heater, plate,and -bias voltage and plate current.
You can calculate amplification factor (u), Plate resistance, and transconductance from that information.