Some of you might be aware that recently, a GEC 4603 CRT became available on eBay; the 4603 is a remarkably short 12" electrostatic tube used in their 1939 models. It was suggested it was NOS. Well, I was the successful bidder on it and yesterday I collected it from the seller.
Today, I decided to try testing it, so a particularly messy rig was created which involved my Radar 202 CRT tester (to provide the heater volts - 4V @ 1A), a Farnell 0 - 30V PSU to provide a grid-cathode potential and a 47kΩ resistor to limit the current from the PSU. The idea is to draw a current between the grid and the cathode, to see if the cathode has emission.
The base of the 4603 CRT is as shown here:
So then the rig was connected, the heater current adjusted for 1A at which point there was 4V on the heater terminals. I had to open up the base connector (two screws) to solder some wire tails to in order for the various croc clips to get a purchase on; it also gave me a chance to freshen up the old solder. The DC power supplied was connected via a kindly-donated Fluke 25 DMM and the 47kΩ resistor.
The DC PSU was switched on, and the voltage slowly advanced. I didn't want to draw too much current from the cathode for fear of stripping it, so I settled on a current of 300µA. I wonder how many volts it would take to achieve this current?
Well, the results are below.
As you can see, just 15V was needed to provide the desired cathode current. For the record, with the resistor missing I briefly drew over a milliamp from the cathode with but a few volts applied - so I think that unless my methodolgy is flawed, that's a win for the GEC!