Here, along with a power supply for heater and screen is the 6200B sweeping a vintage 807 Beam Tetrode. I haven't built a tube test fixture yet so I made some custom shorty test leads instead. The screen lead has a 200 Ohm 2W resistor in line. The plate cap is a 1 3/8 claw grabber from Radio Shack. I have both triode and tetrode curves above. Click for a larger image.

BACK

When testing tubes you will need an external heater supply When sweeping tetrodes or pentodes you will also need an external supply for the screen. I am fortunate to have kept my Heathkit IP-17 tube power supply that I built as a youth (dual 6l6GC in the output regulator!).The green wire running from the top supply to the tube base is the screen bias. There is a 200 Ohm resistor in-line-that's very important. Another important thing to do is supply bypass. Both supplies have a 1500pF 500V silver mica cap right across their outputs. Ideally the leads would be shorter, but I expect that they might get this long in a multi-socket fixture with rotary  switches to switch the socket pins around. Without the bypass the tube went unstable at control grid voltages near 0 (highest gain). This was manifested as the plate V-I curve running around the screen in a very un-predictable manner. It did this tetrode or triode connected without the bypass capacitor(s).

If you do it this way, or any other way PLEASE BE CAREFUL! The chances of getting shocked (go see Old Sparkly in action in The Green Mile) is very high. Be certain to turn voltages down before grabbing the wires to swap tubes, or change configuration. DO NOT, REPEAT DO NOT LEAVE IT UNATTENDED! Kids, pets, and unknowledgeable people are just too touchy feely curious.