The Tektronix Type P500CF is a cathode-follower probe.
It requires +120 VDC for the plate supply and +6.3 VDC for the heater, which can be provided by an oscilloscope (e.g. 517), amplifier (e.g. 1121) or external probe power supply like the 128.
The P500CF is discussed in the Type 128 manual.