For characterising a triode you need 3 Power Supply Units (PSU),

1. Heater / Filament PSU: Wether your triode is a Direct Heated Triode (DHT), where the filament is also the cathode, or an Indirectly Heated Triode, where the heater is used to heat a separate cathode electrode, you need a power source for this.  Heater or filaments have generally low voltages, normally less than 30V, and at currents from a few hundred milliamps up to several amps.  When the heater or filament is cold, it has a low resistance, so it is important to consider limiting the PSU's current to prevent excessive currents on turn-on, which could damage the heater or filament.  (This is not normally an issue in amplifiers as the heater will be supplied by a small transformer which naturally has limited current capability, but if using a laboratory power supply the current capability might be very high.)

2. Anode or Plate PSU: This is the high voltage supply and depending on the valve under test, can range from a few hundred volts up to a few thousand volts, with currents ranging from a few milliamps up to a few amps.  ***Extreme care needs to be taken with these voltages as they can be lethal.***. (The definition of High Voltage or Hazardous Voltages differs somewhat in the literature, and also if it is an alternating current (ac), or a direct current (dc) but to keep it simple, ANYTHING OVER 30V should be considered DANGEROUS and appropriate CARE TAKEN.)

3. Grid PSU: Normally triodes are operated with a Grid voltage that is negative with respect to the cathode voltage and so the Grid PSU needs to be set-up so that it's voltage range can go from 0 to several hundred volts negative with respect to the cathode voltage, and under normal operatin the grid current is effectively zero.  (Triodes can be operated with positive Grid voltages wrt the cathode, but this results in significant grid currents and is not something that I cover here.)