WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
Building, troubleshooting and testing of these amplifiers should only be
performed by someone who is thoroughly familiar with
the safety precautions around high voltages.
I've use NTC thermistors in series with filament on 5C8S and 6C33C/6S33S. Some of these tubes make a rattling sound from the filaments at cold turn on. The thermistor minimizes it.