Some valves on the market today are rare and expensive. GEC Marconi KT66's retail for about £300 /pair today for example. Some others clearly show a bright flash as their heaters initially power up which is worrying when you see it for the first time.
One concern I have is that the traditional constant current approach may not be sufficient to initially raise the temperature of the heater element sufficiently. I understand that some over current is necessary to begin the thermal resistance increase. Without over current, the heater will just sit there and may not get sufficiently hot. I'm not sure about this statement though.
One modern aspect of this question is that I'm not trying to be totally authentic with all of the circuitry. Only the valves. I'm prepared to consider any modern form of protection in what you might call a hybrid design. An anachronistic design featuring relays or micro controllers would be fine as well as DC drive.
What modern or traditional means can be used to reduce the initial current /thermal surge on a valve heater to extend it's life?
NOTE. A single small valve might require 300mA @ 6.3V RMS.
Best Answer
In order to limit the rushing current in the cold heaters + the current in the empty capacitors, use a 33 ohms inrush current limiter like a negative thermistor (NTC) just made for that purpose in the primary circuit of your power supply transformer. Here is an interesting article about that.
When it’s cold (at start) the NTC presents maximum impedance to the rushing current. After few seconds, the NTC heats up and its impedance lowers down to a ohm or so.