[Amps] 8877 heater inrush

[Alan Ibbetson]

I am collecting parts together to build a rugged but low weight 
amplifier for solo DXpeditions. I have an 8877 tube that I want to use.

A 5V switchmode PSU instead of a mains transformer for the heater looks 
a good way to keep the weight down but the low resistance of the cold 
heater (0.05 ohms for the tube I have to hand) means I will need to 
limit the current on switch on, not only to babysit the valve but also 
to prevent the SMPS going into foldback or hiccup limiting.

I can envisage a number of approaches to providing the required step 
start based on relays or solid state switches but they require an 
estimate of the time it takes for the heater element to become hot 
enough at reduced voltage to offer a "reasonable" resistance for when 
the full heater voltage is applied.

Does anyone have data on the heater startup current on an 8877? Curves 
of both steady state current versus voltage and of current versus time 
at fixed voltage would be useful. Please note that I am not asking about 
the time it takes for the cathode to reach operating temperature; that 
is well documented.

73, Alan G3XAQ