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
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