[Amps] 8877 heater inrush

[Jim Garland]

Alan, I wonder if you've considered using an inrush current limiter on
either the primary or secondary side of your filament transformer? For the
secondary side, the NTC B57127P0100M301  (Digi-key pp/n 495-7209-ND)  is
rated at 0.053 ohm resistance at its maximum rating of 15Amps. Its cold
resistance (at turn-on) is 10 ohms, which would limit the initial current
surge to 1 Amp.  At 10 Amps, the normal current draw of the 8877, I'm
guessing the resistance would be 0.1 ohms or less.

On the primary side of a 5V/10A filament xfmr, the steady state current draw
is about 0.200 mA, assuming a line voltage of 250V. A reasonable choice of
ICL would be the Cantherm MF72-200D7   (Digikey p/n 317-1155-ND). This
device has a cold resistance of 200 ohms and a maximum current rating of 200
mA. At its rated current, the hot resistance drops to about 12 ohms. The
only downside of ICLs, in my experience, is that it can take up to a minute
for them to recover their cold resistance when turned off. With an 8877,
however, the heater will still be pretty hot after a minute, so that
shouldn't be a problem. 
73,
Jim W8ZR

> -----Original Message-----
> From: Amps [mailto:amps-bounces at contesting.com] On Behalf Of Alan Ibbetson
> Sent: Sunday, April 24, 2016 09:02 AM
> To: amps at contesting.com
> Subject: [Amps] 8877 heater inrush
> 
> 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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