[Amps] 8877 heater inrush

[Radio WC6W]

Hi Alan,
   Here is my solution to that:

   http://qsl.net/wc6w/wc6wamps/index.html?fr63.html

73 & Good morning,
   Marv WC6W

http://qsl.net/wc6w/

--------------------------------------------
On Sun, 4/24/16, Alan Ibbetson <alan at g3xaq.net> wrote:

 Subject: [Amps] 8877 heater inrush
 To: amps at contesting.com
 Date: Sunday, April 24, 2016, 8:01 AM
 
 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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