From: ABSOLUTELY_NO@SPAM.ORG   
      
   "Big Bad Bob"  wrote in message   
   news:SOadnWoLsZTQ4DnQnZ2dnUVZ_qqdnZ2d@earthlink.com...   
   >   
   > perhaps a switching power supply for tubes _could_ have a 'warmup   
   > cycle' on the filament supply, ramping it up from 2VDC to 6.3VDC over a   
   > period of 10 seconds.  It's a 'feature' worth considering.  A similar   
   > feature might turn the bias supply on FIRST, then ramp up the HV side.   
   >   
   > Anyway, this kind of thing gives me a reason to consider that sort of   
   > stuff in any future designs.   
      
   I have a signal generator with a rather exotic (read as nearly   
   unobtanium) oscillator tube.  To protect this tube, I built a regulated   
   DC supply to power it using the ancient Fairchild uA723 regulator with a   
   pass transistor.  I set the current limit to 50% more than the normal   
   filament draw.  When first turned on, the supply regulates the current   
   for about 10 seconds with the voltage rising slowly.  Once the current   
   starts dropping, the supply switches to regulate the output voltage.   
   This supply not only protects the tube during startup but it also   
   increased the stability of the oscillator about ten fold.   
      
       73, Barry  WA4VZQ      {ham_call} at Hotmail.com   
      
   --- SoupGate-Win32 v1.05   
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