Shipping a Tek 'scope - test-drb@ccmp.vtda.org

12 Nov 2009

Tony Duell wrote:

...
  The 555 even has regulated heater supplies. The
heater
 transformer is fed 
 through a saturable reactor. The current in the control
 winding of that 
 comes from a pentode valve in the PSU unit, the grid
 voltage of that is 
 controlled by a bright-emitter diode running off one of the
 heater lines. 
 So the thing actually gets the RMS value of the heater
 supplies correct 
 (which is what you want, of course). 
That's fascinating.  I'd seen "passive" ballast tubes 
for heater regulation, but not an "active" solution like 
this.  I found a good write-up at:

http://www.radiomuseum.org/forum/2as15_how_it_works.html

Especially interesting to me is the mechanical link in the
diode that shorts it out when the filament burns out, pulling
down the regulated voltage.  I wonder if they learned the 
necessity of that the hard way.

John Finigan