[Milsurplus] Stablizing a BC-221 that has an RA-133 installed

[WA5CAB at cs.com]

John,

You mis-read my post.  The 2500 ohm resistor is NOT the dropping resistor to 
the VR tube.  It runs from the tube +150 terminal to the BC-221 to drop the 
already regulated +150 down to +135.

In a message dated 9/4/2008 12:13:22 AM Central Daylight Time, jfor at quik.com 
writes: 
> Bridging the resistor with a Zener is a really bad idea, UNLESS you pull 
> the VR-150. The circuit is a SHUNT regulator. The problem is that a zener has a 
> really low incremental impedance (a few ohms for a 15 V device) so any 
> increase in line voltage sufficient to make the Zener conduct, effectively reduces 
> the current limiting resistor from several K Ohms, to a very few. This will 
> fry the VR tube very quickly due to excessive current. Not a good thing. 
> 
> If you do pull the tube, the situation is still not good. Any voltage 
> variations in the unregulated supply, will be passed on to the instrument proper, 
> volt for volt. In fact the % regulation is made worse by such a scheme. 
> 
> If you really want to regulate it, I'd tack in a three terminal regulator IC 
> taking the input before the series resistor, and the output to the + end of 
> the VR tube, and the common to ground through the appropriate resistive 
> divider to set the desired voltage. And you pull the VR tube. 
> 
> 
> 
> 

Robert & Susan Downs - Houston
wa5cab dot com (Web Store)
MVPA 9480   
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