by dcgillespie » Tue Apr 10, 2012 12:00 pm
Linn -- I'm sorry, I should have been more specific.
Placing a cap from the junction of the Zeners and choke to ground will help squash any Zener noise, and help absorb any kickback the choke might create. This cap need not be an electrolytic, and frankly, a good film cap between .47 and 1 uF should be just fine.
The zeners can be further protected by making a common ordinary 1N4007/UF4007 diode part of the Zener string (place at either end of the string with the arrow facing away from the rectifier), and another such diode reverse connected across the entire Zener string (arrow towards the rectifier), which will then protect the Zeners from virtually all transients, positive or negative, and make the Zeners basically bullet proof. Finally, another small cap (say .1 uF) placed across the Zener string will finish off any remaining possible noise from the Zeners. Due to their position in the power supply however, any noise they might develop will be extremely small to begin with.
I have used this very approach to minimize the voltage drop usually associated with common screen dropping resistors in pentode amplifiers, and it works exceedingly well. In my case, the choke and Zeners were much smaller since the total current flow in this case is much less. But the principle is the same. This approach provides much better filtering due to the choke, and a very low impedance way to drop the B+ as required, resulting in much better regulation, all versus that of the original resistor.
Keep us posted on your results!
Dave