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In Reply to: RE: How to use dissimilar chokes? posted by DAK on May 13, 2015 at 12:59:41
How much current is needed - i.e what's the idle current of the KT90s?
As far as order, larger choke followed by smaller cap then smaller choke and larger cap will give a lower circuit "Q", and will have a better transient response - less ringing on a current step.
Eli's suggestion of a small input cap is essential. Without it, stored energy in the choke can be delivered into the reverse-biased diode, likely destroying it, on turn-off. The cap limits how high this voltage can go - I'd be inclined to use 0.1 uF myself - a 8 Hy choke carrying 150 mA stores about 0.1 joule (1/2* I^2 * L), and that amount of energy will charge a 0.1 uF cap to 1000V...
Follow Ups:
Hi TB, is the .01 cap required for SS rectification only? My amp has tube rectifier. In my amp I am using the first cap to help set the B+. With the .01 cap my B+ was coming too low. After trying different values 2.0uf was the value that comes up with the target voltage. cheers, Dak
2.0 μF. is large enough to take you out of critical current territory and the good regulation that goes with it. :> ( What vacuum rectifier are you using? A 5R4 drops a whopping 67 V., when delivering its 250 mA. maximum.
If you want critical current behavior, you need to find a lower forward drop vacuum rectifier or switch to SS rectified B+. The value of the "fudge factor" cap. needs to be under 1 μF.
Eli D.
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