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Simplest or easyest dc filament supply for 300B
Simplest or easyest dc filament supply for 300B
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Old 3rd April 2020, 04:53 AM   #91
SpreadSpectrum is offline SpreadSpectrum  United States
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Join Date: Nov 2007
I've had pretty good luck lately buying AC-DC switching modules and adding additional passive LC filtering on the output. This was for some op-amp supplies, but it really cleaned up the output.

I just got some inductors and caps to do the same on my 841 filament switching supply (7.5V, 1.25A buck converter) and I hope to have the same success. I was able to find inductors with acceptable DCRs and good SRF, so we'll see how it all works out.
My Blog: http://tubeswithatwist.blogspot.com/
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Old 3rd April 2020, 09:06 PM   #92
6A3sUMMER is offline 6A3sUMMER  United States
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Low pass filters can be a pain sometimes:

Inductors that have distributed capacitance (all that have turns do; a straight wire in free space does not have much), Capacitors that have series inductance (all do).

One time an engineer built a CLCLCLC low pass filter on a copper clad board using all SMT parts. It had 60 dB of out of band rejection.
When the circuit board was laid out, all the SMT 'solenoid' style inductors were lined up, and close together. Couple, Couple, Couple, was the name of the game.
The out of band rejection was 20 dB . . . time for another board layout!
All it took was to turn the middle L sideways (90 degrees reducing the coupling, and also intrinsically opened up the spacing). Success, 60 dB out of band rejection.

A little attention to details goes a long ways.
Amazing: A little Medicine Makes the Sugar Go Down (A slight Modification of "Mary Poppins").

Last edited by 6A3sUMMER; 3rd April 2020 at 09:11 PM.
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