Radio WC6W wrote:
> Here's a modern idea. Get a surplus 50W switching power supply and
>wind a new output transformer for it. Update the feedback network and
>viola, a regulated screen source
Nice idea!
> I've done the trick myself. I prefer the older low frequency (20
>KHz) supplies and winding the new transformer using a "tape" toroid core.
> 4-400's stay pretty much in positive screen grid current but, you'll
>still need a bleed, either passive or active, just in case.
>
You'll definitely need a bleed, because the results of the screen
driving current back into a regulated switch-mode supply might be a
little, er, unpredictable.
> While you are out looking for switchers... pick up a 5V 75A unit and
>run the filaments off it; regulated! The output transformers in those
>are usually in center tap configuration so, internally it is about an
>11-12 volt CT transformer. If you tap off on the AC side of the
>rectifiers and run the tubes in series parallel you'll be able to better
>utilize the transformer and reduce the required wire size for the
>filament connection. A small bleed on the existing 5V output (may
>already be internal to the supply) will be necessary to keep the
>regulator happy. Then, just adjust the supply for whatever voltage you
>want on the tubes -- there should be sufficient range in the existing
>Vout adjust trimpot.
One problem is that a voltage-regulated supply could cause problems with
current surges at switch-on. Either the supply might try to drive the
maximum possible current into the cold filaments, or it might see the
filaments as a near-short and refuse to start. An electrolytic cap
across one leg of the V_out adjustment divider would probably make the
voltage ramp up gently.
73 from Ian G3SEK Editor, 'The VHF/UHF DX Book'
'In Practice' columnist for RadCom (RSGB)
http://www.ifwtech.com/g3sek
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