On Sun, 8 Oct 2000 16:48:00 +0100 "Ian White, G3SEK" <G3SEK@ifwtech.com>
writes:
>
>Radio WC6W wrote:
>> 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
>
Hi Ian,
I was worried current surges too; Before I tried it!
When I first used this technique in my 4CX1500B amp, I added a
capacitor in series with the remote voltage sensing terminals and
precharged it from the 5V logic supply. This fooled the supply into
thinking (yes, SPS's do think... strangely) it had 6V (the heater rating)
on the output when it was only at 1V, until the capacitor discharged.
I have since used a switcher on a single 4-400A with no special
consideration.
All of the larger switchers I've run examined (your experience may
vary!) have some form of soft start which protects both themselves and
the filaments/heaters.
73,
Marv WC6W
*
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