[HAM] DC filtering to the heater filament voltage of the AO28

[David Anderson]

Kon,

Are you getting the proper 6.3VDC filament voltage from the full wave  
bridge? I ran some calculations at one point to see if I could get  
enough 6.3V direct current from the AO-28's heater winding, and it  
seemed questionable at the time. There's no free lunch on rectifier  
circuits since rectification is not 100% efficient. Every idea I  
tried drew more power from the filament winding than standard AC  
operation.  (I'm building a hi-fi preamp over the holidays, and it  
uses a SS voltage regulator for the tube filaments.)

You could also use a small separate filament transformer with a  
center-tapped secondary for the 6X4 alone and connect the center-tap  
of the secondary to the 6X4 cathode. That way, there would be  
negligible voltage difference between the 6X4's cathode and heater,  
making a heater-cathode short essentially a non-issue. Hammond Mfg.  
makes a small filament transformer that would work perfectly. Costs  
about US$11.

In fact, if I were using DC for filaments to try to reduce noise, I  
would want to have the rectifier tube on a *separate* supply to  
prevent noise coupling from the rectifier cathode to the rest of the  
heater supply. That's the way Dynaco did it in the PAS preamplifier;  
its 12X4 rectifier runs on AC, while the 12AX7s run on filtered DC.

David Anderson

On Dec 19, 2007, at 6:07 PM, Kon Zissis wrote:

> Hi everyone.
> I wrote :
>> I also grounded the - voltage wire so therefore the heater filament
> voltage   is now a basic
>> full wave rectified DV voltage.
>
> That should say " DC voltage" instead of "DV voltage ". Sorry for the
> spelling error.
> All the best .
> Kon