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

[Kon Zissis]

Hi David and everyone.
Because the bridge rectifier loads down the filament winding of the AO28
power transformer, then I wonder if a possible alternative way to get a
DC filament  voltage without loading down the filament winding of the
AO28 power transformer  would be to first reconnect the centre tap of
the filament winding back to the chassis ground and to then wire up two
diodes in series with the two  wires of the filament winding , with one
diode on one of the wires facing one direction and then  the other diode
on the other wire facing the opposite direction thus creating a DC
voltage even though this DC voltage is not as fully rectified as it
would be with a bridge rectifier. Then a smoothing capacitor  could be
wired across the two rectified  + positive and - negative voltage wires
that then  go to the valve filaments. 
 
The  - negative voltage wire would NOT be connected to the ground so
that it remains isolated and thus the wiring remains closer to the
original stock wiring because the centre tap of the filament winding
will be grounded again thus making the filament winding a balanced
source  set up and hopefully the two diodes and the smoothing capacitor
will help  minimise any 50 Hz or 60 Hz hum.
 
Because of the 0.6 voltage drop of each diode , the DC voltage will
still be around 5.3 volts but at least the filament winding will not be
loaded down in the way that it is with the bridge rectifier.
 
Does the above alternative  approach to get a DC filament voltage sound
OK or would there be problems with this approach? 
 
Jon Lord's former tech Mike Phillips wrote the following helpful
information on Hammond Zone :
 
"First problem is the bridge !  use schotky diodes they have much less
drop in them! Second problem is the capacitor! You should have 6.3 plus
volts if you have not then the value of the capacitor is much to low
 
10,000Mfd at 10V is more like it as 6.3V is the RMS voltage and 8.8V
should be the PEAK DC you have to minus the diode losses so 6.5V DC is
easy to get. And watch the DC heater fuse it MUST be suitable for LOW
VOLTAGE or its resistance will be too high."
 
In this light germanium diodes would have even less voltage drop ( 0.2
volts as opposed to the 0.6 volts voltage drop of silicon diodes ) ,
however I do not know if germanium diodes can be trusted to be  reliable
and not break down in this application.
 
Thank you in advance for a reply.
All the best.
Kon
 
David Anderson wrote :
>With lower heater voltages... ....This  
>will mean lower maximum output from the preamp before distortion;  
>My concern was not with the tubes, but rather with the power  
>transformer's filament winding. When you run tubes on DC filaments,  
>you usually need a filament transformer with a higher wattage rating.  
 
.