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

[Kon Zissis]

Hi David and everyone.
 
I just made up  a bridge rectifier made up of four IN5822 40 volt 3 amp
rated Schottky diodes with a 10000 uf smoothing capacitor and I then
connected it between the filament winding ( with the centre tap wire
disconnected from the ground )  and the heater filament wires , and
this produced a 6.32 volts DC reading to the valve heater wires. 
 
When I disconnected the Schottky bridge rectifier and 10000 uf capacitor
from the heater filament wires , the unloaded voltage reading from the
Schottky bridge rectifier and 10000 uf capacitor  was then 10.53 volts
DC.
 
The PBPC 1004 silicon bridge rectifier with a 10000 uf smoothing
capacitor  connected to the heater filament wires produced a 5.72 volts
DC reading and then when I disconnected it from the  heater filament
wires,  the unloaded voltage reading from this silicon bridge rectifier
and 10000 uf capacitor was 9.94 volts DC.
 
I also tried out  the simpler DC rectification method that I mentioned
yesterday, which  had two IN4007 silicon diodes wired in series with the
filament winding wires. 
The first IN4007 diode was wired in series with one of the filament
winding wires and the second  IN4007 diode ( wired in the opposite
direction to the first diode ) was wired in series with the other
filament winding wire.
I wired up a 10000 uf smoothing capacitor across the he positive
voltage wire and the negative voltage  wire I then connected these two
wires then  to the heater filament wires.
I disconnected the filament  winding centre tap from the ground for this
DC rectification wiring set up.
 
After I did this  wiring , I measured  the DC voltage going to the
heater filaments and it was only  4 .3 volts or thereabouts. I then
replaced the two IN4007 diodes with IN5822 Schottky diodes and the
voltage DC reading then went up to around 4.76 or 4.7 volts DC so
unfortunately this voltage is too low  for the heater filaments .
Therefore not only did I not get a free lunch , I did not even get a
free snack with this failed attempt to get a suitable DC voltage
without adding any extra current draw to the filament winding.   
 
As long as the AO28 power transformer can safely handle any extra
current  draw , then the Schottky bridge rectifier with the 10000 uf
smoothing capacitor  seems the best way to produce the DC voltage
because this produces the required 6.3 volts DC for the valve heater
filaments.
 
However I am concerned about any extra current draw caused by the
Schottky bridge rectifier because I am going to add an extra 12AX7 valve
in my 1965 C3 as part of the 12AX7 based EQ/ overdrive unit that I will
build. The extra 12AX7 will draw more current from the filament winding
so therefore I do not know if the Schottky  bridge rectifier combined
with the extra 12AX7 valve will draw too much current from the filament
winding thus overloading the AO28 power transformer.  
 
Do you know what is the maximum safe current draw handling ability of
the filament winding is ?
 
Do you know what the actual total  current draw of  the heater filaments
of the eight valves and the pilot lamp in a stock AO28 is ? 
 
The main reason why I am doing this DC filtering experimentation is out
of curiosity instead of because of any actual hum problem in my AO28
preamp. Now that I am partially rebuilding the AO28 it is the ideal time
to try out this  DC filtering modification . If it turns out that the
Schottky diodes bridge rectifier and 10000 uf smoothing capacitor
combined with the extra 12AX7 valve will end up drawing too much current
from the filament winding , then I will abandon this modification and
rewire the filament winding back to the original stock AC voltage wiring
set up. 
 
The brown heater filament wires look reasonably neat and twisted
properly in my AO28.
All the best.
Kon
 
David Anderson wrote:
You still want a free lunch.
 
What I mean by that is what I said earlier about rectification not  
being 100% efficient. You need a certain number of Watts to run the  
tube filaments, and this power comes from the transformer's filament  
winding; it has to come from SOMEWHERE. There is no getting around  
the fact that running the filaments on DC requires a slightly higher  
rated filament supply winding--unless you've found a way to get power  
from thin air....