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

[Kon Zissis]

Hi David.
In the stock standard  set up the AC filament voltage reading was 6.5
volts  on my voltage meter but now that I have added the full wave
bridge rectifier and 2200 uf smoothing capacitor the DC filament voltage
reading is  5.35 volts so therefore there has been a voltage drop of
around 1.15 volts.
 
The heater filaments seem to light up with a normal looking yellow glow
and the non vibrato channel and the vibrato / chorus channel sounds
normal. 
Because I have not yet finished the work with the AO28 , I have not yet
reconnected the percussion  wires so therefore I have not yet tested the
percussion sound. 
There is  some distortion at full volume level because the gain trimmer
capacitor is turned up to produce the maximum output levels. 
There is less hum because I replaced the 42 year old power supply filter
capacitors.
 
One positive thing about the 1.5 volts loss is that the heater filaments
s are not heated up as much so therefore  they may last longer because
they are running lazily. However I have read that lower filament
voltages can cause cathode stripping and if 5.35 volts is enough to
cause this then it will be better to remove the DC filtering and return
back to the stock AC filament voltage wiring .
 
Interestingly Bill Beer is quoted in the Beauty and the B book as saying
that the valves in the Hammond preamps should practically last a
lifetime because they are running lazily. However the AO28 filament
voltage is 6.3 volts ( 6.5 volts in the case of my 1965 C3 ) instead of
some lower voltage so therefore the AO28 valves are not running lazily
after all. 
As far as I am aware the heater filaments in the Model A , BC ,D,  E etc
organs   used ( if I remember correctly)   something like 2 or 3 volts
instead of 6.3 volts but I think that this might be because the 56 and
57 valves were designed to work with a 2 or 3 volt filament voltage.
 
In the -3 series organs that I have played,  the hum in the AO28 has
never been so bad that I would go to the trouble of adding a separate
filament voltage transformer or a separate power supply for the 6X4 ,
but it is good to know about the various ways of minimising hum. 
I basically added the DC filtering to the filament voltage out of
curiosity. If it turns out that the slightly lowered filament voltage
is potentially harmful  because it can cause cathode stripping or other
problems such as more distortion in the sound then I remove the DC
filtering and  restore the stock AC wiring set up.
All the best.
Kon
 
David Anderson wrote:
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.)