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

[David Anderson]

Hi Kon,

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....

The simplest solution to this is to get a stand-alone filament  
transformer and build a supply based on it. You could even get a  
little toroid for low EM radiation.

I have noticed that the heater wiring in some AO-28s is neater and  
more carefully done than in others. If you have more hum than you  
want, you might try replacing the heater wiring with more tightly  
twisted pairs and routing them as far as possible from low-level or  
very high impedance signal paths. This can make a difference,  
especially around the 12AX7 with the high impedance line from the  
swell capacitor.

David

On Dec 20, 2007, at 7:22 PM, Kon Zissis wrote:

> 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.
>
> .
>
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