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

[David Anderson]

Kon,

Running at 5.35V is not going to cause cathode stripping or anything  
of that nature. It may change some of the basic tube performance  
parameters such as plate resistance. With lower heater voltages,  
you'll get less emission from the cathode and lower bias current,  
leading to lower cathode voltages and higher plate voltages. This  
will mean lower maximum output from the preamp before distortion;  
lower cathode voltage = easier to drive the grid positive with  
respect to the cathode.

Lower heater voltages do also reduce tube noise somewhat; it's a  
trade-off.

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.  
Power transformers are a lot more expensive than tubes, at least  
where I live.

Now, if you put the 6X4 on its own heater supply, you'd take some  
strain off the power transformer's 6.3V winding.

The real purpose of the separate filament transformer for the 6X4 is  
to eliminate the risk of a heater/cathode short, one of the more  
destructive ways a 6X4 can fail, presumably one of the reasons you're  
installing so many fuses. In stock configuration, the 6X4's heater is  
a ground potential, while the cathode is at several hundred volts.  
With the 5V rectifiers like the 5U4 and 5AR4, the cathode either IS  
the  heater (5U4) or is directly connected to the heater (5AR4), so  
there is little to no possibility of a destructive short.

David

On Dec 19, 2007, at 9:24 PM, Kon Zissis wrote:

> 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
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>