FILAMENT POWER SUPPLIES FOR DHTs
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Wholey fat sacred cows, here we go again.
One of the unique traits of directly heated triodes is that their cathodes are directly heated with the filaments, which are the red-hot wires you see glowing in the tubes. This means that the design of the filaments and "how" they are heated, meaning the type of voltage used matters a great deal.
It is well know that the quality of this filament wire, its design and construction technique is a significant factor in sound quality. The microphonic quality of DHTs is directly related to how long these filament wires must be. Tap a tube with your finger and you can hear the wires "sing".
I have just explained why the "big breakthroughs" in DHTs have been in the area of filaments. For example both AVVT "mesh plate" filaments, and KRs developments are one of the reason we are experience such a high degree of refinement.
Filaments in other tubes such as "indirectly heated triodes" and pentodes are not as critical.
In a perfect world AC voltage should be used for the filaments as a way of heating them. It is the resistance to the voltage that makes the wire glow red and boil electrons. This is exactly what happens in your favorite electric bulb or electric stove, in a sense..though it is more complicated because we are talking about "thermionic emissions"; heat causing the flow of electrons.
There a number of reasons why AC is "electrically" superior to DC for operating the filaments of DHTs. BUT the problem is that the AC that is coming from our walls and is being stepped down by your power transformer is not the same quality AC that was available when these tubes were designed sixty years, or more ago. Today's AC is very polluted with noise, hash and high frequency contamination.
You will note that all DHTs that use AC have a "hum balance" pot which is used to adjust the 60 Hz, 2.5, 4, 5, or 10 volts of AC needed for lowest hum. It is best to use an AC millivolt meter to adjust for lowest hum. This hum is readily audible on high efficiency speakers, and you can adjust by ear if you want. The Sun Audio 300B push/pull amplifiers use AC hum balance controls.
It was interesting to note how different brands of 300Bs produced a different noise floor with these amps because of the differing design of their filament wires.
THE PROBLEM OF THE REAL WORLD
The problem of the real world is that it may be impossible to eliminate the AC hum from your DHTs because the AC line is so contaminated. This is certainly true for me, and why I must use DC on my filaments.
The benefit of DC is that it can be filtered, either actively with a solid state regulator, or passively with chokes, and thereby eliminate any noise contamination on the voltage.
PAUSE: David Berning employs an extremely high frequency switching AC supply to provide the filament voltage for his DHTs which yields a spooky quiet background because the human ear can't hear "250 khz hum".
FORWARD; Now that we accept that we are going to use DC filament the question is what type of "filtration" should we use because each has its own unique aural personality. That is right. In the DHT game we must pay attention to details that are not relevant with other tubes.
WARNING: There are certain type of active regulators that will make a DHT sound harsh. Next step for you: Go to www.tubecad.com and read John Broskie's articles about active regulation of filament voltage for DHTs.
What wrong with using chokes? Absolutely nothing if you have the room. But I remind you that you will be able to achieve a much quieter/noise free voltage with active regulation, and this can matter a great deal.
WHY DOES THE QUALITY OF THE FILAMENT VOLTAGE MATTER?
We shape clay into a pot,
But it is the emptiness inside
That holds whatever we want.
Tao Te Ching.
SILENCE BETWEEN THE NOTES MATTER. MEANING SIGNAL TO NOISE RATIO IN DHTs MATTER
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