AC vs. DC filaments

AC vs. DC filaments
Date: Sat, 27 Nov 1999 22:57:40 EST
Subject: Re: 2A3 vs 6A3 vs 6B4G
In a message dated 99-11-27 22:38:15 EST, you write:

Turns out the DC filament excites a higher
order term (than the 3/2 law Ep/Ip) leading
to more upper order terms than AC
filament. Hmmm!

It must be because of the gradient of voltage along the length of the fil,
effectively changing the shape of emitted electrons–the pattern of
emission in a way. It adds yet another variable to the equation that
equals total plate current… but a strange one?
This is pure conjecture.
Ken

I’m still working thru the math, but if you imagine an incremental triode (physically snip a small piece of tube thru the cathode, grid and plate) with AC each incremental piece looks pretty much the same, so the superposition of all the increments is the same as each piece. With DC, the characteristics of each incremental triode is just a bit different, so it’s sorta like paralleling a bunch of similar type tubes. You notice that these defocus just a bit in terms of clarity… same thing’s happening. Structure wise, this causes a slightly larger change in MU with change in bias than with AC. Translated to the frequency domain, the higher order distortion components are subtly intensified, as the curvature around the lower current operating points is slightly higher.
You can also see the same effect by taking a conventional cathoded tube (e.g. 6SN7) and run it at very low plate current (say 50 microamps) and listen to the resulting sound.

Strange but oddly consistent with much seat-of-the-pants wisdom.

Best Regards,
Steve

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3 Comments

1. Gareth, June 26, 2010:

   This sounds like nonsense to me. The AC filament drive also puts a potential gradient across the filament in the same way as DC drive, only it changes with time. So each ‘incremental piece’ as you call it will be at a different potential except for one unique time during the AC cycle. I wonder if in fact DC is better because there’s nothing to intermmodulate with the signal ?

2. Ken, June 28, 2010:

   gareth:

   your assertion that DC filaments are the same as AC filaments does not hold true with established wisdom about DHTs. a great many people go through great pains to provide AC filament power, some even going so far as to push the frequency into the kHz range to avoid AF modulation.

   you are the first person i’ve heard claim that DC heated DHTs sound BETTER than AC.

3. Hugo Gonzales, June 30, 2010:

   A LOT OF THANKS IN ADVANCE FOR THE INFORMATION. WICH SUPLY IS BETTER IN DHT LIKE 300B ? ac or dc? and why.