AC heater vs DC heater?

Hi folks, my dac/amp combo which bought from a local diyer are all using DC heaters so I hadn't heard anything from those AC-heater gears.

Does the good designed AC-heater gear beat the good designed DC-heater one?

Many many thx.

 

Theoretically AC heater is losing up electrons in lower temperature easier than DC heater due to skin effect and peak fluctuations (joule heating is peak-to-peak in nature) on surface. Thus effectively increasing filament lifespan and perhaps increaseing trans-conductance and probably affect some other parameters unknown to us young mere mortals.
Though in practice I imagine it's not trivial to implement. Much, much easier to get clean DC than proper clean sine at 40+ kHz.

 

I've always wondered...how does one make a high frequency heater like that or like Eddie Current uses?

 

Hm, how about a fixed freq sine gen --> Large bandwidth amplifier.

 

Eddie Current uses an inverter. At some point Marv mentioned that the PSU units only output DC, so that means that an inverter in the amp chassis converts that to AC.

This is an option, tough not common for some reason.

 

Skin effect? Typical AC heaters run at 50/60Hz, so probably not. Even RF heaters shouldn't see much of that. Emission is strictly a function of temperature for a given cathode material and structure.

It probably doesn't need to be a proper clean sine. Square wave heating isn't that uncommon.

I've had good luck getting indirectly heated tubes to be silent with an AC heater supply by elevating the heater supply above cathode potential. It reduces the coupling between the cathode and heater.

Directly heated tubes are tricky. DC heating will introduce a bias gradient; since there's a voltage drop across the filament, one end will be at a higher voltage than the other. Depending on the tube geometry, that could mean that the operating point might not be constant throughout the structure. Emission will be constant across the structure, but transconductance might not even if the net effect is averaged out. With constant voltage DC heating, you need to worry about the power supply cap being in the signal current loop as well. Not with constant current, though.

AC does not have a problem with the gradient, but instead is injecting signal-- whether it's 60Hz or 100kHz or not-- right into the cathode/filament. The tube acts as a common-grid amplifier and the filament signal appears on the output with gain. Two ways this could go wrong: it could (and often is) a source of IMD, and it could saturate the output transformers.

Here's a kit.

 

The downside with an AC heater is that you might be able to hear a 50Hz/60Hz tone being generated through the amp. Using a high frequency heater can avoid this.

http://www.pmillett.com/hf_fil.htm

 

Hmm, if a heater is basically cranking a signal through, could one build a "class d" heater design? or is that a sin of audiophilic proportion?
(or maybe I'm displaying my n00bness about tubes)

 

I have plans to make a turntable power supply for a 24V AC motor using a Class D power amp. Adjusting the frequency would allow you to change the speed of the turntable. Heaters have lower voltages so I presume it would work fine.

 

A clean high end amplifier using 50Hz heater?

What is EC reasoning to use 40+kHz heaters?
One of my amp using dc heater probably picks up transformer into its output, causing 50 Hz hum.
Might be nothing with speakers, but with most hp-s it's annoying.

Why sine?
I don't feel comfy about near (or over) MHz squirrels running around my amp

 

Many some of EC amps are using AC + DC hybrid heater(s)?

 

40 kHz is chosen because it pushes the hum far above human audibility.