using DC to power heaters?

[beasleybodyshop]

Why do tube heaters require AC voltage? Just curious. Would be nice if you could just create a separate DC power source to power them - wouldn't that prevent heater hum?

[xtian]

You're correct. You can use DC, and this will reduce hum.

[martin manning]

...but the residual hum in a well-executed AC heater system can be so low that there is really no practical difference.

In the very early days when tube circuits were powered by batteries the filaments were powered by DC. I suspect that when AC power became the norm it was cost prohibitive to provide a second DC supply for filaments. Now that 25 cents worth of solid state rectifiers and a couple of other parts is all it takes, you can easily do it. Most of what improvement there is will come from running the filaments in the first couple of stages on DC, and this has been done in a number of commercial amps where the first preamp tube is the only one that gets DC.

[roberto]

Personally I prefer elevated heaters.
So a 40-50V DC reference for the heaters' AC power supply.

Absolutely no noise even with high gain design.

[Leo_Gnardo]

...but the residual hum in a well-executed AC heater system can be so low that there is really no practical difference.

True 'nuff however I've found some pre tubes seem to pick up hum no matter what ya do. I don't discard those that do this, just save them for use in a DC-filament amp.

[martin manning]

Personally I prefer elevated heaters.
So a 40-50V DC reference for the heaters' AC power supply.

Absolutely no noise even with high gain design.

That would be part of "well executed," especially if you have a cathode follower in the mix ;^)

[Cliff Schecht]

Something I've heard but never experienced (probably because I haven't been using tubes for 20+ years) is that when using DC heaters, some tubes will develop hot spots in certain areas of the heater when powering with DC. These hot spots will eventually weaken and the heater will fail at that point. Again something I've never experienced but I've heard before. Maybe it's only an issue with thoriated tungsten or barium oxide but I'm not sure as I'm yet to find any reliable sources on this yet.

Also I have a similar notion to Roberto in that DC heaters are a band aid fix to a whole multitude of problems (bad building technique, too much gain too early/or just too much, noisy tubes, non-elevated heaters, etc). I've found in practice that there is NO substitute for a quality layout when it comes to heater noise introduced from lead dress issues. There is also a capacitive coupling effect between the heaters and cathode that the DC-elevated heaters will alleviate. Using purely DC heaters is usually a last ditch effort in my book and IME is not necessary for all but the most demanding applications (in which case, why the hell are you using tubes?!? ).

[Leo_Gnardo]

Something I've heard but never experienced (probably because I haven't been using tubes for 20+ years) is that when using DC heaters, some tubes will develop hot spots in certain areas of the heater when powering with DC. These hot spots will eventually weaken and the heater will fail at that point. Again something I've never experienced but I've heard before. Maybe it's only an issue with thoriated tungsten or barium oxide but I'm not sure as I'm yet to find any reliable sources on this yet.

I think this has more to do with directly-heated cathode tubes. When you pay big $$$ for 300B or 845, like that, you wanna make 'em last as long as possible.

Using purely DC heaters is usually a last ditch effort in my book and IME is not necessary for all but the most demanding applications (in which case, why the hell are you using tubes?!? ).

As I see in Peavey 5150, JSX, & other amps of that series. 4 of 6 pre tubes DC filament. Fizzy-tone!

[beasleybodyshop]

Leo, think those amps use DC because they are high gain?

[Leo_Gnardo]

Leo, think those amps use DC because they are high gain?

Yes, DEAFinitely. At first they foxed me, because if you pull out one pre tube V1,2,3,or 4) the others go dark. Then I had a peek at a schematic and had an a-ha moment. DC supply, series wired filaments. A bit bizarre, but, hey it's Peavey.

Also just as well because it would be difficult to find a batch of preamp tubes that run quietly enough on AC.

[David Root]

DC heaters can be useful on higher mu pre-tubes that don't have humbucking heaters eg 6SL7, 6SQ7.

Someone touched on big $$ hifi tubes. Some direct heated triode power tubes with heater voltages above 2.5V can be noisy even with a carefully designed and laid out AC supply eg 6A3 and the mentioned 845 & 300B, and most especially in single ended designs.

But otherwise I agree that if you get your AC heater supply done right you don't generally need DC heaters.

[Cliff Schecht]

6SL7's benefit just as well from elevated DC heaters IME. While there is always a capacitance between the heater and cathode, elevating the heaters even just by the power tube cathode will reduce the effective capacitance and reduce that capacitance-induced hum. The 6SL7 amps I've built have a lot of gain on tap but are still quiet. The other trick to that however is finding a 6SL7 that is quiet and has low microphonics for the first stage. Good luck with that.

[labb]

Hey Cliff, have you tried any of the new production 6SL7's if so what did you think of they.

[renshen1957]

Personally I prefer elevated heaters.
So a 40-50V DC reference for the heaters' AC power supply.

Absolutely no noise even with high gain design.

+1

[John_P_WI]

Personally I prefer elevated heaters.
So a 40-50V DC reference for the heaters' AC power supply.

Absolutely no noise even with high gain design.

+1

Have to agree +1