dc for the heaters

[Jered]

On a high gain pre amp will dc to the heaters (12AX7) significantly reduce noise compared to AC, with correct wiring.

[Stevem]

It's not just the wiring!
Other things like if the heater winding has a center tap and or a fax grounding thru two 100 ohm resristors , or a wire wound balance pot, a shielded tube socket or even the use of a 12ax7 tube with a wound filament will all effect 60 hz from the heater!
I have found over the years that if a amps layout is right then simply biasing up the filaments with about 45 volts of dc works just as good as running them on straight dc without the work of doing such!

[hans-jörg]

Hello,
is it necessary to use a "wire wound pot", or does a 200 Ohm trim pot does the job as well (3x12AX7, 2xEL84) ?

Hans-Jörg

[dorrisant]

Wire wound pots are usually rated for more wattage than normal pots or trimpots. Thus the suggestion.
Also, you must use adequate filtering when using DC for the filaments. If not, you could end up with more noise than the normal AC configuration.

Tony

[hans-jörg]

Hi,
I don´t use DC. But trim pots for the AC heaters. I know that wire wound are for higher ampere ratings. But is the trim too less? Is this a sleeping dragon?
I done this often and had never problems, but I´m aware that it´s on the edge.
I measured several times that its not exactly the middle of the trim/pot where the amp is most silent but with 2/3 by 1/3 or 70Ohm by 130Ohm. In this case I´m with 2x100 Ohm not at the right point too.
Therefore the trim pot.

Hans-Jörg

[matt h]

(deleted)

[Merlinb]

On a high gain pre amp will dc to the heaters (12AX7) significantly reduce noise compared to AC, with correct wiring.

Yes, but only if it's clean DC, and only if the rest of the amp doesn't have significant hum problems that mask the heater hum. For example, in some cases the magnetic coupling between PT and OT is enough to dominate a properly balanced AC heater supply.

[vibratoking]

If not done correctly (which is pretty freaking involved and costly in parts and real-estate), DC will be substantially crappier than properly routed AC. My understanding is that lifetime of certain tubes will be reduced with DC heaters as well.

Generating a proper DC supply isn't that costly or involved IME. Pretty simple actually. It is SOP for many builders. That being said, the end result may not be worth it. All in all, a DC supply is better from a noise perspective. That difference may not be audible unless all other masking noises are reduced to a low enough level.

Also, I am not aware that tube life is shortened by supplying the heaters with DC. Cathode life can be shortened by over or under heating, but this has little to do with whether the supply is AC or DC.

[matt h]

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[VacuumVoodoo]

Want hum free hi-gainer? Use separate transformer for preamp heaters. Well filtered DC and put the heaters on some 50-70V DC above ground.

[Tone Lover]

Want hum free hi-gainer? Use separate transformer for preamp heaters. Well filtered DC and put the heaters on some 50-70V DC above ground.

He might not have seen that before,maybe a schematic would be helpfull.
First time I tried it I didnt realize it was easiest with 12-0-12 transformer . I had a 9-0-9 transformer had to do a voltage doubler after trying to find out why it wasnt giving enough voltage for 12volt DC . Lesson learned.
But then maybe you could be talking 6 volt DC. I dont know , Im not home or I would post one. just a thought.
I will try and remember to do so when I get home if no one else does.
Thanks Bill

[ampdoc1]

I've been using only DC heaters for 10 years. It's almost "idiot proof" and the parts required are cheap. I use a large FWB and mount four small 3300uf caps to it. Output is marginally below 6VDC, but that's only a 6% drop from optimum.

Check it on my current build. (Blue Caps mounted on the FWB and the module mounted thru the FWB screwhole to the chassis) on lower right)

[hans-jörg]

A beautiful straight build.

I think AC/DC is a personnel preference.
I think too that the PT/OT hum is a main problem of hum in an amp (a proper heater wiring assumed).
I use AC with ~50 V to ground - or a separate heater tranny. Never had problems of this kind of hum.

Hans-Jörg

[Jered]

Thanks guys great stuff.

[surfsup]

Check it on my current build. (Blue Caps mounted on the FWB and the module mounted thru the FWB screwhole to the chassis) on lower right)

What's under that electrocap stack?