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Old 1st February 2008, 12:29 PM   #1
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Default Tube Heaters Ac Or Dc

a simple question
AC or DC heater supplies. i have heard good and bad things about both. could anyone help me understand the advantages and disadvantages of the 2 methods.
cheers
ed
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Old 1st February 2008, 12:58 PM   #2
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I also read a lot...

In Valve Amplifiers Morgan Jones states that with good practice even a RIAA stage can be made quiet using AC heaters. And the RIAA stage is the most senstivive to pickup hum...

So, why DC? Although Morgan Jones speaks about an AC fed RIAA stage, he presents a schematic for the regulation of the heater supplies for the RIAA stages. I think that it is meant to avoid disappointed people that could not get it 'humfree' with AC. Following that line of though, I remember a diyaudio member once stating that DC is for people not skilled enough to use AC... (Possibly right, but I don't expect everyone to be willing to learn perfect AC dressing for a small number of projects they will be doing throughout life)

DC heaters require quite some additional efforts in the PS. Basically one needs:
- more components: bypassed diodes, elco's,
- without a regulator, higher order harmonics are added, so a regulator is a good thing,
- with a regulator comes the need for heat dissipation: is the chassis enough, or should one add a heatsink?

The common 6.3V AC from the power tranny can only be used for 6.3VDC if one uses a low drop out regulator...to obtain the 6.3VDC from a normal regulator (317, 350) one needs about 9VAC, which in most cases means a separate or custom wound transformer.

In my recent projects I am trying to use AC. In my opinions it is better to tear some hair out now (I am young and have lots of hair) figuring out, in the 'learning by doing' way, how to work with AC, than to always have to recur to the hardest and more expensive DC way in future projects.

DHT tubes are a different matter, which I leave for people as Kevin, who is using CCS to feed 300B's!

Hope this helps a little bit.
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Old 1st February 2008, 01:49 PM   #3
oshifis is offline oshifis  Hungary
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AC works hum-free without any special precautions at line level stages. It has very low, frequency-independent source impedance, which is a good thing.
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Old 1st February 2008, 05:45 PM   #4
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Quote:
Originally posted by ErikdeBest

I think that it is meant to avoid disappointed people that could not get it 'humfree' with AC. Following that line of though, I remember a diyaudio member once stating that DC is for people not skilled enough to use AC...
yup.....the good old analogue skill is a fast disappearing asset.

page 265 M.J. 3rd edit regarding EF86 and spiral heaters.


NOS small signal tubes are gradually diminishing and many were designed to work on AC heater supplies with the hum and noise in the 1960's spec i.e equipment noise floor capable of -60dB down..
Today, with D/A conversion, the anticipated hum noise floor has got a min -90dB down. To get a tube preamp down to these levels and be better than the main amp, the cue for DC front end heater supplies may seem obvious.

I had a hum problem using recent made EF86 Svets, where an "up and over" heater construction is used and by using an AC heater supply, the anticipated low hum performance can't be met when changing tubes. By designing front ends to run on a DC heater supply, generally one will get a more consistent performance regardless of the same type tube.


richj
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Old 1st February 2008, 05:47 PM   #5
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I have never had to use DC for heaters in any of my amplifier projects. Proper lead dress, and twisting of heater pairs is quite sufficient in my experience.
Glenn
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Old 1st February 2008, 05:58 PM   #6
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Quote:
I have never had to use DC for heaters in any of my amplifier projects.
Does that include amplifiers that use DHT?
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Old 1st February 2008, 07:25 PM   #7
m6tt is offline m6tt  United States
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not to mention filament damage on indirectly heated tubes (and possible directly heated tubes, the only ones I can afford are rectifiers) with dc...I find even with somewhat imperfect lead dress hum will not be a problem so long as the heaters are referenced and input cables etc are not too long. Bad tubes hum more too. I had a modern 12ax7 that sounded like crap and hummed more, compared to some special quality preamp tubes I've seen that you can't even tell the amp is on.
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Old 1st February 2008, 08:25 PM   #8
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Default 300b with AC

I breadboarded a 6sn7, 300b amp using only AC for heaters and managed to get the hum down to 4mv at the speaker. Had to twist wires tightly, move them around a bit, use a humpot and limit the voltage on the 300b to slightly less than 5v (4.6 to 4.8 vac).

I did try to use dc on the 300b but did not like the sound at all.

I also have a 6sn7, 45 power amp that is virtually hym free using ac on the heaters.

FWIW
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Old 1st February 2008, 09:46 PM   #9
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Quote:
Originally posted by Bas Horneman

Does that include amplifiers that use DHT?
No, I guess I should have clarified that

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Old 1st February 2008, 10:06 PM   #10
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Quote:
No, I guess I should have clarified that
O no..but I was curious. Last time I tried AC on my 300B it made a hell of a noise. It seemed as if the hum pot's did nothing...no matter how I turned them.
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