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Old 28th December 2011, 11:14 AM   #1
Aiace is offline Aiace  Italy
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Default Filament AC or DC?

I'm wondering what is the best solution for an indirect tube (in particular for 6AS7).
What's the pro and cons form both the solutions?

Thank you in advance for sharing your experience and opinion.
Aiace

Last edited by Aiace; 28th December 2011 at 11:15 AM. Reason: typing mistake
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Old 28th December 2011, 11:27 AM   #2
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Look at the last post in the 6V6 linestage. A properly implemented DC heater will reduce background noise. You can see the 60Hz and subsequent harmonics associated with the use of AC on indirectly heated tubes. Noise is noise and the less of it the better.

Here:6V6 line preamp
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Old 28th December 2011, 12:25 PM   #3
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AC on a 6AS7 is fine and doesn't generate any noise to speak of.
I have a PP 6AS7 amp and it is whisper quiet. I have always been of the opinion that there is some strong evidence that a poorly executed DC filament supply degrades the sound. It is quite a challenge to make a DC heater for a juice hungry tube like the 6AS7 - lots of extra heat in an already hot tube - will shorten amp life. Lots of extra components and big vulnerable caps to degrade in all that lovely extra heat.

If you are asking this question - you are unlikely to end up with the correct DC supply and so will end up with a worse sounding amp that cost you a packet more to build.

Don't do it !!

Shoog
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Old 29th December 2011, 03:50 PM   #4
Aiace is offline Aiace  Italy
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It seems that the opinions are diveded fifty-fifty

In the end, the concept is : a bad DC is worse than a good AC. This is sound like a Lapalice sentence to me

Is there any other experience out there ?
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Old 29th December 2011, 04:16 PM   #5
GloBug is offline GloBug  Canada
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It's good to hear the full story, helps to make an informed decision.

Both sound reasonable to me, like a lot things, not so black and white.

I would verify what Shoog said and go from there. Just weigh it out the benefits vs. efforts and compromised components for your particular project.
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Old 29th December 2011, 05:21 PM   #6
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Quote:
Originally Posted by Aiace View Post
It seems that the opinions are diveded fifty-fifty

In the end, the concept is : a bad DC is worse than a good AC. This is sound like a Lapalice sentence to me

Is there any other experience out there ?
That of course assumes that in all instances all advise is equally applicable. That most certainly is not the case here.

A few minutes searching Google will show that no one is using DC supplies on tubes like the 6AS7, which should tell you plenty about the utility of going down that path.

There are obvious benefits for using well designed DC heater supplies with all Direct heated tubes, there are no obvious benefits with Indirect heated tubes which have reasonable heater to cathode insulation. All that is needed to ensure that this is so is to raise the heater supply by about +20V above cathode potential. This can be derived with a simple resistor network from the B+ (which serves the dual purpose of been your B+ bleed).

For a full discussion of the issues;

http://www.freewebs.com/valvewizard/heater.html

Last time I looked into this in any depth there was a general consensus that a voltage regulated DC supply was inherently inferior to AC. The feeling from those who had tried it said that it was absolutely necessary to use a CCS DC supply - which is a little bit more complex to implement than a simple voltage regulator.

Shoog

Last edited by Shoog; 29th December 2011 at 05:51 PM.
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Old 29th December 2011, 06:07 PM   #7
Yvesm is offline Yvesm  France
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My position is to go DC only when all other options was unsuccessfull.

With indirect heaters, remember to "bias" the heater above the cathode, this may save you for that expensive, inneficient and even noisy rectifier, even at very small level like in a phono preamp.

Yves.
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Old 29th December 2011, 07:02 PM   #8
Aiace is offline Aiace  Italy
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Quote:
Originally Posted by Shoog View Post
All that is needed to ensure that this is so is to raise the heater supply by about +20V above cathode potential.
Quote:
Originally Posted by Yvesm
remember to "bias" the heater above the cathode
This is not clear to me at all. Reading on the first few pages of Philips data sheets (I found them in this forum elsewhere) the filament must be at lower voltage than the catode. So, +20V sounds very, very, strange to me. Please explain me why.

Quote:
Originally Posted by Shoog View Post
it was absolutely necessary to use a CCS DC supply - which is a little bit more complex to implement than a simple voltage regulator.
Absolutely not more complex. You need 1 resitor less using a CCS than VCS using an LM1085.
But obviusly I agree with you: AC is easier then DC.
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Old 29th December 2011, 07:46 PM   #9
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Quote:
Originally Posted by Aiace View Post
the filament must be at lower voltage than the catode
If a filament negatively biased relative to a cathode signal can possibly flow from a filament to a cathode and from a cathode to an anode make AC filament audible.

In case filament positively biased only leakages from a cathode to a filament would be possible. Actually positively biased AC filament common practice in old tube gear like EL84 based.

You can check maximum possible voltage filament-cathode in a tube specification.

IMHO solid state linear stabilized DСpreferable nowadays since it’s very simple and inexpensive using modern ICs.
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Old 29th December 2011, 08:20 PM   #10
Aiace is offline Aiace  Italy
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Originally Posted by suntechnik View Post
If a filament negatively biased relative to a cathode signal can possibly flow from a filament to a cathode and from a cathode to an anode make AC filament audible.

In case filament positively biased only leakages from a cathode to a filament would be possible. Actually positively biased AC filament common practice in old tube gear like EL84 based.

I admit this confuse me furthermore.....
As far as I figured out, it's the opposite.
Cathode emits electrons and they go stright to a positive voltage. So if filament is positive a few electrons go to the filament generating a leakage current modulated by tha AC of the filament.
If the filament is negative respect the catode no one electron can goes to a more negative potential.
The most of you say the opposite, so I think somenthing is running out of my mind,but what ????


(if I've counted right, we are still 50%-50% )

Last edited by Aiace; 29th December 2011 at 08:40 PM.
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