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Old 21st April 2008, 12:03 AM   #1
diyAudio Member
Join Date: Jun 2004
Location: Indiana
Default 6au6 Cf

When using a pentode such as 6AU6, or EL84 for that matter, for CF service is it really worth the hassle of using pentode mode or is triode wiring nearly as good? I am thinking of using a 6AU6 as a 3rd order LP ala Steve Bench for a subwoofer output but I also have a spare Chinese EL84 clone that could be used also I just don't whether all the bother of pentode mode is worth it.

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Old 21st April 2008, 07:23 AM   #2
diyAudio Member
Join Date: Oct 2004
Location: Adelaide South Oz
absolutely worth it.
The screen grid (g2) MUST be tied at AC to the cathode by a capacitor.

Weird but true - if you don't do that, then it is the triode mode characteristics of the pentode which apply to its analysis and performance.

In order of performance,
triode CF
triode CF with current source load (either tube or SS current source)
pentode CF

The definitive text / maths etc can be found in Wallman and Valley.

If your a bit of a heretic then MOSFET Source Follower is theoretically even better.

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Old 21st April 2008, 07:31 AM   #3
AKSA is offline AKSA  Australia
diyAudio Member
Join Date: Sep 2001
Location: Melbourne, Australia
Hi Ginger,

How about driving the screen grid on a CF at around 50% of the AC voltage at the cathode?

In one sense, this is half way between triode and CF follower, wonder how that would sound?


Aspen Amplifiers P/L (Australia)
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Old 21st April 2008, 10:19 AM   #4
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Join Date: Jun 2004
Location: Indiana
So you would connect screen grid to b+ (same B+ as Plate?) via a smallish resistor and to the cathode with a cap that is of sufficient size to short all AC to the cathode?

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