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Old 12th December 2012, 10:30 AM   #1
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Default Double feedback pentode Power Stage

I came up with this concept for a power stage, using pentodes.

The idea is to get maximum power from two pentodes, and keep the feedback only in the output stage, while keeping distortion down. As I read here, partial feedback output stages are best driven by high output impedance driver stages.

-Anode to grid feedback
-Balanced feedback from the output trannsformer
-Fixed biasing

G2 power is omitted for clarity. Possible options for feeding G2 are a properly stabilised power supply, or a plain simple resistor a-la RH84. Pleas comment on that too.

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Old 12th December 2012, 11:12 AM   #2
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Penthode + CFB (using secondary) is nothing new. Take a look at Sansui AU70 schematic. That will work for sure.

Anode to grid feedback - what is it for ???

Additionally, if you want to implement CFB (cathode feedback), its better to use separate tertiary winding instead of secondary.
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Old 12th December 2012, 11:39 AM   #3
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One of the advantages of NFB is to low the output impedance when properly used.

Cathode feedback is voltage-series feedback, and then it effectively reduces output impedance.

Anode to grid FB is current parallel, which increases output impedance, so since this point of view, they cancels out together or almost counteract.

The advantage in respect to lower the distortion continues to work of normal way.
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Old 12th December 2012, 12:30 PM   #4
Jaap is offline Jaap  Netherlands
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Local feedback 50 years ago
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Old 12th December 2012, 12:33 PM   #5
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Quote:
Originally Posted by Osvaldo de Banfield View Post

Cathode feedback is voltage-series feedback, and then it effectively reduces output impedance.

Anode to grid FB is current parallel, which increases output impedance...
Are you sure about this?
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Old 12th December 2012, 12:38 PM   #6
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I am pretty certain that anode to grid reduces output impedance too...

By the way I never said I invented this, just exploring the possibility of combining these two kinds of feedback plus fixed bias.
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Old 12th December 2012, 01:30 PM   #7
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Cant you use a normal output transformer with an additional cathode winding for the feedback?
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Old 12th December 2012, 01:57 PM   #8
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Quote:
Originally Posted by SY View Post
Are you sure about this?
Are you sure you are alive or you are dreaming?
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Old 12th December 2012, 02:35 PM   #9
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Audio Research did this cathode feedback scheme with the ST 70.

ARC ST70C3

The Tango XE-45-5 PP output transformer has a secondary tap for cathode feedback.

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Old 12th December 2012, 06:54 PM   #10
oshifis is offline oshifis  Hungary
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And the Quad II did it
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