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Old 29th August 2009, 03:18 AM   #31
ja2dhc is offline ja2dhc  Japan
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The attached schematic is the original McIntosh Amplifier's
published in 1949. It uses Bi-Filer winding for
P and K windings. Therefore 100% feed back is applied to the
Cathode. Since the Cathode should be the referenced potential,
resulting 100% feed back is applied to both G1 and G2.
The attached schematic uses 100% positive feed back to G2
to cancel G2 negative feed back. Therefore only 100% feed back
to G1 is remained as the beam connected 6L6.

E-Linear uses 50% tapped feed back to both G1 and G2 if
UL tap is 50%.
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Last edited by ja2dhc; 29th August 2009 at 03:21 AM.
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Old 29th August 2009, 06:15 PM   #32
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Quote:
Originally Posted by ja2dhc View Post
The attached schematic is the original McIntosh Amplifier's
published in 1949. It uses Bi-Filer winding for
P and K windings. Therefore 100% feed back is applied to the
Cathode. Since the Cathode should be the referenced potential,
resulting 100% feed back is applied to both G1 and G2.
The attached schematic uses 100% positive feed back to G2
to cancel G2 negative feed back. Therefore only 100% feed back
to G1 is remained as the beam connected 6L6.

E-Linear uses 50% tapped feed back to both G1 and G2 if
UL tap is 50%.
The McIntosh design sets out to deliver large cathode FB while maintaining pentode operation of the output tubes. They also ran the driver tubes B+ supply from the opposite end of the plate winding( positive FB ) to deliver the required swing to the output tube grids.

E-Linear tapping is up to the output TX winder. I am using multi-tap designs so the amout of FB can be adjusted. 20-30% is adequate, but with OPT designs running 40% U-L taps it is not a large handicap.

The larger tap percentage requires more voltage headroom from the amplifier stage, and would work best with AB designs instead of the B+ limited Class A designs...unless we go to large power finals that can take Class A dissipation at higher voltage...
cheers,
Douglas
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Old 29th August 2009, 10:58 PM   #33
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Quote:
Originally Posted by ja2dhc View Post
The attached schematic is the original McIntosh Amplifier's
published in 1949.
A nice advantage of the McIntosh circuit is that it allows great bandwidth cheaply, by using a low-ratio output transformer. It scores you a huge amount of feedback, but it's just as easy to give some of that back with positive plate-to-grid feedback, via a bootstrapped driver stage. Or, run it without positive feedback and you have what behaves like a ~mu=2 triode output stage with vanishingly low distortion and an acceptable open-loop damping factor. Just need a ballsy high voltage driver stage.

There's a Krohn Hite unity-coupled amp out there with a bootstrapped pentode driver good for ~0.0015% THD at 35 watts, using 80 dB total feedback.
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Old 30th August 2009, 12:13 AM   #34
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Quote:
Originally Posted by jon_010101 View Post
A nice advantage of the McIntosh circuit is that it allows great bandwidth cheaply, by using a low-ratio output transformer.
I'd go one further, they do it with simple bobbin geometry too; no complex interleave needed.
cheers,
Douglas
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Old 30th August 2009, 02:38 AM   #35
ja2dhc is offline ja2dhc  Japan
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Quote:
Originally Posted by Bandersnatch View Post
E-Linear tapping is up to the output TX winder. I am using multi-tap designs so the amout of FB can be adjusted. 20-30% is adequate, but with OPT designs running 40% U-L taps it is not a large handicap.
As folks explained, E-Linear has similar concept of G1, G2
feed back to McIntosh without special winding transformer,
but needs UL tap. As Bandersnatch says, value of feed back
can be adjusted by the percentage of UL tap. In my case,
I added two pieces of 39Kohm to the plate of the driver,
it reduces G1 feed back to the half, resulting, the driver's
operating condition could be relaxed. Please refer to my schematic
as below.

http://ja1cty.servehttp.com/E-LINEAR/E-Linear-amp.png

'73 de JA1CTY & JA2DHC/1 Kazuo Ohashi
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