Chris Hornbeck wrote:

On Wed, 28 Sep 2005 14:47:22 GMT, Patrick Turner
wrote:

But the 4.84% of applied CFB is not a very significant amount of FB.

Bingo. This connection was a regular feature of the
earlier Audio Research amplifiers for years. No good
reason at all to bother.

Thanks, as always,

Chris Hornbeck

Where you have a 500 watt amp, and the outout voltage into an 8 ohm
centre
tapped speaker secondary is about 63.24Vrms, then you have 31.6V
available for
series CFB for the two halves of the PP circuit using say 20 x 6550.
If you have 250Vrms at the anode, and the grid to cathode voltage is say
25Vrms,
then you only need 55.6Vrms of grid drive to each output tube grid,
and very easy to do without the extra stage and bootstrapping that is
employed by
EAR and McIntosh.


This amount of voltage FB is far more than in the case of the parsimonius
amount you'd have with a
50 watt amp using just 2 x 6550, some 20Vrms with a CT for 10V CFB to
each cathode.

You still have bothers trying to arrange the speaker sec to suit
a range of loads;
I never bother to try to match to 8 ohms because nearly all speakers
have slipped down to 6 ohms nominal with dips to 4 ohms or less.

Patrick Turner.