Cathode feedback and input impedance

abj1 · 30th January 2007, 11:05 PM

Quick question for the less technically challenged...... What does cathode feedback (ie putting the output tranny secondary winding under the cathode of the output valve) do to the input impedance of the output valve.

The context is an 811 in SE driven with positive grid bias (at all times). Currently direct driven with trioded 6L6 CF and wondering whether the input impedance of the 811 is raised by the cathode feedback (which works very well, I might add). And if so, would it rise enough to consider replacing the 6L6 with a 45 (needs to be ST14 size or smaller - the 6L6 is a beaut metal specimen at present)....

Cheers,
Andrew.

SY · 30th January 2007, 11:48 PM

It does raise the input impedance, but at the same time it increases the drive voltage requirements.

abj1 · 31st January 2007, 02:06 AM

Thanks SY, this is good news. I have used cathode feedback in a variety of projects and am comfortable with the increase in drive requirements. They are not too severe - with a 5K to 16R tranny (speaker to 8R tap) and a 100 to 900V swing on the anode of the 811 this amounts to an increase in required voltage swing at the cathode of the CF from (very approximately) +/- 20V to +/- 38 which should be easily achieved.

The question really amounts to whether the input impedance of the 811, which I very roughly guestimate to be 2K at +20V on grid at 500V anode to filament would rise enough to make a 45 CF workable at its relatively modest Gm. I know there are other valves with much higher transconductance but I like a bit of historical consistency in an amp (the voltage gain is currently handled by an 01A before the CF) and I also have severe real estate limits on the chassis.

Any further thoughts on the magnitude of the rise in input impedance of the 811? Two areas of amp design I still find to be dark arts are CF's and feedback (which I rarely use except local NFB - either cathode feedback or plate to grid) and a good intuitive understanding I just don't have.

Cheers,
Andrew.

30th January 2007, 11:05 PM	#1
abj1 diyAudio Member Join Date: Jan 2005 Location: Melbourne	Cathode feedback and input impedance Quick question for the less technically challenged...... What does cathode feedback (ie putting the output tranny secondary winding under the cathode of the output valve) do to the input impedance of the output valve. The context is an 811 in SE driven with positive grid bias (at all times). Currently direct driven with trioded 6L6 CF and wondering whether the input impedance of the 811 is raised by the cathode feedback (which works very well, I might add). And if so, would it rise enough to consider replacing the 6L6 with a 45 (needs to be ST14 size or smaller - the 6L6 is a beaut metal specimen at present).... Cheers, Andrew.

30th January 2007, 11:48 PM	#2
SY diyAudio Moderator Join Date: Oct 2002 Location: Austin, TX Blog Entries: 1	It does raise the input impedance, but at the same time it increases the drive voltage requirements. __________________ "...we stumble and get up, we are sad, confident, insecure, feel loneliness and joy and love. There is nothing more; but I want nothing more.” - Christopher Hitchens 1949-2011

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31st January 2007, 02:06 AM	#3
abj1 diyAudio Member Join Date: Jan 2005 Location: Melbourne	Thanks SY, this is good news. I have used cathode feedback in a variety of projects and am comfortable with the increase in drive requirements. They are not too severe - with a 5K to 16R tranny (speaker to 8R tap) and a 100 to 900V swing on the anode of the 811 this amounts to an increase in required voltage swing at the cathode of the CF from (very approximately) +/- 20V to +/- 38 which should be easily achieved. The question really amounts to whether the input impedance of the 811, which I very roughly guestimate to be 2K at +20V on grid at 500V anode to filament would rise enough to make a 45 CF workable at its relatively modest Gm. I know there are other valves with much higher transconductance but I like a bit of historical consistency in an amp (the voltage gain is currently handled by an 01A before the CF) and I also have severe real estate limits on the chassis. Any further thoughts on the magnitude of the rise in input impedance of the 811? Two areas of amp design I still find to be dark arts are CF's and feedback (which I rarely use except local NFB - either cathode feedback or plate to grid) and a good intuitive understanding I just don't have. Cheers, Andrew.

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