ampage
Tube Amps / Music Electronics
For current discussions, please visit Music Electronics Forum. New: view Recent Searches.
New: visit Schematic Hell!
The sunn still shines online!

 
Listen to great tunes streaming live right now!

ampage archive

Vintage threads from the first ten years

Search for:  Mode:  
previous: Ben N My bad--I didn't mean Rk, I meant t... -- 7/29/2004 5:38 AM View Thread

Re: Separate NFB for BF/SF channels?

7/29/2004 5:32 PM
Wild Bill
email
Re: Separate NFB for BF/SF channels?
Ben, you've got a good idea but you should keep in mind that the nfb is fed from a voltage divider - the 820R and the 100R from the PI input to ground. If you use a drastically different value in place of the 820R you may not have any nfb at all if the 100R is not changed.  
 
The ratio usually is about 10:1, like 1k:100R or the Marshall 47K:4K7, which gives the same ratio in nfb voltage of the speaker tap level and what goes into the PI input.  
 
One trick I sometimes use is take that centre-off ground reverse switch that's doing nuthin' since you went to a 3-wire power cord and bring the speaker nfb wire to the centre tab. Then you take the 820R and maybe a 1K6 from either tab and put their ends in common and run it back to the nfb input at the PI.  
 
What this does is give you 3 positions of nfb - stock, none and "half". This allows some interesting tonal variation. You can play with the 1K6 for taste - one jazz/blues player liked a 470R to increase the nfb and make the amp even more damped.  
 
.02  
 
---Wild Bill

 
Replies:
Don Symes Is that a setup you could switch 'h... -- 7/29/2004 11:35 PM