Johan;
On your 1st point; I tried on several designs to obtain a succesful solution to curing the slight rise in the LF response below tranny cutoff; On some circuits a 10uF electrolytic cap in series with 470K was used; however with the extra components I never gave this chassis space.
On your 2nd point; I done some of work on this; a common cathode configuration is far better, gives better balance and with some cathode feedback degeneration to the grids drastically reduces IM thd. However, I haven't done this on 6SN7's as it is already the best tube for this application so the benefits may not be worthwhile.
On your point 3: The reason that the 47K resistors were used is that this was the recommended value with the NOS 6550A version; however, considerable improvements have been made to subsequent versions which up this value in fixed bias as you say around 100K. I recon there aren't many NOS A versions about now.
Trimming the overshoot squarewave response with snubber is done on the 1st stage anode. I will readup some old trade notes about this.
richy
On your 1st point; I tried on several designs to obtain a succesful solution to curing the slight rise in the LF response below tranny cutoff; On some circuits a 10uF electrolytic cap in series with 470K was used; however with the extra components I never gave this chassis space.
On your 2nd point; I done some of work on this; a common cathode configuration is far better, gives better balance and with some cathode feedback degeneration to the grids drastically reduces IM thd. However, I haven't done this on 6SN7's as it is already the best tube for this application so the benefits may not be worthwhile.
On your point 3: The reason that the 47K resistors were used is that this was the recommended value with the NOS 6550A version; however, considerable improvements have been made to subsequent versions which up this value in fixed bias as you say around 100K. I recon there aren't many NOS A versions about now.
Trimming the overshoot squarewave response with snubber is done on the 1st stage anode. I will readup some old trade notes about this.
richy