west wrote:

Why do Dynaco Amps and even Uncle Ned's schematic of a Hi Power Williamson
Amp use the 16ohm OPT tap instead of the 8ohm for feedback? What would
happen if you took a stock ST-70 or MKIII and moved the feedback over to the
8ohm? Thanks.

Its a very fair question.

The arrangement where FB is taken off the 16 ohms tap, and usually at the end of

a tapped secondary winding is that it allows free movement of loads from one
tap to any other without having to re-arrange the values of feeback R and
compensation cap often needed in other amps to keep them stable.

The 16 ohm FB point works when 8 ohms is on the 8 ohm tap,
at 0.7 of the S turns, and when 4 ohms is on 0.5 of the S turns.
The signal voltage at the 16 ohm tap is the same for all 3 load values.

But a tapped S winding is an appalling practice imho, since the
leakage inductance rises badly, 4 times, when 4 ohms is connected to the
CT of the S winding.
The HF response can be less than when 16 ohms is used at the 16 ohm connection.
HF stability can be a problem...

Patrick Turner.



west