Jon Yaeger wrote:

I've been following with interest this thread about the use of the 16 ohm
tap as a FB point.

I once rebuilt an Eico ST-70 that had poor bass response and a lot of
instability and distortion. It had a peculiar output configuration where
the 4 ohm tap was grounded, and FB came off of the 16 ohm tap.

If there is a single secondary winding, and there is a CT, then
all the winding is 16 ohms, and 1/2 the winding is 4 ohms.
So if the CT is grounded, 4 ohm speakers can be connected
across 1/2 the sec, or 16 ohms across all the sec.
No alteration for the amount of FB is needed, since the
voltage gains of the amp remain unchanged.
Its a clever way to amke things simple, but the 4 ohm
connection still wastes 1/2 the winding, which should, imho,
be able to be in parallel with the other 1/2 of the winding.




I don't know if the configuration caused the problems (there were a few
other things going on) but when I grounded the "C" tap and lifted the 4 ohm
off of ground, things improved markedly. I don't recall if I also shifted
the FB point to the 8 ohm tap, but I think I did.

Then the amount of NFB is doubled, +6dB, and stability
would been an issue.
I am assuming the C tap is the "common", which it isn't really.
C is just one end of the winding, then you have the CT, then you have the
4 ohm end, where the NFB is derived.

The existing set had two ways to connect 4 ohms, from the 4 ohms to 0V, or CT,
or from C to 0V, and then you get a reverse phased output signal.
The latter means the FB is still in correct phase, but at the distant end of a
winding
of which 1/2 has no speaker current.




For what it's worth . . . .


A dime?, a dollar?

Or just the time it takes to know.

Many amps are slightly unpredictable when different arrangements of OPT
secs are tried, so watch out for the DOIG, Demon Of Instability Gremlin.
Diplomatic relationships between the GOT and DOIG are at an all time low,
since so many are doing so much with so little knowledge.

Jon