wrote:

Has anyone ever seen this topology covered in any text?
I would like to learn more about how to use it.
What are the benefits, and what's required of the transformer??
I like the idea having the transformer in a feedback loop with the 6550.
Any comments? anyone used it before?

view schematic he

http://www.nashaudio.com/Public/6550_OUT.pdf

Thanks
RonL
I

As Phil suggests, the schematic is almost identical to the Leslie amp.

The extra cathode feedback from the speaker secondary is a rather
ineffective use of NFB.

Let us suppose the OPT Z ratio is 6k to 16, then the TR = 19.36:1.
then if the typical signal voltage from cathode to anode is 250Vrms per
tube,
then we would have +237.1Vrms at an anode, and -12.9Vrms at the cathode,
so the amount of NFB applied is 4.84%.

The 4.84% = 100% of the secondary winding voltage.

However it isn't as simple as this.

There is also some NFB applied via this connection between the cathode and
screen.

So its a very complex equation indeed to work out exactly what the
final amount of applied NFB is in terms of dB of FB, because the amount of
applied FB due to the
UL connection has to be taken into consideration, and we don't know the
% tapping in this case.
I have some notes in Radiotron booklets of 1955, and indeed
equations for gain/feedback are beyond the scope of this email for me to
explain
further.
But its all been thought about a lot 50+ years ago, and the best minds
did try to work it all out.

But the 4.84% of applied CFB is not a very significant amount of FB.

One would definately find that since UL gain is rather low, applying some
5% of CFB isn't going to have any significant effect
on Rout, or thd, and in fact it amounts to only about 2db of NFB.

For any real benefit with CFB in the output circuit the
% of the anode to cathode total signal must be over 10%
And then one finds the tubes work best when the screens are taken to a fixed

supply voltage.
Quad found all this out before they released the Quad II amp,
which is why they use a dedicated tertiary cathode winding which equals 1/9
of the turns of the
anode winding.
I have followed in their footsteps and provided ppl
with amplifiers using say Ea = +470V, Eg2 fixed at +330V, and
12.5% of the primary turns devoted to a CFB winding, and with a
separate speaker winding, all somewhat better implememnted than
the bean counters at Quad would allow.
My most recent 300 watt amps using 12 x 6550 use 20% of the primary
as CFB.

The use of the speaker secondary for FB results in an insignificant
further improvement, and makes it very awkward to implement
with regard to arranging series/parallel windings on the speaker secondary
to allow for impedance matching to 4,8,&16 ohms while
retaining symetrical windings each side of a centre tapping,
and the same current density in each wire of the speaker secondary,
and no wasted or unused sections of windings in the speaker secondary.

I think the Leslie connection is BS, and the above website pdf
also is a case of poor design.

Good design includes for plenty of interleaving, low losses,
and scope for load matching without waste or any increase in
winding losses if the amp is to be matched to say 4 ohms instead of 16 ohms.

Sadly, many amps have much increased winding losses when selecting
the 4 ohm tapping.

Its a lazy crummy way to build an amp.

Musical Reference also used a centre tapped speaker
secondary as a CFB winding in an amp which used a pair of EL84
with Ea = 700V, Eg2 = 350V, and it put out 36 watts
into a 14k a-a load.
The output tubes were working in nearly pure
CFB, but because the voltage gain of the EL84 is much greater than many
other larger output tubes, the small amount of CFB was slightly more
effective than
the above mentioned case.
This amp worked in nearly class B conditions, and sure it made impressive
power,
but it wasn't as good a class A amp which
made 36 watts.


Patrick Turner.