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


Yes. It is a form of distributed load that is widely used but more
often the speaker winding is separate from the cathode winding. This
makes a big difference to how it looks in the drawing which is
perhaps why you don't recognise it.

The ins and outs of distributed loading are many...used in different
proportions by Quad, Mackintosh, van der Veen, and various current
manufacturers. There may be a paper on Plitron's site. It is quite
well covered in Menno van der Veen's "Modern High-End Valve
Amplifiers".

Using the speaker winding as shown means that the ratio of anode to
cathode winding, and hence the amount of feedback, depends on the
winding ratio between primary+secondary and secondary. It is
unlikely that both can be optimised at once. Handy if you only have
simple transformers to hand though.

This limitation can be overcome by using the optimum ratio for the
cathode, and then tapping that winding at the best point for the
speaker.

The common alternative is to use separate cathode windings
altogether. I think this makes it easier to minimise leakage
inductance between anode and cathode, and also avoids attaching the
cathodes to the speaker terminals, which could pose problems for the
average user and makes global feedback a little more awkward to
implement.

For an illustration of how cathode feedback alters the
characteristics of the output valves:
http://www.ivesonaudio.pwp.blueyonder.co.uk/modal.htm

For the circuit of my amp, wired for the "UL30CF-10" option:
http://www.ivesonaudio.pwp.blueyonde...6ch6schema.gif

cheers, Ian