Bell Audio 2325 Output Transformer

[tdlunsfo]

So I had a couple of Bell Audio units land in my lap and both have bad power transformers. The customer asked me to test the output transformers so he can part them out as the units are not in the best of shape after spending a few decades in an attic. I was getting strange readings with my NE-2 transformer test and decided to download the schematic and saw a setup that was pretty strange to me. Any idea what they are accomplishing tying the cathodes to the OT?

[nickb]

The cathode windings are just for negative feedback. There was a fad for doing it that way it at the time.

[Helmholtz]

It's a cathode feedback winding used to improve linearity, also employed in the legendary Quad-II hifi amps.

Such local NFB (another example being screen NFB via UL taps) allows for less global NFB to achieve comparable performance.
Less global NFB was assumed to cause less stability problems.

[tdlunsfo]

Thanks for the replies, I learned something new. I'm going to assume that it wouldn't be easy to substitute this OT in another amp unless it had a similar setup?

[Enzo]

WHy? If you don't connect extra windings, they just sit there unused. You don't have to use the feedback windings, you don't have to connect the UL taps.

[tdlunsfo]

Your right, I see it now.