Repurposing vintage Fimosound OT with strange "extra" secondary.

[Carlsoti]

Being the stubborn idiot that I am, I'm adding a PT to a Filmosound 183, along with other significant changes to the circuit. It wasn't the original goal, but it turns out that I'm headed in the general direction of a low-powered Matchless Clubman. Parallel 6SL7 input>5879>6SL7 PI>6V6 output. I'm having a bit of trouble understanding what the "extra" secondary on the OT does in it's original configuration and how to handle it moving forward.

FWIW, cheap and dirty voltage tests of the OT give me impedance ratios of 275:1, 525:1, or 1850:1 through the "normal" output secondaries. Primary (plate side) resistance is 340ohms; the full secondary, black to green, is about 1 ohm (275:1); secondary black to grey(~0.7ohms) is the 525:1 ratio.
 
As it was late at night and I was quite tired, I didn't take any voltage measurements from the "weird" secondary, but the resistance measurement is 26.8ohms.

I guess what really matters is, do I just leave these leads unconnected (capped off, obviously) or can I re-purpose them for additional functionality?
 

[astronomicum]

I have no experience with it but looks like a feedback circuit to me. I would say it could be reused once you know how it works and how it differs from a standard NF circuit. Interesting.

[kagliostro]

Davoli Krundaal, old italian Guitar amp brand used that kind of NFB separated secondary

You can see schematics here on the schematics database of the forum

Franco

[Merlin]

It's called a tertiary winding, they're pretty common in circuits that use NFB around a transformer. The tertiary winding will be wound with fine wire to acheive close coupling with the speaker winding, while allowing the speaker winding to remain floating (important in some audio equipment, but not the average guitar amp).

You can leave it unconnected, or use it for feedback as it was intended.

[Carlsoti]

Thank you all. Yet another instance of me working while sleep deprived and firing off a stupid question while hustling out the door for work. Another glance shows the NFB feeding the cathode of the driver for the splitter.

I had considered calling it a tertiary winding, but wasn't sure that was the correct terminology.

[scstill]

Is there an advantage to tertiary NFB vs tapping Secondary for NFB?

Seems that we often make arguments that manufacturers in the day were intent on saving $$ for mass production. This tertiary NFB approach seems to fly in the face of that logic.

[Merlin]

Is there an advantage to tertiary NFB vs tapping Secondary for NFB?

Here's a good article by none other than P J Baxandall that may answer your question:"High Quality Amplifier Design" Wireless World, Jan 1948, page 2:
https://www.worldradiohistory.com/UK/Wireless-World/40s/Wireless-World-1948-01.pdf

[scstill]

Here's a good article by none other than P J Baxandall

Thanks for introducing me to Mr Baxandall.

Wikipdedia mentions his tone control circuit that, at least in 1952, "blew all others away"
https://learnabout-electronics.org/Amplifiers/amplifiers42.php
Is this still correct today? Are you aware of any guitar amps using it?

His article on Tertiary FB is interesting but difficult for me to follow and extract practical knowledge.
I guess I have forgotten how to be a college student.

[Merlin]

Basically the tertiary winding can be wound to acheive wide bandwidth, allowing feedback to be applied with less stability problems than taking it from the speaker winding.

Almost every hi-fi amp that has tone controls uses a modern version of the Baxandall tone control. The JCM800 Bass uses it too