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Questions About This Local NFB Scheme
Questions About This Local NFB Scheme
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Old 1st August 2020, 06:28 PM   #21
mondogenerator is offline mondogenerator  England
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Originally Posted by 6A3sUMMER View Post
The issue of multiple windings to use them for negative feedback is the Leakage Reactance and the Distributed Capacitance of the windings.
The plate winding and negative feedback winding may not be accurately in phase (or may not accurately be out of phase).

It can even be an issue for Ultra Linear primaries. If the leakage reactance is high, from the UL portion of the primary to the rest of the primary winding (to the plate), then there can be a delay from the plate action to the UL screen. That means the phase of the UL negative feedback is not accurate.

How about that?

I can see that there are good reasons why it may/may not give an identical signal, and why something that at first glance looks like a brilliant idea, may not be so bright.

Thank you. I guess the proof is in the pudding, and as usual the OPT plays a pivotal part.
It's HiFi Jim, but not as we know it
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Old 1st August 2020, 11:59 PM   #22
FlaCharlie is offline FlaCharlie  United States
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Originally Posted by Tubelab_com View Post
As stated, not all OPT's have the same phasing, so you must try CFB one way, then reverse the secondary and try again. Choose the connection that gives the lowest gain (volume level for a given volume pot setting) to ensure negative (not positive) feedback.
This is especially helpful to me since I don't have a scope or any other sophisticated measurement devices. Thanks.
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Old 2nd August 2020, 12:51 AM   #23
Tubelab_com is offline Tubelab_com  United States
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When testing for phasing in this manner I have seen an EL34 actually oscillate when the phasing was wrong with some OPT's. The amp will make a loud squeal or hum. Reverse the wires and try again if this happens.

Any testing on a brand new amp should be done with junk speakers. I use a pair of Cerwin Vega's that I got at a Goodwill in Florida for $10. Won't cry if I blow them up, but they're still alive after 2 years of abuse.
Tubelab, I blow stuff up so that you don't have to.
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