circiut help

[shooter]

I'm following the W44 progression, didn't want to highjack it.
the section below has got my head hurting.

What I’m “seeing” is an overly complex local NFB? 
 Not sure by a long way if it’s also freq./tone shaping the mix tube inputs downstream?

I was trying to find the math, but 90% discussed Hi pass, low pass series RC, with comments like math for parallel RC filters is boring!  So I’m unsure if Fc = 1 /2piRC in a || RC circuit is correct.

Thanks

dave

[PRR]

It can't be NFB because there is no Back involved.

V1 makes two signals 180 deg in polarity; call them + and -.

There are two Y networks with three RC legs in each. Different R and C everywhere. They take signal from both + and - signals. I would assume that some frequencies come more from the + side and some frequencies come more from the - side. It's too much to figure. It is not any standard textbook problem.

So having two signals with various frequencies at various phase, we come to V2, a 2-input differential mixer with trem oscillator forcing V2a then V2b on. It will alternate between the two Y networks and deliver to an output.

[sluckey]

Here's a good circuit description...

     http://www.thermionic.info/dorf/Dorf_TheWurlitzerVibrato.pdf

BTW, this is the same circuit used in the Vox AC-15.

[shooter]

I bookmarked the read, it was a late night 

so the cathode is the + (inphase) output   
that's where I "assumed" nfb from the plate feed

thanks Gentlemen

dave

[PRR]

> nfb from the plate feed

You generally can't force NFB to a cathode from the plate of the same tube. Plate and cathode work at the same current, so there is not enough "ooph" available to wobble the cathode. (Different if you add a transformer, and there are output stage which do this.)

[shooter]

Plate and cathode work at the same current

AH, my thought process was on the AC signal side, since the cathode "sees" a small outta phase signal (plate) from the inphase sitting at the cathode.

[sluckey]

I believe that feeding a signal from plate to cathode would be positive feedback. You would have to feed the plate back to the grid to have negative feedback.

[shooter]

 
I said it was a long night 

ok, I'm going to read your link before I speak again   

[sluckey]

My last comment was generic. Has nothing to do with that link. 

[shooter]

Ya, I just walked into it, once PRR pointed out it was a PI, and a pi has 2 outta phase signals..............
anyway;
to regain some integrity  , I point to the article;

 “Such a vibrato, with it’s ease of construction and compactness, does an almost impossible job which has puzzled many people…….”

while I normally DON't side with "many people", for this I'm coping to being a member in good standing 

[PRR]

I believe that feeding a signal from plate to cathode would be positive feedback.

Well, yeah. That too.

(Good catch.)