Tube NFB

[emrr]

Non linearities of the transformer can certainly be reduced by including it in the FB loop, but the poles and phase shifting  inherent in the transformer may cause instability and require adjustment of the forward path and feedback loop in the form of added zeros, lead and/or lag networks to compensate.  Generally speaking though, I found this less necessary if the loop gain isn't  too large.

Paraphrasing the Langevin catalog, those phase shifts could become a problem with cascaded amplifiers, which they invariably were back then. 

 

[bluebird]

Non linearities of the transformer can certainly be reduced by including it in the FB loop, but the poles and phase shifting  inherent in the transformer may cause instability and require adjustment of the forward path and feedback loop in the form of added zeros, lead and/or lag networks to compensate.  Generally speaking though, I found this less necessary if the loop gain isn't  too large.

Thank you, that makes sense.

P.S. We have something in common, I worked for David K. after you did. You have quite the reputation over there...

 

[Winston OBoogie]

P.S. We have something in common, I worked for David K. after you did. You have quite the reputation over there...

Hi Bluebird  I know two David K's.  Assuming you mean David in Burbank then yes, I imagine I do.  I went very much off the rails while there and will forever be deeply sorry for what went down.

Edit:  rest of reply removed by me, unimportant.

 

[Winston OBoogie]

Paraphrasing the Langevin catalog, those phase shifts could become a problem with cascaded amplifiers, which they invariably were back then.

Interesting that they go into details like that.  Was this info to assist folks purchasing their stuff such as 5116 amps etc.  to build custom systems? 

 

[boji]

Between the book that just arrived and this thread, I'm in tube info heaven.    Thanks everyone! <3

 

[Tubetec]

Radio Designers Handbook you should look out for too Boji , nearly 1000 pages of good valve stuff .

 

[boji]

Radio Designers Handbook you should look out for too Boji , nearly 1000 pages of good valve stuff .

Yes, thanks Tubetech.  Promise RDH is next on the list, but this Cyclopedia...so fat...so full of topics!

I use conventional types of  nfb almost all the time, no question  - except in some single-ended output tube (spud) applications, where it is interesting to try to see (and hear!)  the changes  from a 'no nfb'  transitioning to a  'some nfb'  etc.

Edit: Coil Audio's FB knob on the CA-70 sounds like it's there for low pass HS / tone shaping.

 

[alexc]

Between the book that just arrived and this thread, I'm in tube info heaven.    Thanks everyone! <3

Such class ! 

++Like

 

[alexc]

Can you be more specific about exactly what you mean by  the phrase "alters the harmonic structure". I assume you are not talking about the myth that NFB adds harmonics not present in the original output.

Cheers

Ian

Speaking very much in the context of 'single tone testing', albeit with good precision as is the case in the hands of a good 'tester' and some reasonable 'pc based REW kit' ...

I think that nfb certainly lowers the amount of energy in 'low harmonic distortion products'  while raising somewhat the 'energy present in the high harmonic distortion products'.

High harmonic distortion products are raised much less than the low harmonic ones are reduced, so it's a huge gain no doubt  ..  but the higher ones are nonetheless  more noticable 'than without nfb'  ...  in these simple spectrum snaps - meaning 'more overall energy' in the high bands is present than before.

...

If I could find the best of both, I would - so far, that's as best as I can  describe it.  I'm not discounting romantic notions attached to 'really sparse spectrum snaps' and 'love of spuds'  and so on.  I have all the above.

...

Respectfully that's my experience - not so much 'adding' harmonics as 're-jigging' or 're-binning'    the energy present  (or npv lol)  ...  amongst the available harmonic bins,  in terms of a spectral display sys.

In some of my 'REW' based posts and snaps one can see some history of my measures of these things. 

Basically, as you turn up the system gain, the low order products increase - then you turn on and up the nfb and that low order noise  decreases  ...  repeat until you get your best measures of 'the bottom end' thd, hum etc  vs the top end.

Take a snap, make some notes, make some posts.

Repeat.  Perhaps you can 'dial it in' some more, depending on your constraints or 'scenarios'  .  which can really pile up!

Eventually you get the feeling that 'at the limit' of one's  'gain/distortion/level' envelope [attention span] and yes, even one's 'even/odd'    bias-es  [sorry]    ..    you get the best compromise you can  make

between    'low end  noise'  [energy-wise]    vs      'high end noise' [energy-wise]

Take some more snaps  then call it a days work and go home.    In the context of  'rew snaps'  and 'cro  watching',  that is. 

Sadly for some, that is the realm of the 'scope jockey'.

Not so, me! I lerz it.