Nfb question

[catalin gramada]

Hello. There are functional diferences in nfb applications for this paraphase circuit ? (Meant a floating parahase with full bypassed common cathode resistor) .Ok, the values should be tailored a bit to match precisely, but I wonder from a functional point of view the circuits works absolutely identical please ? Thanks.

[Helmholtz]

Another tricky question .

I think the lower circuit has less cathode separation.
You want the NFB signal to go to the left triode only.
But considering that the impedance of the 100µ cap is 23 Ohm at 70Hz, some bass content of the NFB signal will get to the right side cathode where it means positive feedback.

[catalin gramada]

Another tricky question .

I think the lower circuit has less cathode separation.
You want the NFB signal to go to the left triode only.
But considering that the impedance of the 100µ cap is 23 Ohm at 70Hz, some bass content of the NFB signal will get to the right side cathode where it means positive feedback.

Hey. Thank You. Allright that mean for same amount of positive feedback injected in second cathode the cap value should be also 10 times order, meant 1000 uF please ?

[Helmholtz]

Yes, that should work.
Also make sure the power rating of the 1k series resistor is sufficient.

[catalin gramada]

Thanks, I.ll try it.

[catalin gramada]

Hey Helm. You observation is absolute hundred percent and even in original design the amount of positive feedback is substantial. This nfb design in paraphase inverter looking like an improvisation from this point. Still I don't see nothing to destabilize the stage and put it in border of oscillation. Now, the second stage have gain factor of 1 as it use a lot of local negative feedback into a plate follower configuration. It is possible for this reason the effect of the positive feedback into second cathode to be suppressed and from this point to not count, please ? Thanks.

[Helmholtz]

Don't forget that a signal fed to the cathode is not attenuated by the voltage divider at the grid, so gets full gain.

It is essential that the NFB dominates.
You can check by disconnecting the feedback.
If that increases gain at all frequencies there's net NFB.

Personally I prefer a LTPI.

[catalin gramada]

Personally I prefer a LTPI.

Me too.

[catalin gramada]

Hey Helm. Thanks you opening my eyes. There is such of very interesting point regard the positive feedback induced by actual original arrangement Ampeg used. It is completely responsible for the amp character. They used a 25uF bypass cap. Switching to 100uF do a difference in sound (different harmonics content). Make a optimal paraphase with individual bypassed cathodes and nfb applied in first cathode only and get a completely different amp. So yes, the tone of this amp is stucked by original nfb arrangement as well as they did it. I think it worth a analysis to find why. Thanks.