Negative feedback problem Help

[catalin gramada]

Two pics to show what is happen by adding nfb into circuit with signal out of phase:


No nfb.

Nfb.
Yellow is output signal.

[catalin gramada]

So despite is out of phase adding nfb it looks like positive...

[Helmholtz]

Two pics to show what is happen by adding nfb into circuit with signal out of phase:


No nfb.

Nfb.
Yellow is output signal.

Sorry, scope pictures are not self-explanatory. Please elaborate what they show and which trace is which.


Did you check if the screen taps go to the same tubes as the respective plate wires?

[catalin gramada]

Blue is input signal, yellow is the output on 8 ohm tap. First image no nfb applied, it clearly shows the signals are out of phase. Second pic. the same but nfb applied to first stage 4.7k series/ 10 ohm shunt. Nothing to substract, it goes positive as you see...Thanks

[catalin gramada]

I will reverse the driver phase to show how it goes. One sec...

[Helmholtz]

Two pics to show what is happen by adding nfb into circuit with signal out of phase:


No nfb.

Nfb.
Yellow is output signal.

Still not clear where the blue signal is taken.
As said before the signal at the output must be IN phase with the power amp input signal for NFB. Reason is that the input tube's cathode is an inverting input.

If the the output is out-of-phase with the input, result will be positive feedback, causing increased output. I guess that's what your scope pictures show?

[catalin gramada]

I reversed the driver leads to output tubes. Same thing. No nfb / nfb.

[catalin gramada]

The blue signal is generator signal at input of the amp, first triode grid on schematic.

[Helmholtz]

Again: What does the blue trace show?

And did you make sure that screen taps and respective OT plate wires are connected on the same sides of the OT primary?

Sorry, simul-posting.

[catalin gramada]

The blue signal show the input signal from my generator. It is applied at the input of the amp into the first triode grid (same triode which get the nfb). Yes the wires are correctly tied.
Get some progress: I reversed the output taps : common OT to hot/ 8ohm tap to ground. Now input and output signals are in phase...

[Helmholtz]

If you interchanged power tube grid signals/PI outputs, the output should now be in phase with the input. Your scope picture still shows out-of-phase signals, so something went wrong.

[Helmholtz]

Get some progress: I reversed the output taps : common OT to hot/ 8ohm tap to ground. Now input and output signals are in phase...

Ok, as said reversing output taps is just for testing.
Your result means the the OT primary wiring (both plate and screen taps) should be reversed and the secondary should be wired as shown in the OT specs.

[catalin gramada]

No. reversing pi outputs to power grids didn.t do any difference, the input signal and ot output are out of phase no matter how reverse. But I reversed the OT output with 8 ohm tap to ground and now the signals are in phase....

[catalin gramada]

Well, is not quite "ground", my OT is floating now, just I reversed OT outputs : common and 8 ohm tap. Now I want to add nfb return, what OT terminal I have to declare as "ground return" please ?

[Helmholtz]

No. reversing pi outputs to power grids didn.t do any difference, the input signal and ot output are out of phase no matter how reverse.

That doesn't make sense. Output phase should invert.

But I reversed the OT output with 8 ohm tap to ground and now the signals are in phase..

Please see post #27.