Negative feedback problem Help

[catalin gramada]

Hello. I got recently an OT Hammond 1650 WA, 1,9k Raa 280w. I used it into a classic UL circuit but I cannot apply any global nfb. It goes positive no matter how reverse the phase. What I did wrong please ? Thanks.

[pdf64]

There must be too much open loop gain and phase shift within the loop for the degree of feedback. Note that you’ve got an extra gain stage (with RC coupling) than normal for a tube guitar amp circuit.
Maybe try increasing the value of the series feedback resistor (4k7?).

[catalin gramada]

There must be too much open loop gain and phase shift within the loop for the degree of feedback. Note that you’ve got an extra gain stage (with RC coupling) than normal for a tube guitar amp circuit.
Maybe try increasing the value of the series feedback resistor (4k7?).

Hey, thanks
I built this circuit few times with different OTs half by Hammond size. Never happen to get any issues with nfb down to 15- 12 db.

[pdf64]

Hmm, maybe verify the values of the components then?

[catalin gramada]

Except the fact I used some toasted power tubes I have at my hand for test and the OT coil axis is close straight oriented to the tubes there is nothing suspect. The signal is well balanced from driver output....still get good amplification but crossover distortions are noticeable without nfb.

[pdf64]

Maybe the 10 ohm shunt feedback resistor is actually 100 ohms?

[Helmholtz]

I used it into a classic UL circuit but I cannot apply any global nfb. It goes positive no matter how reverse the phase.

What are the symptoms?
There must be a fundamental difference if you reverse the output leads.
(Caveat: Interchanging OT output leads is only meant for a quick test and refers to swapping the 8 Ohm and 0 Ohm wires.
As your OT has 4/8/16 Ohm taps, there is only one correct wiring order, because e.g. the impedance between the 8 Ohm and 16 Ohm taps is much smaller than 8 Ohm.
So you cannot ground the 16 Ohm tap for correct impedances. Please ask if that sounds confusing.)

Did you actually determine the NFB ratio by measuring open and closed loop gains of the actual amp?

[Helmholtz]

For correct OT phase use your dual channel scope and see if the signal at the 8 Ohm output is in phase with the power amp input.

[Helmholtz]

Please note the Caveat I added in post #7.

[catalin gramada]

Hello. Thanks Helm.
Open loop:
The tubes are crap, but still working.

[pdf64]

...There must be a fundamental difference if you reverse the output leads.
(Caveat: Interchanging OT output leads is only meant for a quick test and refers to swapping the 8 Ohm and 0 Ohm wires....

Just wondering if it may have been the OT primary that got swapped around but without also swapping the screen grid UL taps? I can't work out what effect that might have, though it's not obvious that it would cause oscillation.

[Helmholtz]

Just wondering if it may have been the OT primary that got swapped around but without also swapping the screen grid UL taps? I can't work out what effect that might have, though it's not obvious that it would cause oscillation.

Yes, I also thought about that. I think it will cause instability. As normal UL operation works by negative screen feedback, that would result in positive screen feedback because of phase reversal of the screen signals.

[catalin gramada]

The tranny is a massive piece of iron : 12.5kg/28lb not easy to manage. The coil axis is oriented to the tubes as was happen to have room onto the chassis

[Helmholtz]

Hello. Thanks Helm.
Open loop:
The tubes are crap, but still working.

I see 2 out-of-phase signals.

[Helmholtz]

The tranny is a massive piece of iron : 12.5kg/28lb not easy to manage. The coil axis is oriented to the tubes as was happen to have room onto the chassis

Not quite sure what this is referring to.
But coupling effects always depend on relative phase. So if this orientation is adverse, just turn the OT by 180° to reverse the polarity of the leakage field.