ringing problem. please help

[catalin gramada]

Hello. Trying to stabilise the circuit below I don.t know how is better to cure so I need an advice please. The problem starts once I aplied the NFB and is related by maximum open position of potentiometer, no mater how big is the signal in front. No oscillation before the pot is maxed open. I.m interested by your opinions How do you think is better to cure? Thanks



Lte edit: solved. My mistake working at too small signal level. Excited the transformer a litle bit more and the ringing is pretty consistent. hooked 47pF and is done

[Chuck H]

If applying NFB caused the problem then you may have the primary leads of your output transformer on the wrong plates. You can try reversing them.

[catalin gramada]

Thanks Chuck. The amp is stable, just a minimal incovenient asked for a phase compensation at high frequencies,(there are four stages in losed cloop and OT is a cheap one) but was fixed.I.m pretty happy how looks like from 40 to 10000cps. Thanks

[Chuck H]

I recognize that we're working with a language barrier. Are you saying the problem is fixed? Did you swap the OT leads? I think you are saying that you believe the problem is a HF phase error above 10000cps but that it's fine otherwise. So I'm going to address that. only now did I notice the red scribble on the diagram is a scope waveform. So this is the actual electronic description of ringing and not an audible ringing tone. I'm with you now. I don't know what the purpose of the amplifier is but with guitar amp circuits it's pretty common to roll off some HF for stability since they are very high gain amplifiers. You can try a 100pf cap between the phase inverter outputs. It's a standard circuit on many amps and may reduce the ringing.

[J M Fahey]

If he had OT windings the wrong way, it would continuously oscillate; some ringing or instability means that phase is basically "right" but there is some shift at frequency extremes, normal with transformers of any kind, just some are worse than others.

[mikepukmel]

Sorry for a novice question here, but re:
"In fact it start to ringing (no oscillation) little bit before the pot is maxed open."

What do you mean by "ringing (no oscillation)" Doesn't ringing mean oscillation?

[catalin gramada]

Hi. Understood. Supriming components. But I did a little bit investigation and think I found the ringing source in primary OT side. Think is better to start here snubing somehow. Related by this, could be a coupling problem or a mediocre OT ?

[catalin gramada]

Sorry for a novice question here, but re:
"In fact it start to ringing (no oscillation) little bit before the pot is maxed open."

What do you mean by "ringing (no oscillation)" Doesn't ringing mean oscillation?

Yes it is, ringing is a form of oscillation. Semantical problem with my broked english. No osscilation before pot was open at max meant. Sorry

[mikepukmel]

Yes it is, ringing is a form of oscillation. Semantical problem with my broked english. No osscilation before pot was open at max meant. Sorry

Thanks C.G. (thanks for answering my rudimentary questions, Im climbing a long learning curve!! )

[Chuck H]

100pf across the PI outputs.?.

[dstrat]

only seeing part of the picture , guessing your driving a lot of signal into the second tube possibly causing grid blocking?