5v333
Well-known member
my tube makeup circuit proceeds in snail speed. i have managed to use two windings of my OT for global negative feedback. bass is much cleaner.
the input transformer of the tube circuit shows some ringing and has to be damped with some kind of RC.
how ever when this is done, the phase response differs between the secondary halfs. i can for ex see a somewhat predictable point where the phaseshift is 180º on one of the halfs, while on the other half the phase goes up and down some more and finaly reaches 180º one or severeal octaves from the other half.
i have tried some different transformers and only one showed very good symetry however that one was not very happy with the source impedance.
yesterday i just tried something new.
i injected some local feedback around the first triode. taken after the interstage caps and applied to the secondary windings. by lowering that resistor i could get rid of the ringing and maintain phase symmetry. i was very pleased even though i lost1-2db.
i then noticed that this changed the flatness of a squarewave in low frequencies when monitoring the output of the circut.
so i tried the same thing with a small cap in series with the feedback R. tuning the right RC takes some time but this seem to work quite good aswell.
the results are very good when listening to music through the system . less mess, better balance and less fatigue.
also the hum is almost 10db lower.
since this is local feedback i thought i could do the same by making the cathode resistance larger but it didnt work...
i would like to understand what could be the case here.
is it somekind of impedance missmatch between the transfomer and the grids?
why is a zobel not as good as feedback?
am i making a dominant pole that puts things in stability also at the input?
any thoughts?
the input transformer of the tube circuit shows some ringing and has to be damped with some kind of RC.
how ever when this is done, the phase response differs between the secondary halfs. i can for ex see a somewhat predictable point where the phaseshift is 180º on one of the halfs, while on the other half the phase goes up and down some more and finaly reaches 180º one or severeal octaves from the other half.
i have tried some different transformers and only one showed very good symetry however that one was not very happy with the source impedance.
yesterday i just tried something new.
i injected some local feedback around the first triode. taken after the interstage caps and applied to the secondary windings. by lowering that resistor i could get rid of the ringing and maintain phase symmetry. i was very pleased even though i lost1-2db.
i then noticed that this changed the flatness of a squarewave in low frequencies when monitoring the output of the circut.
so i tried the same thing with a small cap in series with the feedback R. tuning the right RC takes some time but this seem to work quite good aswell.
the results are very good when listening to music through the system . less mess, better balance and less fatigue.
also the hum is almost 10db lower.
since this is local feedback i thought i could do the same by making the cathode resistance larger but it didnt work...
i would like to understand what could be the case here.
is it somekind of impedance missmatch between the transfomer and the grids?
why is a zobel not as good as feedback?
am i making a dominant pole that puts things in stability also at the input?
any thoughts?
