positive feedback, what does it do?

[Tdale]

If I wire a cap, or a cap and resistor in series, between the plate and the grid of a tube, that's called positive feedback, right?

What does it do to the gain and the sound? What sound different when using positive feedback compared to not using it..

Tommy

[VacuumVoodoo]

If I wire a cap, or a cap and resistor in series, between the plate and the grid of a tube, that's called positive feedback, right?

Wrong. That's negative feedback since the signals on plate and grid are in opposite phase.

What does it do to the gain and the sound? What sound different when using positive feedback compared to not using it..

Tommy

A capacitor between plate and grid will lower the gain above certain frequency depending on the cap and tube's internal capacitance and resistance. Above this frequency gain will drop at a 6db/octave rate. This cap connects in parallell with the tube's internal grid-plate capacitance and is therefore subject to the Miller effect. A simplified explanation is that it will have same effect as a capacitor with it's value multiplied by "Stage gain" connected between plate and ground. i.e. a 10pF cap between grid and plate gives roughly same effect as 500pF between plate and ground if gain stage is about 50.

A cap and resistor in series will give a shelving response i.e. at certain frequency the gain will start dropping 6db/octave and further on it will flatten out.

Soundwise it's comparable to a hi-cut control. The effect on OD will depend on which gain stage this is applied to. Early on - less OD on treble.

A small value capacitor between grid and plate can be used to tame a high gain amp that has a tendency to oscillate. One trick is to connect 2 pcs of about 2 inches of hook-up wire to grid and plate on every gain stage and twist them together until oscillation stops. This way you can easily identify tha gain stage that is most sensitive and responsible for causing oscillation.

Hope this helps.

PS. Out of lurking phase and hoping to be of some help.
Disclaimer: I'm building amps commercially. I'm not involved in building copies or clones of any past or present design.

[Tdale]

Thanks!

Isn't this local loop also used to make a tremolo? How does that differ from using only a cap, or a cap-resistor in series?

Hmm, sweden, commercial amps... Mystiqe?

Tommy
Norway

[Tdale]

tubewonder......I saw you homepage right after I posted my first reply

Your amps looks nice


Tommy