Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Colas LeGrippa on March 16, 2019, 07:05:41 pm

Title: Parasitic cancelling tricks ?
Post by: Colas LeGrippa on March 16, 2019, 07:05:41 pm

Hola amigos !


In one of my builds I have put a pre PI MV.
It works very well until I crank the preamp volume all the way up while reducing the MV in order to get low level distortion. Until now everything is perfect but all of a sudden I hear crackling noisesand the volume raises by itself before starting to howl like a million wolves even with my guitar unplugged....Seems parasitic oscillation to me. What would you suggest, doctors ????


Colas

Title: Re: Parasitic cancelling tricks ?
Post by: shooter on March 16, 2019, 07:46:16 pm

some thoughts;

try some homegrown
use a distortion pedal
try some homegrown
add LOTS of caps in your PS
try.......
quit adding knobs to an otherwise great amp
try some......
 :icon_biggrin:

Title: Re: Parasitic cancelling tricks ?
Post by: Colas LeGrippa on March 16, 2019, 08:11:17 pm

Yeah I know how to get a good tone with mmy Hot Cake pedal, ( made by Paul Crowther, drummer of split enz band, remember? )


But you see I just wanna succeed to build a high gain amp with stability. Many succeeded. I miss only that ninja trick to chase the oscimlation in my 4 gain stages .


Talking of knobs, I may have added a master vol pot but I removed terb, mid and bsss....

Title: Re: Parasitic cancelling tricks ?
Post by: pdf64 on March 17, 2019, 06:09:43 am

That seems to be free running oscillation, rather than parasitic oscillation.
Parasitic requires there to be a regular input signal in order for the oscillation to manifest somehow, whereas your amp quite happily oscilates all on its own, given suitable control settings.
Whichever, oscillation is caused by later stages coupling to earlier stages (positive feedback), and requires sufficient gain and phase shift between the two (a non-inverting / 360 degree phase shift gain of 1).
Of course, capacitence and EM / electrostatic fields couple everything to everything else, but usually one or two feedback loops dominate.
Any implemented circuit will have a secondary, usually largely unintended ‘overlay’ of couplings, feedback loops which may be positive or negative. The physical layout of the components,lead dress etc largely determine the significance of any of these couplings.
Without circuit and layout detail, it’s back to you to assess your build in the light of this info.

Title: Re: Parasitic cancelling tricks ?
Post by: Colas LeGrippa on March 17, 2019, 11:19:53 am

Thanks for the info.


Let me tell you though that your knowledge and your nice theory doesn't help me. I wanted to have tricks to tame the parasitic oscillation.
Raising the filtering cap values or adding a small cap here and there, that kinda info, see ?


Colas