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previous: Doc I think Andy moved on to the questi... -- 9/21/1999 4:43 PM View Thread

Re: Cap across driver tube

9/21/1999 6:19 PM
EthRe: Cap across driver tube
That tiny capacitor always confused me. I've tried it , but never left it in when putting my amps together.  
I would like to heer comments about the tone addition of such a capacitor. The gain of the power amp/ phase inv. is determined by the feedback voltage divider, so Figuring out the roll off frequency of this capacitor also includes figuring out open loop gain. Right? Right!  
Secondly, in feedback controlled amplifiers that contain alot of capacitance, oscillations can become a problem. This capacitor adds capacitanc, it creates a roll off inside the feedback controlled amplifier! It does limit the total open loop gain of the amp, But most amps have feedback! This cap will then cause some problem  
Slugging the dominant pole is something that can be read about in a few books, How I understand it is: The feedback signal is, ideally a 180 degree phase shift of the input signal. This ideal 180 deg. signal then limits power amp gain in the ratio of the fedback divider resistors. However, when there are capacitances in a feedback controlled amplifier, the feedback signal may not be a 180 degree signal, it will be phase shifted! at some frequency it will be 180 degrees, total of 360 degrees! Now it is not really negative feedback, it is an oscillator. I don't see how this wouldn't be possible, and by figuring all of the rolloffs of the circuit you could figure at which freq. it is... Above hearing level most likely.  
So are all those old Marshalls and fenders, etc. Oscilating ? There are some other variables that ... Save? them.  
So Back to slugging the dominant pole, Basically you will want to correct the amplifier phase shifts with equel phase shifts in the feedback. ie capacitor in paralell with the feedback resistor. An exact value requires figuring out the open loop gain of amp and tatal of all circuit phase shifts.  
If you need a high freq rolloff, paralell a cap across the feedback resistor. Not accross pins 1 and 6. It only will add to the chances of oscillation, and performance (dependant on o.l.gain) is less than straight up anyway.  
I feel blasphemous posting this!  
I would like to here of tonal ideas regarding this cap. Also, has anyone tried the 'cross-line' tone controls in Kevi O'Conners book 2? -Eth sorry, the editting was kina making a mess of this document, bare with me

SpeedRacer I suppose I'm guilty of "Leo did so... -- 9/21/1999 7:11 PM
Steve A. Eth:  &nb... -- 9/22/1999 2:24 AM