capacitor reactance

[alejo]

Looking for info about like calculates capacitor value to bypass cathode in bias automatico; I find that:
when I now the value of cathode Resistor in ohms. it formulates:

Reactance=(1/50).Ohms

....to the Low Frecuency operate the circuit (for example at 20Hz.)

now, as the reactance of a capacitor for a given frequency calculates?

 

[Steve Eddy]

Looking for info about like calculates capacitor value to bypass cathode in bias automatico; I find that:
when I now the value of cathode Resistor in ohms. it formulates:

Reactance=(1/50).Ohms

....to the Low Frecuency operate the circuit (for example at 20Hz.)

now, as the reactance of a capacitor for a given frequency calculates?

1/(2 x pi x f x C)

Where f is frequency in Hertz and C is capacitance in Farads.

se

 

[alejo]

Thanks steve.

I confused because:

In one PushPull EL84/6BQ5 use for comon cathode resistor arround 140 Ohms
I want Bypass this resistor, and calculate:

Reactance=(1/50)*140
Reactance=2.8

Now find capacitor for 2.8 Ohms reactance at 20Hz

1/(2*pi*f*c)=

1/(2*3.141516*20* "test value capacitor in farads")=

for 2800uf

1/(2*3.141516*20*0.0028)=

2.84 Ohms


Ok. now;.... for correct bypass cathode resistor 140 Ohms at 20hz need 2800uF capacitor ?????

Please help !!!!!

 

[fdegrove]

Hi,

Actually the formula you should use is:

f = 1/(2*PI*R*C)

or: C = 1/(2*PI*F*R)

f = -3dB point.

R= Rk'//Rk

Rk' = (Rp+ra)/(mu+1) (Common-cathode stage)

Rp = Plate resistance

ra = Ri or internal resistance of the tube.

Rk = cathode bias resistor.

Note that C should always be much larger than Rk but not much larger than necessary for the lowest -3dB point wanted: t = R*C

Cheers,