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Old 20th September 2004, 12:29 PM   #1
alejo is offline alejo  Argentina
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Default capacitor reactance

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?
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Old 20th September 2004, 03:46 PM   #2
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Default Re: capacitor reactance

Quote:
Originally posted by 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?
1/(2 x pi x f x C)

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

se
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Old 20th September 2004, 08:15 PM   #3
alejo is offline alejo  Argentina
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Location: bs.as.
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 !!!!!
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Old 20th September 2004, 09:49 PM   #4
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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,
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