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previous: Craig Enzo, Can you help me under... -- 4/9/2004 4:56 PM View Thread

Re: Can I Change the Cathode Bias Cap Value?

4/9/2004 11:33 PM
Enzo
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Re: Can I Change the Cathode Bias Cap Value?
The "bias" varies with the signal from instant to instant, but the net average doesn't change.  
 
As a thought experiment, imagine a speaker line with no signal. You can measure the voltage there with a meter set for DC and get zero. Now imagine a large signal on the speaker. MAybe 20 volts peak either way. If you measure that with a meter on AC, you get about 14VAC. But switch the meter over to DC and you still get zero because that is the average voltage.  
 
If you have a cathode biased stage with no signal present, the bias voltage will establish at whatever the current dictates. Measuring DC, if you then add signal, the reading will stay the same. I am speaking theoretically, ignoring such things as power supply sag. SO in the absence of signal, if you switched a bypass cap in and out of the circuit, the bias voltage wouldn't change.  
 
In terms of signal, the unbypassed cathode resistor will drop a varying voltage that follows the signal. So as the signal at the grid goes more positive, the voltage at the cathode also goes more positive, and so it tends to act against the signal - a sort of negative feedback. WHen you put the cap in there, the cap maintains a steady voltage equal to the average. Now as the grid goes more positive, the cathode does not because the cap charges. So the tube amaplifies the signal more than it would. The gain increases in the stage. (Look at any number of tube amps where a cathode cap in a preamp stage is switched in and out as a gain boost.) SInce the resistor and cap form an RC circuit, there is a time constant to it wich translates into a frequency. So the bypass effect will be somewhat frequency dependent.  
 
Does that help? WHen I use the term "bias" I am referring to the steady state. When we discuss the signal, it is not incorrect to discuss the "bias" changing, but it is a different sense of the word.

 
Replies:
Craig Enzo,Thanks a lot for takin... -- 4/12/2004 7:48 PM