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In Reply to: Re: Basic answer on Cathode bias. posted by Pär on August 18, 2002 at 22:15:16:
Redshift (into Astronomy, too?),Were you asking if the resistor which ties the cathode to ground resistance should be factor in the total current flow through the tube?
If this was the Q... Yes, is the answer. I believe this is one of the reasons most cathode biased tubes have bypass caps. The use of a parallel capacitor and a resistor tied from the cathode to ground lowers the value of resistance needed to set the idle bias of the tube---via the time-constant equation.
Too much resistance alone, will limit the ultimate current flow though a tube. The combination of cap and resistor becomes a frequency-dependent circuit---so you need to choose the correct range of cap and resistor to give you the idle level you want---yet not lose too much low frequency response, via the cap acting as a high frequency filter. In other words, there are set values of resistors and caps you'll see in many pre-amp, gain stage, phase-inverter tubes, and power tubes---depending on the intended use of each tube.
One bad thing about bypass caps is it introduces this time-constant factor into the operation of the tube. This tends to cause non-linear shifts in feedback control. Probably why a lot of hi-fi gain stages prefer no bypass caps, to maintain linearity of operation---via feedback control.
For us guitar amp nuts---linearity is not a big issue. Primary frequency range is limited to say 100 Hz to 14,000 Hz. So the extended range of linear frequency response is not needed. But, the overdriving of the gain stage tubes is essential for the harmonics and overtones guitar freakazoids are after. Thus, most gutiar amps have both caps and resistors together, in the cathode biased tubes.
Hope this wasn't too much yaddy-yaddy-yaddy.
Follow Ups:
Not sure I understand your statement following:"I believe this is one of the reasons most cathode biased tubes have bypass caps. The use of a parallel capacitor and a resistor tied from the cathode to ground lowers the value of resistance needed to set the idle bias of the tube---via the time-constant equation."
As far as setting bias goes, I've always viewed the cathode resistor as operating independently from the cap. However, the unbypassed resistor also creates a current feedback situation for AC signals that lowers the AC gain of the stage and increaaes the effective AC plate resistance. The bypass is there to increase AC gain and/or lower AC plate resistance in most applications. This is true for single ended or individually biased PP stages. Where a PP stage uses a common cathode R, the use of the bypass becomes less clearcut if class A. If PP Class AB or B operation is desired then the bypass cap becomes less optional (at least for reproducing amps). For geetar amps it seems that anything goes.
There is a time constant set up by the C/R combination that may or may not be a factor in an amps LF response.
Steve,Pretty sure about the generalization of bypass cap and resistor used in concert, allows the manufacturer to use lower value resistance for the cathode resistor.
Anyhow, check out Kevin O'Connor's TUT 1, for a much better description of cathode biasing using resistors alone & resistor/capacitor combos.
You have it right. The cathode bypass cap puts the cathode essentially at ground with regard to AC signals, but does not effect the DC bias voltage across the cathode resistor, as it simply charges up to this voltage.
Hi steve .
I see a friend of mind take off the by pass caps on his amp in the cathode bias , he said it was good but then he put the caps back on. he was scare of some things may go wrong in a long term use .
Taking them off will lower the gain a bit, but will not cause any harm. If your friend likes the sound better (many do), he should leave them off.
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