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Tubes Asylum Questions about tubes and gear that glows. FAQ |
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Tubes Asylum Questions about tubes and gear that glows. FAQ |
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In Reply to: Re: Basic answer on Cathode bias. posted by steven oda on August 18, 2002 at 22:44:52:
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.
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Follow Ups
- Are you sure about that??? - Steve O 08:58:04 08/19/02 (2)
- Re: Are you sure about that??? - steven oda 13:20:24 08/19/02 (0)
- Re: Are you sure about that??? - Jim Carlon 13:06:07 08/19/02 (0)
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