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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 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.
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Follow Ups
- Re: Basic answer on Cathode bias. - steven oda 22:44:52 08/18/02 (5)
- 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)
- Re: Basic answer on Cathode bias. - vintageguy 05:05:23 08/19/02 (1)
- Re: Basic answer on Cathode bias. - Jim Carlon 13:00:42 08/19/02 (0)
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