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In Reply to: RE: No. posted by Tubenstein on July 26, 2014 at 09:23:16
The value of the cathode bypass cap is determined by more than just frequency response. Things like the class of operation, topology, and even the designer's philosophy regarding issues such as overload recovery must be considered. That aside, typical values to bypass a 270 ohm cathode resistor in a AB1 PP amplifier would be 47-100uF. The exact value isn't critical, as you will see if you research schematics of commercial equipment online. Note that while the cathode should have sufficient bypassing so as not to create significant degeneration or phase shift within the audio passband, frequency response is usually determined during the design phase primarily at the grid.
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Edits: 07/26/14Follow Ups:
A class AB1 push-pull amplifier should not have a cathode resistor at all beyond something for current sensing.
The mention of overload recovery above deserves some attention. In the context of guitar amps that are frequently driven into distortion, a too-large cathode bypass will result in very unpleasant sounds during the time it takes for the output stage (for example) to return to normal operating points following a momentary overload. It has to do with the time constant of the resistor and capacitor. If the cathode bypass is smaller, the recovery may be barely noticeable.
Of course, we don't want HiFi amps to be driven into distortion, but a very large cathode bypass can make the amp more fussy about transient overloads. Some bandwidth limiting at the lower end could also make an amp less susceptible to subsonic signals like record warps.
It's usually not that difficult to experiment with different cathode bypass values to determine which one sounds best. Use your ears :-)
TK
Andy has a input 6GK5 CCS loaded
with a 270R cathode resistor.
That makes a 120uFd for 5Hz F3.
DanL
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The Mind has No Firewall~ U.S. Army War College.
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