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Tube DIY Asylum Do It Yourself (DIY) paradise for tube and SET project builders. |
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In Reply to: rectified cathodes and "ground" posted by hifitom on October 31, 2001 at 22:56:18:
This subject strikes at a concept that has been the source of some of my real "Eureka" moments in understanding what is going on in a circuit. Namely: An AC ground does not have to be located at the 0 V DC reference.Take a typical implementation of a bypassed bias-setting resistor. The cathode of the tube is bypassed for AC to ground, so the cathode is an AC ground. The DC voltage at that spot is not at 0V DC.
With the diode providing your bias, yu now have an interesting hybrid - the impedance of the diode is different for AC than it is for DC, so you have a DC voltage at the plate of the diode tube, but the AC impedance is much, much lower than the "DC resistance" given by the voltage across the tube divided by the current. The plate of the diode is a quasi-AC ground here.
Now you can take a look at everything else in your circuit that is "grounded" and decide - what DC voltage is useful here? And you can listen to what using different sorts of bypass components do to the sound, the small AC impedance of the diode sounds different from the small AC impedance of a cap, and the resulting cathode degeneration from each approach sounds different. Another way to play with your sound.
You can also bypass UP - B+ is another AC ground.
This is the exact thought process that got me working on the Free Lunch amp I designed and - you know what? - I like what I was able to do with the sound using this approach.
Other examples of this type of exploration are the Ultrapath bypass cap, the Monkey amps, and Steve Bench's No R No C amps.
-j
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Follow Ups
- Re: rectified cathodes and "ground" - J Epstein 12:26:57 11/02/01 (0)
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