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previous: jbrew73 brucei know what the bypass cap... -- 7/11/2004 8:44 PM

Re: low volume sustain

7/11/2004 9:00 PM
Bruce /Mission Amps	Re: low volume sustain As long as the cathode is higher voltag ethen the grid and lower voltage then the plate, the resistor's value is only for DC bias. In this application, the bypass capacitor is what's allowing AC current to fluctuate through the power tube with no impedance by the biasing resistor.... if the cap is big enough in value, with the right amount of low Z reactance, it's supposed to completely bypass current around the resistor, so, the power tube's idle current level stays the same while the AC current "rotates" around it. I think, as the power supply B+ voltage drops from more current delivered in and out of the OT, the amount of plate B+, screen B+ and the relative DC bias changes in the power tube, creating the uncompressed, compressed or tonal changes. To really hear these things, try the same amp with no bypass cap and try to ignore the gain boosting from the cap in vs out. In other words, pick a volume level that the non-bypassed version and bypassed version are the same power output and experiment with small value caps to big value caps. Bruce

7/11/2004 9:00 PM

Bruce /Mission Amps

Re: low volume sustain

As long as the cathode is higher voltag ethen the grid and lower voltage then the plate, the resistor's value is only for DC bias.
In this application, the bypass capacitor is what's allowing AC current to fluctuate through the power tube with no impedance by the biasing resistor.... if the cap is big enough in value, with the right amount of low Z reactance, it's supposed to completely bypass current around the resistor, so, the power tube's idle current level stays the same while the AC current "rotates" around it.
I think, as the power supply B+ voltage drops from more current delivered in and out of the OT, the amount of plate B+, screen B+ and the relative DC bias changes in the power tube, creating the uncompressed, compressed or tonal changes.
To really hear these things, try the same amp with no bypass cap and try to ignore the gain boosting from the cap in vs out.
In other words, pick a volume level that the non-bypassed version and bypassed version are the same power output and experiment with small value caps to big value caps.
Bruce

Replies:

Bruce /Mission Amps oops... forgot the other part... wh... -- 7/11/2004 9:12 PM