by ricochet » Thu Jul 10, 2003 12:57 am
No, it's true. When you turn down the volume, the wiper of the pot that's carrying the signal on into the following amp stage is moving toward the ground end of the pot, with more resistance between it and the "high end" of the pot that the signal from the previous stage is coming into. There's a certain input capacitance of the following stage, whether tube or solid state. For a typical 12AX7, the input capacitance is on the order of 100-150 pF. (The tube's static grid-cathode and grid-plate capacitance are multiplied by the stage's amplification, due to the Miller Effect.) When you've got a resistance and capacitance in series, their product is a time constant. The bigger the time constant, the longer it takes to charge up to a given voltage. The high frequencies just don't have time to do it before they reverse direction. Effectively, the high frequencies are being held back by the resistor and bled off to ground by the tube's input capacitance. It's a low pass filter. The lower you turn your volume, the lower the high-frequency rolloff point drops. Since you can't reduce the input capacitance of the tube, the only things you can do to counter the effect are to drop the total resistance of the pot (as by adding that parallel resistor to the wiper that I mentioned), which will also reduce the gain, or add a small cap bypassing the volume pot for high frequencies as Fender did on the "bright channel" of their old Bassman and other amps. Neither is a perfect solution. But hey, we're talking about blues guitar, not hi-fi audiophool stuff!