Electron tube grid rectification effect on changing gain

At your risk. Take all safety precautions.


Electron tubes can detect the average value of incoming signal and adjust bias accordingly so that signal is prevented from clipping.


They can detect the envelope of the signal. For example what a VU meter would show. This much lower frequency DC varying information is extracted by the grid rectification effect. What sound engineers call the side chain signal of an automatic variable gain amplifier or compressor. A volume control that adjusts itself.


This lower frequency DC information is extracted in exactly the same way that audio information is extracted from an AM (amplitude modulated) RF (radio frequency) wave. What radio engineers call demodulation of an AM wave. It is non linearity put into infomation extraction use.




It may happen so elegantly that nothing is noticed expect perhaps a great sound. The effect can be monitored by a DC milliampere meter through anode circuit or a DC voltmeter on anode. The effect had been also seen on oscilloscope while overdriving a Neumann microphone V14 amplifier. After the strong signal was suddenly reduced, the gain started to increase, and sinewave amplitude was observed to increase. (At your risk. Take all safety precautions when working with any voltage or current).




See also [Armstrong]


Older posts on Neumann U47


Yesterday's posts on WE417a with free grid.


Reference:


Operating Features of the Audion - E. H. Armstrong