- Posted by Tre' (P) on July 01, 2009 at 10:24:52
In Reply to: RE: Reading Plate Characteristic Charts - Question posted by Maxamillion on July 01, 2009 at 09:00:23
The grid voltage is grid to cathode. When the signal is present on the control grid the grid voltage changes. The operation of the tube follows the black load line (22K load in this case).
This 6sn7 example has an idle current of 11.3ma and a idle plate voltage of 188vdc. The grid is 4 volts negative with respect to the cathode.
If you place a 4 volt peak to peak signal on the grid, the plate voltage will fall to 155vdc on the positive half (as the grid voltage is forced by the input signal from -4 to -2vdc) of the input wave form and rise to 220vdc on the negative half (as the grid voltage is forced by the input signal from -4 to -6vdc) of the input wave form.
The tube and operating point is not totally linear but not far from it. The plate voltage increase is 32vdc (220-188)while the plate voltage decrease is 33vdc (188-155).
This difference in the plate voltage change from the positive half of the input wave form to the negative half of the wave form causes harmonic distortion.
The trick is to find a tube and a operating point where the positive plate voltage change and the negative plate voltage change are equal.
Tre'
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