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Great article, very thorough. It's worth noting (though it may be a bit out of the scope) that your audio amplifier tube is actually wired as a triode. The screen grid tied directly to the plate makes the tube operate as a triode instead of a pentode. No big deal in this case, but in higher power/voltage circuits, you would just need to make sure that you aren't exceeding the voltage and power ratings for the screen grid.