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Any setting available in the full range of the trimmer pot is safe to use. It isn’t possible
to set the screen voltage to a level that will be harmful to the amp. However, you need
to keep in mind that the bias current will increase as you increase the screen voltage,
so you will need to re-calibrate the bias current every time you change the screen
voltage.
If you are significantly increasing the screen voltage, keep in mind that the bias
current will increase as well, so you want to make sure not to let the amp sit running
for very long after adjusting the screen voltage, before you re-calibrate the bias voltage.