Screen Current and Grid Driven Tetrode Linear Amplifiers - Tom, K1JJ
I just found out something the other day and wanted to pass some info along in case others have missed it too:
One of my amplifiers is a grid driven tetrode linear -and, I've been tuning it incorrectly...
It seems that for a grid driven tetrode linear amplifier, a certain amount of screen current is REQUIRED to make the tube operate at optimum linearity.
I've always thought that ZERO grid current AND zero screen current were best, but this is not true. Zero control grid current is best (AB1) but the recommended tube screen current allows the tube to operate more cleanly. This screen current is set by the adjustment of the LOADING capacitor, and also by the amount of screen voltage for a given bias and drive. (More plate loading capacitance = lighter loading = more screen current) SOME screen current is a good thing.
In contrast, a grounded grid triode linear amplifier achieves best linearity by loading the final more heavily (less loading capacitance) to reduce output by about 7% or so. This "loading heavily" requirement does not apply to a grid driven linear tetrode amp... ding, lice all done, ah ha!
The bottom line is that if you run a grid driven AB1 linear amplifier, you should be seeing zero grid current, recommended screen current, and using a regulated screen supply for the cleanest result.
Here's some detailed info on Richard Measure's website on the subject: