It should be remembered that if the parasitic resonant frequency of the grid
circuit is ABOVE the parasitic resonant frequency of the anode circuit,
then it doesn't oscillate - which is an explanation of why external anode
tubes with relatively large output capacities can be (not necessarily are)
stable without parasitic suppressors.
I will agree with Tom in that in my experience, 572B's are a MAJOR pain in
the butt to stop parasitics in. Looking at the construction, with those
long grid leads, it's hardly surprising.
73
Peter G3RZP
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