Varying the tuning cap does indeed help check for VHF
oscillations, because the tuning cap greatly changes the anode
load impedance at VHF.
Varying tank components on the load side of the tank does not
change VHF impedance a significant amount, because of the low
pass filter effect of the tank.
To test for HF stability you need to move the input and output
impedances through all load resistances and reactances, including
those below the operating frequency.
That's because the input and output networks are transparent at
low frequencies.
Raising HV about 25% above normal, if safely possible, is a good
idea. It's also a good idea to check at low plate current bias, and
near rated dissipation.
I've never found a PA that would pass these tests and oscillate in
the real world end-use.
By the way, if you have Clipperton L or other amplifier that is un-
neutralized and uses 4 572 or 811A tubes in parallel, don't try
these tests without safety glasses. The HF oscillations will cause
components to fail.
73, Tom W8JI
w8ji@contesting.com
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