HF instability, or instability at the operating frequency of the amplifier, is the most damaging type of problem. This is because the primary energy storage system, the tank circuit, is resonant with high Q at or near the operating frequency.

If you look at network analyzer swept measurements of a tank circuit, you'll see the maximum voltage distributed through the tank occurs at or near the operating frequency.

Some amplifiers have designed-in HF stability problems. The Yaesu FL2100 is one example of an amplifier with poor RF design. Un-neutralized amplifiers using tubes with high feedthrough capacitance are another source of problems. Any amplifier with high feedthrough (or feedback) capacitance is a candidate for HF instability if the load is removed while the amplifier tube or tubes are drawing quiescent current. Dentron amplifiers with four 811A and four 572B tubes, as well as the Collins 30L1 with four 811A's, are examples of relatively unstable amplifiers (along with the FL2100).

The cure is to remove intentional undesired feedback or to neutralize the PA!