For those of you preparing to take the plunge into QRO power for your ham shack, you are advised to take an hour and read this web site…
The site discusses what to look for when the dreaded VHF parasitic oscillation attempts to destroy your tubes. If you look at E-Ham reviews you will notice amplifiers do work pretty well, but there are enough folks that see these damaging oscillations to cause some alarm. The variable appears to be with the tubes themselves. Their mu rating varies a bit so some are more susceptible to run away current during a run away oscillation.
The Eimac 3-500Z is one tube that has amplification well into the VHF range.
There are some reports from E-Ham the Ameritron 811 or 811H may have these issues even though the tubes are more benign 811.
The point is, before you purchase an amplifier read the information on the web site above so you will know what to look for should it happen.