RF source for variable load

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Hi,  I'm attempting to put together an RF source for a non-transmitting
purpose, requirements as follows:

Frequency: Constant at about 800 KHz (external oscillator will drive)

Output voltage:  Anywhere between 40 and 60 v p-p, but must always be inside
this range.

Load: Variable, capacitive and lossy. Xc between infinity and -j300 ohms, in
parallel with resistance infinity to 600 ohms.

Waveform:  Needn't be pure, as long as plenty of fundamental present.

Other: The device will be used repeatedly with different loads. It has to
deliver immediately when connected, without tuning.

The output stage is proving troublesome,  standard mosfet HF configs from
the cookbook & datsheet sites I've checked out generally assume that there's
a reasonably stable load, which is matched to the source using a tuning
network. Audio designs don't seem to like hundreds of KHz. I've tried
several different setups, all suffer either from voltage output variation
that's way outside bounds as the load varies, or outright instability.
Getting awful tired of winding and rewinding toroidal coils.

Does anyone have any suggestions on a simple, robust configuration? TIA

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