Had a look at the old quad GG 807 type linear.
COR! that's a real old bit of ham radio.
My own experience with 1625/807's is that with more than 1KV on the
plate they didn't survive long. Any mismatch or offtuning and flashover
happened pretty fast. Back in the 1960's I had quite a bin full of duds
that went just that way.
Recently I had a fault in my modified kenwood 820s which destroyed 2x
6146B's in a similar way. That's with only 850v plate voltage too.
However it is true that linear amps tend to work better in a high
voltage/low current mode, provided the finals can take it.
My old home brew 100watt linear used a TT21 (KT88 with a top cap plate)
and that worked fine with 1050 volts. Nice and linear and 66% effecient
on 14 Mhz (measured).
I once built an 811A GG linear but never needed the extra power so it
nevergot on the air.
Regards Cliff Wright ZL1BDA
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Re the anode voltage used for PA valves ; the older (1968-1973
generation) Yaesu transceivers (FT 40x & 50x)using TV Line Sweep valves
(tubes) had 2 HV voltage transformer taps resulting in either 650 or
850V-DC .
I always used 650V since 850V caused the valves to be too hot ,due to
the limited (1 small fan ) cooling .
Although I have seen designs including 4 line sweep valves in
parallel,running on 1000 or even 1250 V ,but only with enhanced cooling.

When licenced in 1985 my first transceiver was a FT400 , an excellent
learning piece of kit,involving some 'fried' PA valves.

Frank GM0CSZ / KN6WH