Just an addition to what others have replied:
> The SWRs on my antennas are below 2:1, tube plate current around
> 450-500 mA (which I guess is stretching it a bit), and grid current
> around 200 mA. Output according to a Daiwa meter is between 700 to
> 1000 watts depending on the band.
You don't say how much below 2:1 the SWR is. With a PI output, your SWR
is probably good enough to be no concern at all, BUT you cannot trust
your Daiwa meter in this case!!! The reading of a normal RF power meter
depends critically on a precise load resistance of 50 Ohms. As soon as
the SWR is not precisely 1:1, the accuracy of the power meter suffers.
It might show more or less power than the real one, but in average it
will typically show more power than real.
So, maybe the actual power getting out of the amp is a lot less of the
700 to 1000 Watts you think! And any power that's drawn from the power
supply but not delivered to the antenna, must be dissipated by the poor
tube! That might explain excess heating, if you actually have excessive
heating in your amp.
You should try loading it into a dummy load, and then measuring output
power, plate input power, watch how red it gets, etc. At least with a
dummy load your Daiwa will show something close to the truth.
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