Everything I've ever read about these old sweep tubers says don't overdrive them. In the original paperwork for the K-500 (and it's sister, the F-500) it specifically states NOT to use any type of driver with these amps.
Yup, that's what it says.
With a 4-watt drive I'm getting a 200 w DK and 300 swing out of mine with tubes testing in the 95% range.
Between the radio and the K-500 I run a swinger 205 set to the 'swing' mode. With the swinger running a pair of 6lq6 tubes that test in the 60% range and a slight detune it provides about a 6 watt DK with a 20-watt swing. Running this into the K-500 gets me about a 220 w DK and close to a 450 watt swing./QUOTE]
Do you think the receiving station can hear the difference between 300 watts and 450 watts?