Agree with everything Geoff posted but the 3-500s should show color to maintain vacuum. 73 Rob K5UJ On Fri, Aug 24, 2018 at 7:25 PM ARS W5OMR <ars.w5omr at gmail.com> wrote: > > I have run as much as 200w > out but the tubes start showing color. The dissipation level is high on > those tubes in linear AM mode. >