I'm with Rich, if I have a material with high dissipation factor on the
charts, and one with low factor, and I expect a some kV of RF in there, I
would select based on the numbers (and cost or availablity, of course). My
experience has shown that a plastic with higher loss at even 1 MHz or 1
KHz, will also be higher loss at 30 MHz, and it will be a matter of design
whether it gets hot in use or not. At VHF and up, this is a rule. Otherwise
you end up with the 'wandering tune' amplifier, as it heats up.
I forget if I ever posted this story of the wandering tune / 4CX1500A I
tested. After a few minutes at 1 KW at 100 Mhz, I noticed that I had to
keep tweaking on the plate capacitor, the tune was moving off frequency.
Since I had a PLL as an exciter, I could not just track the PA tune with my
frequency - not to mention this was a commercial TX and there are FCC regs
about this..... So I kept turning the crank to keep the output peaked.
After a short while, the cloud of gray smoke came from the cavity vents.
Upon inspection, the tube was very tarnished (blue looking oxide on the
cooler structure) and toasty. We had neglected to remove duct tape from the
air cooling inlet under the socket. It was there to prevent me from
dropping screws into the blower while working. Begin a prototype, there was
no air switch to protect the thing from this sort of stupid mistake.
Interestingly, the tube was still functional, but was a bit out of spec as
far as the output capacitance goes. I suspect that it was throughly warped
inside (screen grid probably) too. I think it was my first trophy. I hate
to even tell about my last trophy in 1993 due to failure while testing.
73
John
K5PRO
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