[AMPS] PA loading and tube life

[Graham Jones]

	Thanks to Carl, Rich, Dan and others for replying so promptly to my
enquiry. 

	From your combined inputs I think I get the picture. I gather that
recommendations for "heavy plate loading" in the HF region are mainly for
reduced IMD at the cost of a small sacrifice in efficiency, provided grid
dissipation is not exceeded.   Obviously the term "heavy loading" does not
mean overloading to the point where efficiency is poor or where grid
current, anode dissipation or maximum cathode current ratings are exceeded,
otherwise tube life will be shortened rather than increased. At VHF and UHF
there may be other significant issues relating to secondary screen emission
etc.

	In the case of the amp I use, the problem of excessive grid dissipation is
a moot point. The 4x 4CX250Bs are grid driven by a low power pentode in
class A, incapable of driving the PA grids very positive. I think trying to
over-drive it would produce only bad IMD (or dirty rotten splatter) rather
than excessive grid dissipation.  

Thanks again :-)

73 Graham VK3BKG

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