[R-390] Re: tube shields?

[wli]

Guys:

This has been an interesting thread. We discussed this
before in some detail. One nice article was published
in ER vol 66 page 10 (1994) by Bill Kleonomos entitled
Electron Tube Survival Primer. He referenced a study
by GE Owensboro in the 50's on tube life vs envelope
temperature, and republished the survival curves on
observations made employing 6AQ5W's and 6AK5W's.

Here are some salient points of the Klernomos article
for this group:

1) Number one enemy of tube life is excessive heating
of the internal structures

2) Number two enemy was excessive heater voltages

3) Soda glass used in tubes is 'opaque' to infrared
energy, so that heat is not 'paased thru'

4) Soda glass has poor thermal conductivity leading to
local hot spots

5) Heat transfer from the internal elements to the
envelope glass occurs by conduction via internal
trapped gases and thru the tube pins. 

6) A thermally conducting path from the tube shield to
the chassis is required for cooling

Thanks

W. Li
Mercer Island, WA


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