Update: Thermal Grease for Vacuum Tubes/BeO Thermal Blocks

[Steve Harrison]

Howdy all,

My query last night about the proper grease to use on conduction-cooled
tubes and thermal links, such as in the Heath SB230 and other commercial
amplifiers using the Eimac 8560A tube, yielded a number of responses, for
which I thank you all.

To summarize: in many cases, ordinary heatsink grease has been used by most
of you with no problems. However, several respondents, one of whom had
experience with radios used in high-ambient-temperature applications such
as the deserts of the Middle East, replied that a common failure mode was
that the ordinary grease would "drip" out at the very high temperatures at
which power tubes operate.

Remember, for example, that silicon transistor junctions are typically
rated to operate at 150 degrees Centigrade for an expected lifetime of
dozens of years. A mere 25 degree rise in junction temperature can cut that
down by several times! So, the hottest temperature at which you might
expect to operate a power transistor flange is around 125 to 150 degrees
Centigrade. And even then, few of us will feel at all comfortable with that
"HOT" transistor.

In contrast, RF power vacuum tubes, such as the 8873 in the SB230, and the
8560A in commercial V/UHF FM equipment, may operate at up to 250 degrees on
the envelope, nearly twice as hot!

The real answer was found when my buddy happened to "read the instructions"
packed with one of his new 8560 tubes. The specification sheet said that
Wakefield type 120 thermal grease was recommended. As it happened, I had
bought a 4 or 5 ounce tube of that stuff some years ago. I'd always thought
it was a little "funny" because it seemed to be much thicker than the usual
stuff. That is the case: Wakefield 120 is REALLY thick stuff: SO thick that
it took me 15 minutes to squeeze out a big dab into a plastic envelope to
mail to my buddy without breaking the thin aluminum tube! (I don't wanta do
that again very soon!)

So, folks: there really IS a special grease for those conduction-cooled
tubes, and you really SHOULD be using it instead of the ordinary stuff we
all use for transistors. If you MUST use the ordinary stuff, you should
examine its condition periodically (every couple of months, I'd say) and
relube when it appears to have run down the thermal block, or has dried up.

73, Steve K0XP

P.S. Where to get Wakefield 120?? I no longer know  since I got mine years
ago! Probably, however, Newark or Allied have it. Look under
"Manufacturers" in the indexes for "Wakefield". Wakefield is still alive
and well, by the way.

Sponsored by the City of Tempe 

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