> Do external anode tubes have a similar problem of element outgas
> during lengthy storage? Do they have a getter on the inner surface of the
> anode which can be activated by arranging to raise the anode temperature?
> I have several 8877 tubes which have been stored for several years and
> would like to put them to use.
The gettering in the 8877 is primarily on the heat dam of the
heater/cathode structure.
It reaches temperature from the normal heater voltage on the tube,
so a long period of filament activation will getter the tubes. The
normal problem with 8877's isn't gassing though, it is misalignment
in the grid causing a short or causing the grid to "hot spot" and
gold to be boiled off. Grid wires are located between bands of
cathode material, so the electrons miss the grid. The electric field
between grid wires controls anode current, so the grid normally
doesn't get heavily bombarded with electrons.
If the grid is misaligned, areas of the grid will be struck by
electrons and the kinetic energy will gradually cause the gold to
migrate around inside the tube.
In severe cases, the grid may touch the cathode.
8877's had periodic problems with grid misalignment and heat dam
problems over the years, but the tubes from San Carlos have been
excellent. No problems at all.
I think it was around the later mid-90's when production moved, I
can't recall the exact date.
In the late 80's, for a period of time, it was almost impossible to get
a good 8877. About 90% or more failed a cold test where only the
heater was turned on and off for a few days. The grids would move
over and touch the cathodes! Avoid 85XX to 89XX code dates!
Some factory rejected tubes made it into the market over a period
of time. A fellow in California was obtaining rejected tubes and
replating and relabeling them, and selling them as new tubes under
various brand names...including Eimac. (It wasn't Rich Measures
doing this, it was a person associated with Penta Labs)
Eimac started embossing their logo in the top of the anode cap to
reduce the ease of relabeling old tubes.
If you have the tubes, no sweat..try em. If you buy them, beware
old code dates.
73, Tom W8JI
w8ji@contesting.com
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