[AMPS] 8877 Rejuvination

[Tom Rauch]

>   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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