At 03:41 AM 10/26/2004 -0700, R.Measures wrote:
Thus, when a higher-mileage grid becomes warm from the flow of normal grid
current, the barium coating on it begins emitting electrons in the Wrong
stinkin' direction - and the anode-current starts decreasing. The barium
that sticks to a gold-plated grid of an 8877 can apparently be partly
dislodged by vertically and firmly tapping the top of the tube with a 4oz
hammer -- which is the same procedure that is used to dislodge gold
meltballs from the cathode and from the anode insulator.
I am confused, how does the barium emit current in the wrong direction? The
direction of electron flow is determined by the electric field direction.
And secondly why does it emit electrons at all? If gold poisons the cathode
and makes its emission drop why does the barium on the gold emit
electrons? 1. does not gold poison electron emitters? 2. The temperature of
the gold grid should be much less than the cathode. Also, the amount of
surface area on the grid that has been covered by the cathode material is
very, very, mall compared to that of the cathode.
I have to admit I have not used 8877s and prefer big glass tubes. I love to
see the anodes glow. But I have had lots of 4-125, 4-250, 4-400 and 4-1000
tubes and found few that were actually gassy. The only exception has been
some new Triton tubes and Penta Lab tubes. Even the 1960s GE, RCA and
EIMAC 4-1000 tubes that I have had have not been gassy. And the place to
get 4-1000 and 3-1000 tubes is from France now. The AMPEREX (Covimag) ones
are great. But you may want to allow longer blower cool down time due to
the fact that the massive graphite anode takes a while to cool down.
73
Bill wa4lav
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