>In a message dated 4/2/98 6:29:36 PM Central Standard Time, measures@vc.net
>writes:
>
><< Zero emission meaning that the filament is not lit? If the filament is
> not lit, the solder on pin 1 or pin 5 may have melted and dripped out of
> the pin. Resoldering the pins using rosin flux usually fixes the
> problem. Oiling the fan helps. Sometimes, one needs to tighten the
> helper-spring tension on contacts 1 and 5 on the sockets.
> >>
>Rich,
>
>Okay, well perhaps not exactly zero. The tube seems to be in excellent
>condition other than the emission is significantly less than the other tube.
>Here are my observations:
>
>1. When transmitting the plate of the "good" tube glows a nice dull red. The
>"bad" tube doesn't light up at all. If I swap tubes, the dull red stays with
>the "good" tube.
>
>2. When I operate the amplifier with only one tube in place (for short
>periods of time) the idling current is much less on the "bad" tube.
What are the numbers?
>Once again, socket position does not matter. I get output from the "bad"
tube, but
>it is much less than the "good" tube.
>
ok
>3. The tubes look identical from a physical examination. I can see no
>evidence of internal damage. With power applied, the filaments look
>identical. (Try to do that with your 8877 :-)
>
>From this, I have concluded that for some reason, the thorium in the filament
>of the bad tube has been depleted.
The thorium content memains c. 1.5% during the emissive life of the
cathode. The thing that depletes is the tungsten-carbide layer.
However, this depletation takes place slowly, not suddenly.
>Perhaps there may be other explanations?
>
My guesses are that either the filament helices are bent in one of the
3-500Zs, or that the tubes just happen to have very different
characteristics. . One test for a bent filament can be done with a
C-meter, by measuring the grid/fil. C using very short test leads.
Normal C is roughly 8.5pF. If the C measures much more, the filament
could be bent. . However, the best test is with a high-pot. Normal
grid/fil. withstanding ability is 7 to 9kV - provided that the tube has
a good vacuum.
cheers
Rich...
R. L. Measures, 805-386-3734, AG6K
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