>Greetings,
>
>I'd like to know if their is a reliable way to test the emission of
>the power tubes we hams generally use. Tubes like CX800s, 8877s etc.
>without having to actually run them at full power in an amp.
>
One needs to operate the tube/valve in some kind of amplifier. An
oscilloscope with a 1000 to one HV probe is quite useful. The signal
frequency can be 60Hz or 50Hz -- which makes things a bit easier. The
goal is to measure peak emission versus filament voltage. If minimal
filament voltage is needed to produce normal peak emission, much emissive
life remains. If maximal filament voltage is needed to produce normal
peak emission, the tube is on it's last legs.
- If the anode supply voltage exceeds 12kV, it's a good idea to get
behind a lead shield whenever anode current is flowing.
Before placing any transmitting tube into a test setup, one should
probably high-pot. the tube at double the anode v rating. A tube that
contains a short, or a tube that has gas due to a bad seal can damage the
tube tester.
An inexpensive source of tube testing is commercial broadcast tube
rebuilders. However, a second opinion is not a bad idea. . I know of a
number of instances where customers were told that a tube needed to be
rebuilt - but a second test (from a different source) indicated that
emission was normal.
- Here's how the scam works: The customer is told something like
this: "Your tube is a dud, sir, however we can recarburize the
filament for you and restore full emission." If the customer declines to
have the filament recarburized, he is told: "We can either return the
dud to you, or purchase the core, and send you a cheque." . . Don't
fall for it. A core is worth much more to someone who needs a core --
and the second test may indicate good news.
- If you need a padded factory-style shipping container, they are
available at reasonable cost from tube rebuilders.
- To avoid mixups, record the serial number of tubes that you send to a
rebuilder for testing.
Rich...
R. L. Measures, 805-386-3734, AG6K
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