Bill is absolutely right. In addition, keep in mind the cathode current
peaks are about three X the average current. So, if you expect to run 1A plate
current the cathode has to emit about 3A for the plate and another .5A or
so peak grid current.
More would be better to allow for additional aging.
The trick is how one measures peak cathode current. Not everyone has access
to curve tracer. You can do it with seat of the pants methods if you
understand all the principles involved.
73,
Gerald K5GW
In a message dated 1/17/2016 7:27:24 A.M. Central Standard Time,
dezrat@outlook.com writes:
------------ ORIGINAL MESSAGE ------------(may be snipped)
On Tue, 12 Jan 2016 20:33:25 -0500, KA9P wrote:
>
>Can anyone explain what a good/bad emission number is on a used
>transmitting tube like an 8877? Or just point me to an explanation?
REPLY:
"Bad" is mostly in the eye of the beholder. A "good" tube should match
the plate current curves shown in the data sheet. That is, for a given
plate voltage and grid bias, the plate current should be xxx mA as
shown on the graph, especially at the higher end of the graph. A tube
that has low emission will be lower than that, but how much lower can
vary a lot. If it is just one or two percent lower, for all practical
purposes, I would call it a good tube. Below that, the tube is
probably still useful but it depends mostly on how hard you want to
drive it.
In other words, there is no strict definition of a "bad" tube. It all
depends.
73, Bill W6WRT
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