When using 4CX350A tetrodes, be sure to check and maintain the
filament voltage, below the Eimac data sheet value, unless you need
high pulsed current. Reason for this is that there was a mistake in
the original design which caused them to need to run excessive
filament power, which will limit the lifetime. This was found with
certain military amplifiers by a reputable supplier. It has also been
discovered independently(this spring) by Brookhaven National
Laboratory, where crates of these tubes are used in quantities as an
RF preamp. Instead of revising the design and the datasheet, the
manufacturer recommends the cutback. Use prudent voltage management
techniques, such as watching the plate current or output, while
slowly decreasing the filament voltage, until you see the knee in
emission. Then raise it about this by a few %. In SSB emission limit
will probably show up in the peak power. I believe that at least a
half volt reduction is permissable but will depend on individual
tubes. An old tube may not pass the test if it has already run at
datasheet voltage for a period of time.
73
John/K5PRO
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