[Amps] using 4CX350A

[John T. M. Lyles]

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