[Boatanchors] LINE VOLTAGE, FILAMENT VOLTAGE and SANITY

[Gary Peterson]

“It is well know in the broadcast industry that excessive heater voltage 
_greatly __reduces_ tube life.
Ray, W4BYG”

Having worked in the technical end of radio for 50 years, this next month, I can verify the validity of this statement.

Proper filament voltage management can double the useful life span of an expensive tube.  I took over a site that had, traditionally, gotten ~20K hours out of a 4CX20,000D.  This tube has a thoriated tungsten filament (directly heated cathode).  I began getting 40,000 hours out of subsequent replacements (running 25 kW output at 100 MHz).  There was an excellent article, published in the ‘80s, written by an Eimac employee, named Artigo, as I recall.  He outlined the procedure that follows:  

Now, this advice is for new, thoriated tungsten filaments:

Run at data sheet specified filament voltage for the first, approximately, 200 hours.  The RMS filament voltage must be accurately measured at the socket.  This requires the use of an accurate, analog iron-vane type of instrument or a DVM that measures true RMS.  I use a Fluke that measures true RMS by virtue of integrating the AC waveform.

After the first 200 hours, or so, the filament voltage is reduced until the tube no longer operates properly.  In FM service, this is when the output power begins to drop.  In AM service, this is when the audio distortion begins to increase.  In either case, locate the “knee,” where the output drops or distortion increases and operate about 0.2 volts above that point throughout the life of the tube.  A tube with a nominal, data sheet filament voltage of 7.5 volts may run for a couple of years, at full output or low distortion, with 6.7 volts at the socket terminals !  When a replacement PA tube costs around $3K, this is worth doing.  I did this with my 3-500Zs and they are still going strong after 20 years.  And, yes, I am fairly active.

Do not attempt this procedure with indirectly heated cathodes, such as the venerable 4CX250B.

Regards & Happy New Year,

Gary, KzeroCX (because this list will not replicate a slashed zero)