Henry RF AMPLIFIER

3-500G Filament Voltage

 NE7X - 01/22/2012

There has been lots of discussion, emails and chat groups about what the proper filament voltage for the 3-500G tube should be to extend its life. The Eimac data sheet for the 3-500G says filament voltage must be 4.75 to 5.25 volts for maximum tube life. That's 5 volts, +/-5% which is fairly common for thoriated-tungsten transmitting tubes.

 

For maximum 3-500G life, the ideal situation would be for the filament voltage to be exactly 5.0V in operation, which would require a regulated filament supply. The next best, and the more practical condition, is for the filament voltage to stay within the 4.75 to 5.25 volt range under all conditions, standby to full load.

 

One thing you also need to take into consider is the AC line-voltage input. Fluctuation from 210 to 250VAC input will have a direct correlation of what the filament transformer puts out.

 

There was an article published on the internet by IW5EDI stating running the 3-500G at higher filament voltage would reduce the live of the tubes. This article circulated around the internet causing quite a concern.

 

If you are considering lowing the filament voltage for your tubes because you feel or heard it will make the tubes last longer, think again! Here are three things which will happen if the filament voltage is lower then the specification.

1) loss of linearity during peaks, causing splatter

2) Greatly accelerated emission loss from filament poisoning. This is a more rapid failure than excessive voltage can cause

3) Limited power output (premature cathode current saturation) and/or reduced plate efficiency

 

There is an excellent article written by Jim Perry, KJ3P (kj3p@arrl.net) in the January 2012 QST, pages 56 & 57, about 3-500G tube filament voltage. Jim found no clear evidence that running your 3-500G at a slightly higher filament voltage (within reasonable limits) would cause any major shorting in life of the tube. Jim explains in very clear detail how he tested and validated his findings. In the same QST article, Charles Rankin, WA2HMM (crankin@dialup4less.com) responds to KJ3P's comments and validates Jim's findings. This is an outstanding QST technical article and if you have any additional concerns or questions, I recommend you get a copy of this article and read it.

 

The key thing which I found which shortens the live of tubes quicker then anything else is high grid current. Driving the grids above what the tubes are design will shorten the life of the tubes faster then having a half of volt higher on the filament voltage. For a pair of 3-500Z tubes in ground-grid configuration, the Ig should be around 220-240ma.

 

The bottom line, unless you have a shorted filament transformer secondary or extremely high AC line-voltage and the filament voltage is running >6+ volts, DO NOT worry about it. Just run your amplifier at the filament voltage the tubes are currently running and enjoy the amplifier. 99% of the time, the tubes will most likely out last "you" before they will burn out due to high filament voltage.

 

For additional reading about RF vacuum tube filament voltage, read the information Tom W8JI has on his web site, Filament Voltage Life.

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