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Harris Visit

I had the pleasure of a visit from Mr. Geoffery Mendenhall of Harris Corp. the other day. We had a nice lunch and chat. We even visited the transmitter site to discuss the tube and life of these beasts in the HD world.

I have submitted the tube to Econco for an evaluation on how this tube fared. I will keep you posted on this. Our conversation focused on the fact that the failing tube caused IPA overloads. As the filament carbonizes we see a higher resistance and need more power to maintain emissions. Also we are thinking there is some increase in reactance withing the grid circuit. Somewhere a point is attained where the IPA cannot provide the necessary drive and overloads.

Once we have some evaluation on the state of the tube after removal we will not really know how far along or “burned out” the thing is. As the digital carriers require higher peak current, this tube in an analog transmitter would probably last a bit longer. All very interesting things.

Stay tuned for any other information that I may come across.

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  1. Bill Eisenhamer
    June 15, 2009 at 14:25

    Well, I’ve been ratted out. Actually, no, I was brain dead on a little description. Geoff caught it and I am now posting his description of the filament aging process. Thanks Geoff for catching my neuron misfire!

    “Just a clarification on the filament depletion process called “de-carburization” where the carbon is slowly cooked out of the thoriated tungsten filament.
    As the de-carburization process continues, the hot resistance of the filament goes down causing the filament current to increase for a given filament voltage. For example, a new 4CX20,000 filament would draw about 140A @10VAC while the same filament nearing end of life would perhaps draw as much as 160A @10VAC. This is why the filament voltage had to be lowered when the new tube was installed, to compensate for dynamic impedance of the filament transformer designed in to limit the cold start in-rush current.”

    And there you have it!


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