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Author Topic: Yellow powder on the heat shield of a 3-500Z  (Read 1822 times)
N2AXZ
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Posts: 90




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« on: March 31, 2006, 08:06:40 PM »

I am hoping that someone can tell me what has happened here ---- hopefully it is just a defective tube.  Last week I purchased a matched pair of 3-500Z tubes from RF Parts, and installed them in my homebrew amp. I am using the standard Eimac SK410 sockets, and glass Eimac chimneys.  The tubes are cooled by forced air.  After installing the tubes I applied filament power (4.9 volts, total current draw about 30 amps), and both filaments lit  up nicely.  Prior to applying any plate voltage to the tubes I intended to let the filaments run for about 10 hours total (I have read that it is a good idea to do this with these tubes to allow the getter material to scavenge up any residual gas in the tube).  I put about 2 hours on the filaments last night before powering down.  This evening I powered the filaments back up and noticed that one tube considerably dimmer than the other.  To determine whether it was the tubes or the wiring to the sockets that was bad, I reversed the tubes, and discovered that the same tube was still dim ---- revealing that something was wrong with the tube.  I pulled the tube and noticed that there were numerous particles of fine yellow material scattered over the heatshield of the tube, and all over the bottom of the interior glass.

What is this material?  I presume it is used as a coating on the filaments.  I also presume that this tube is defective, and I plan to call RF Parts on Monday to report the problem and get an RMA from them so the tubes can be replaced. Can anyone here explain what happened, and did I do anything wrong by leaving the tubes running as I did?

Thanks for any help that you can provide!

73,

David
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K0BG
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Posts: 10248


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« Reply #1 on: April 01, 2006, 08:33:48 AM »

I've seen this before. Occasionally, one of the pin seals leaks, lets in a little oxygen, and presto. I'd call RF Parts, or better yet, send an e-mail and then call them.

Alan, KØBG
www.k0bg.com
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