A Somewhat Unusual Failure

[K2PG]

While trying to put my Collins 20V-2 on the air last night, the high voltage immediately tripped off. There was a modulator overload. After resetting the overload relay, I noticed that the plate of one of the modulator tubes was glowing abnormally bright and the static plate current of both tubes (combined) was 400 mA! All of the biasing components checked out. I then discovered that the 4-400 had developed a dead grid-to-filament short. This was a Penta Labs tube, made in the Czech Republic, that I bought five years ago.

Have any of you had such a failure with a tube that was always used in audio service? I have heard of these shorts developing in RF power amplifiers that developed VHF or UHF parasitics, as the parasitic oscillations cause the tube to draw an enormous slug of grid current, causing the grid wires to heat and warp.

I replaced both modulator tubes with a pair of NOS Eimac tubes. The transmitter now works normally. If only someone would resume making glass power tubes in this country, as the foreign ones are generally of poor quality.

[k4kyv]

The same thing happened to one of my 8005 tubes that I use in the  final of the smallest of my homebrew rigs, presently tuned to 40 m.

The problem in my tube is intermittent.  When the tube functions, it works better that most of my spares, including a couple of NOS ones I acquired.  For that reason, I have been reluctant to toss it out.  If I could find out exactly where the short is occurring, maybe I could tap the tube with a  hard blow at just the proper angle, and eliminate the short once and for all without destroying the tube.

[Vortex Joe - N3IBX]

Phil - I think 4-400C's are now made in China. Most of the "Penta" branded tubes now come from that country. I thought the Eastern Urine-Peein jobs to be of better quality. Considering the amount you've used them in the past five years, a crap out or grid to filament short is definitely premature!

Regards,
           Joe N3IBX

PS: Were they the tubes with the pretty "Pagoda" style graphite anodes?

[Tom WA3KLR]

Phil,

Several years ago I found a press release on the web mentioning that Eimac had sold it’s glass envelope tube line to the Electron Technology Division (ETD) of Triton Services, Inc.  If I recall correctly, the release was a couple years old at that point.

ETD was/is? located just on the south side of Route 22, just east of Route 33 in Easton, PA.  They were primarily involved with traveling wave tubes and high-power thyratrons, I believe.  Years before, the side of their building towards Route 22 used to say - ITT Electron something or other, as I recall.

What happened to the tube line, I don't know.  Maybe ETD turned right around and sold it to someone else.  Or maybe this is another case of a business killed by poor American management.  I just don't know.

[John K5PRO]

Triton had difficulties producing the Eimac designs of glass tubes, I was told by someone in the industry. Also, getting glass blanks of the size needed for big bottles isn't as easy as it used to be.

Covimag in France still makes reasonable Amperex 3-500G and 4-400 tubes. They were captive to Richardson Electronics (as sole supplier) but I am not certain where that stands now.