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From: rjr@mgweed.UUCP (Bob Roehrig)
Newsgroups: net.ham-radio
Subject: TUBES and broadcast equipment comments
Message-ID: <22395@mgweed.UUCP>
Date: Tue, 17-Dec-85 12:47:57 EST
Article-I.D.: mgweed.22395
Posted: Tue Dec 17 12:47:57 1985
Date-Received: Tue, 31-Dec-85 03:45:22 EST
Organization: AT&T Information Systems - Montgomery Illinois
Lines: 40


I also was in broadcasting and most of my experience with tubes involved
Eimac 4-400 types. In 5 different transmitters, I naver had to replace
one due to failure of any kind. One time, within a 2 year period, I did
swap some around to better balance the currents in the plate modulator
pair. At one FM station I was at, The idiot owner had shorted out the
air sensor switch and had installed toggle switches to turn on the 
blower and exhaust fan. One morning (after a very late night out) I
forgot to turn on the air switches when I fired up the transmitter
and it ran for 6 hours without air coolong at all. These 4-400A's were
operating at 3825 volts at 325ma and the plate color was close to white,
but even after the no-air episode, those tubes were still functioning
properly 18 months later when I quit the job. On the other hand, the
driver stage consisted of a push-pull pair of 6146B's and even though
they were loafing along, I had to replace them every 3 weeks! 
(The B's lasted a week longer than the A's).

With the good experience I had over the years with Eimac tubes, I would
not use anything else in my higher power ham gear. They should last a
lifetime in amateur service on HF.

The only "bad" experience I had with tubes in broadcast equipment was
with the 5879 pentodes used in the preamps in a Gates "Dualux" console.
These tubes were so noisy that I had to hand-pick them to get the board
to pass the proof-of-performance test.  RCA boards I worked with used
12AY7 dual triodes and always were quiet and reliable.

(An interesting thing happened when doing a proof test on the previously
mentioned FM station when the hum level was too high. We had to end up
cutting the lacing cord on a wiring harness in the Gates "Level Devil".
This unit is a compressor/expander. The idiot designers had put the 
115 volt AC wiring in the same harness as the unbalanced input level
control wires behind the front panel!)

I always advise ham friends that replace tubes in their equipment to
test all tubes in a good (Hickock) transconductance tester first.
One time when I bought a pair of RCA 6146's and a 12BY7 for my Kenwood
820, One 6146 was shorted and the 12BY7 was gassy. Never trust a new
tube these days (the big transmitting ones are an exception).