rjr@mgweed.UUCP (Bob Roehrig) (12/17/85)
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).