John Beckman - W4BTX HAM RADIO "A PAST TIME FOR A LIFETIME"
I thought I knew everything about amateur radio. But that's not possible because, like a living thing, it is always changing. I learned something new about tuning linear amplifiers just recently. Linears really came into common use when SSB took over from AM in the late' 50s and early'60s. That is not to say that Amplitude Modulation is dead, there are still nets made up of guys who, thankfully, are maintaining the tradition of AM. But, for the most part, hams now use transceivers followed by a linear amplifier.
My first linear was a homebrew pair of 813s in grounded-grid for one band only, 75 meters. My second linear was a Heathkit SB-220 which I built in the early '70s. I used it on RTTY for 15 or 16 years in the days when we still used those wonderful old teletype machines. With the 220 which used two 3-500 tubes, I just tuned for maximum output and that was that. I still think that is the correct way to tune "glass tube" amps. But with the modern ceramic tubes it is a different story. For instance, I have an Alpha 86 which uses a pair of 3CX-800A7 tubes. The 86 has a "tuning bar-led." I always wondered why the led showed correct tuning which was NOT the same point if I tuned it for Maximum Output. I didn't understand it, but I stayed with the led bar and trusted that the amp was correctly tuned. BTW, I still have that Alpha and it still works like a champ.
Later I added an Alpha 91 to the shack. That amp uses the "Russian Tank Tubes" and it was hard to hold it back About 20 watts of drive would send it over the legal limit. It had the same tuning type led bar and I used that as guidance. I didn't dare try to tune it for maximum output, instead of using the led for guidance. If I had done that I would probably have drowned out the 50kw broadcast station about 4 miles from where I live.
Moving right along I sold the 91 and decided that I wanted to try an amp with the famed 8877 tube. So I bought an Ameritron AL-1500. The small "manual" doesn't really give much guidance on tuning the 1500 accurately and what it gives is not the best advice. I tried tuning it for maximum output and it would peak at about a KW output with 30 watts of drive. I knew the 8877 tube was better than that so I dug into some research on linears. Just what is the proper way to tune one? I found out two important parameters. First, the grid current is the most important meter to monitor proper tuning. Second, the 8877 wants to be hard driven. So my 30 watts of drive and tuning for max output was ALL WRONG. So here is how I tune the AL-1500.
I turn the loading capacitor for maximum capacitance. Then I tune the transceiver/driver to deliver about 40 watts of drive. I watch the plate meter and use the tuning capacitor to dip the plate current to a minimum reading.. Then I rotate the loading cap to drop the grid current to 60 ma while observing an output meter. 3600 volts on the 8877 with 60 ma of grid current gives me well over 1500 watts ouput. I could keep reducing the load and the output would keep rising, but why push it any further? And, you know what? The ramp runs cooler, the tube is happy, the guy on the other end of the contact is happy and I am happy. And, I have learned something new about Amateur radio. I call that a win-win-win situations, don't you?
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