Socket details, 815
the Twoer, because after all, it is a limited output unit. Three inches on each side for the grid line is not bad at 144 megacycles, considering the listed 13,3 mmfd input capacity of the 815 and the parallel tuning used.
Grid bias oF minus 15 volts is listed for class AB2, and this just about what we wind up with finally. Putting a very large capacity across the grid resistor takes care of a sufficiently ''stiff" bias for an AM linear, to an astonishing degree. My first contact with this amplifier was in West Bath, Maine, some 135 away Down East, airline miles. He reported, "Modulation very fine. Filling carrier. Even in deep fade to noise could still read you-" 1 was limning about 20 watts out then. After all, this is just a single $1,75 tube driven by an unmodified Two er.
The screen is a nice place for a power control il you would like one. Put a high power pot (five or ten watts rating) in place of the screen resistor indicated. Actually what are we worrying about? At these power levels? Tee Vees run 200 to 300 watts all day long with—in my opinion—morons watching them for the most part, so the 10 to 50 watts input of our little "booster" linear is OK for mv money. After all, it does operate around 60% efficiency on maximum Iwo*er modulation. SSB linears call for a stifF screen supply voltage. Do we need one here? Let us see just how rigid this must be for an AM linear which does not operate in the same fashion as an SSB linear. Checking the screen voltage under modulation with just an 8 mfd to ground ior regulation, we find practically no variation in voltage* it should be remembered that in an AM linear stage with symmetrical modulation voltage, the average plate current remains constant. The proper positioning of the drive and bias is more important here, as we will see, although the whole rig appears remarkably uncritical. In fact, as you listen to it while varying screen voltage and grid bias, there isn't much difference. The rf drive lias perhaps the greatest effect. I finally settled for 20K series resistor in the screen; putting
190 volts on it with 450 volts on the plate and 40K in the grid, which puts some 12 to 15 volts bias on it. Again, remember that information on how to operate a linear for SSB does not apply here. That is, not all of it. In SSB, no rf is applied (if your carrier is really suppressed) til you talk. With an AM linear the carrier is always there.
As shown "flat-out" the unit is some 15 inches long. The plate lines may be run back over the tube if a good shield of copper-clad bakelite is installed over the tube and under the ^bent-back" plate lines (not shown). In this case the length could be kept under 8 inches. If you really want to make it small, this can be done, but the Q, output, and rf filtering effect may drop a little,
A diagram of two-switch send receive operation is shown in a previous section in the 2C39 linear article-
Set t ting down with a steady 20 watts output, driven by the unmodified (as-yet!) Two'er into the little 4 element beam, results were almost identical to operation with the 2C39 linear. "Modulation sounds excellent'; "If 1 had a Two'er I would get right to it"; "Sounds great,"
Personally, I'm getting quite interested in trying this rig out mountain-topping, my favorite recreation,
Perhaps I miss«"] the corrections, or perhaps you missed the errors.
In the article "Let's Keep It Simple" (Jan, '64), the 1 fj0 microhenry choke should connect to the collector rather tlinn the emitter. This was compounded by \ K2AUB's statement that 1Lthe circuit may appear strängt: at first." Hi. A similar circuit appeared in "Hints Sc Kinks" a whQe back. For me, both circuits had the same defect. Tire crystal signal seems to swamp the signal from the antenna, Any surest ions?
In the sweep generator article "A New Broom" hy K6JHJ (May *62)t the oscillator grid leak was omitted. Anything else?
A very practical accessory for the "Broom*' can be found in the Oct, *6I issue of "CQ„" A two tube log-rithmic amplifier and detector circuit can he "lifted" from the "Spectrum Analyzer" by K2BAJ and W2QZJV
Gordon C. LaGrange, W5AKQ
Bill Hoisington K1CLL
We will plunge right into the thick of AM linear theory without delay because I feel that the young starting out amateur (1 was one once, too) lias been had on this question. For the life of me, 1 can't understand why, but maybe you can form your own conclusions, Let^s see.
Most of my amateur life I have shied away from AM 1 in ears because "everybody" said they were ''only 30% efficient/' It wasn't until I really delved into the subject recently and actually built several on 2 meters and on 6, that I began to wonder "what cooks here?"
Starting in naturally with "The Radio Amateurs Handbook," there is about two columns on the linear AM amplifier to be found on pages 296-7, What does it say? "The plate efficiency of the amplifier (linear AM) varies with the instantaneous value of the modulation envelope. The efficiency at the unmodulated carrier level is only of the order of 3035%," "Because of this low efficiency, linear amplifiers have not had much application in amateur transmitters, and only about one-third (of the input power, dc) is converted to useful carrier output."
There are some odd things about the above, ilie part about the 30% is true, but only for an unmodulated carrier, which "Who needs?" The second part is an opinion only. The fact is, a "sin of omission" has apparently been committed, for many long years.
For example, what does Terman say? "The linear amplifier is a form of Class B amplifier, and its most important use is as a power amplifier of modulated waves." Now read the next carefully. "The plate efficiency at full output is usual iy of the order of 50 to 65 percent under practical conditions. The peak power that can be developed by a tube operating as a linear amplifier is approximately the same as the power that can be developed bv the same tube in class C operation." At about this point I began to realize the "why" of some of the on the air reports I had been getting. Any more? Yep! "The linear amplifier is a class C amplifier modified by adjusting to make the output proportional to the exciter voltage. Such amplifiers are used extensively in the amplification of modulated waves since they preserve the modulation without distortion." Well! Also, no wonder! "With the maximum allowable excitation the plate efficiencies of linear amplifiers normally lie between 50 and 65 per cent , , , So that's why I got those reports on the air! Any other "Authorities" on the subject? Yes. The Radio Corporation of America puts out a useful little tome, "Technical Manual TT5." This handbook "will be useful to engineers, . . . , amateurs, and many others technically concerned with transmitting tubes," On page 19 we find . . class B amplifiers are particularly suitable for use as output amplifiers employing "low-level" amplitude modulation/' " , . the maximum efficiency varies from approximately 33% for an unmodulated carrier, to approximately 66 per cent for a fully modulated carrier/'
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