Pi Network Coils

Now—Pi-Network inductors specially tailored for your needs. Here are highly-efficient, super compact tank coils incorporating the unique feature of integral band switching.

Model 850A and Model 852, now complement the famous B&W Model 851. All are designed for single or parallel tube operation on 80, 40, 20, 15, 11 or 10 meters, with top efficiency in Class "C" or linear operation- Windings give ample current carrying capacity with optimum "Q*' over the entire operating range.

See these superior B&W inductors at your dealers nowt or write B&W direct for detailed information.


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MODEL 650A $35. OO

problems for a few minutes and then the XYLs broke it up by suggesting we go for a drive around Rimini,

We wound up in San Marino, which is a tiny republic surrounded by Italy on a mountain top. Its one souvenir shop after another, and while there I met MID, the only ham in San Marino and Galliano's first contact as a ham. He is no longer active or I would have made a schedule then and there as I could use another country toward that DXCC.

Back to Rimini for the "stuffing" which coi> sisted of the Italian equivalent of a New England Shore Dinner. Rimini is on the Adriatic Sea, and the fish is excellent, especially when eaten in a romantic restaurant where the ceiling is a grape arbor and where you wash down each course with the juice of the grape.

Finally it was time to part, and we took our presents and said goodby as they dropped us off at the hotel Of course we gave them presents too; some nylon things lor his XYL, a doll and dresses for Monica, his daughter, and some radio parts for Giuliano. As for our gifts, we now have a beautiful miniature marble statue of Paui^ne Bonaparte (Napoleon's sister) on our mantle, a pair of beautiful gold frame 17111 century style pictures on the wall, and one of the best bottles of after-dinner cordial I ever tasted safely stowed down the hatch. Plus, of course, the best gift of all—the fond memories of an evening spent in cementing international relations that neither the XYL nor I will ever forget.

The XYL, normally no more enthusiastic about ham doings than any average XYL, remarked afterward that she wished we had other spots on our tour where we could visit hams I'd contacted. Hmm—maybe I should have written that French ham I sent a copy of "My Old Kentucky Home" to on request,

DX contacts are brief and often difficult, but find the common ground if you can; whether it be your gear, families, work, hobbies, or what have you. Find it—and you find a friend, not just a contact. . . . K4ZQR

Controlled Carrier Screen Modulation

Terrence Banks K3LNZ 426 Oronge Sr., S.E. Washington 20, D. C.

( *ne of the problems confronting either the home constructor or the kit or commercial equipment buyer is the recent popularity of the controlled-carrier method of screen-grid modulation. Judging from the on-the-air discussions I have heard about this system, some of the basic facts are being ignored by those interested, either pro or con, and the advertising claims of several manufacturers serve only to add more confusion to the issue. It is the intention of this article to present the basic facts in such a way that they can be considered minus the prejudices and misconceptions usually brought up.

First, let's set our ground rules so we will know what wre are talking about and, equally important, what we are not talking about. By screen modulation we mean only the controlled carrier variety. Without this feature, screen modulation requires much larger tubes than would normally be used for the same power level, and is best left for those with large tubes and small power supplies. By plate modulation we mean the standard transformer-coupled modulation of the plate (plus screen grid in tetrodes and pentodes) of a Class C final amplifier. Linear amplifiers, sideband, etc,, are not under discussion. Neither is audio quality which Heath and several other companies have proved can he excellent with screen modulation .

Let us use the popular type 6146 tube as a guinea pig. We will supply adequate grid drive and run 600 volts on the plate, First we will take a typical set of screen-modulated conditions, and then will compare the corresponding maximum ratings with plate modulation, We will then discuss the relative output obtainable, the ease and economy of applying the two methods, and finally some operating considerations.

Power-wise, most manufacturers will rate their 6146 transmitters as "90 watts input on CW and 90 watts peak input on phone/" Trans-

Iated into English, this means that the maximum plate current readings obtained on voice peaks are equal (150 ma) to the key-down CW conditions. However, no meter yet produced will keep up with the variations in the human voice, so meter readings of 150 ma actually mean that the peaks are running in the neighborhood of 250 ma, or about 70% higher, This is about the highest they can run without the opposite peaks driving the screen so negative that cut-off of the tube (and therefore splatter) occurs. Presuming peaks of 250 ma and the opposite peaks of slightly above zero, we have an average input to the final of about 125 ma, which at 600 volts will give us 75 watts.

In a plate modulated final, the meter wilt read a steady 112 ma for the maximum rated phone input of 67)i watts, whether modulated or not. However, the secondary of the modulation transformer will have an audio (ae) voltage across it, and therefore in series with the dc plate supply voltage, exactly equal to our 600 volt plate voltage. This gives us a voltage varying from zero to 1200 volts. We assume the modulating impedance, which is the combined resistance of the final tube, tank coil and other circuit components, to remain the same (about 5500 ohms) so our plate current actually varies from zero to double the average meter reading. Ins gives us peaks of four times the unmodulated input, as the voltage and current both double at the same time. 67K watts times 4 is 270 watts on peaks, which would make us think the average input is one-half of that, or 135 watts. Due to factors we won't discuss here3 this is not the case and the average power input under these conditions is about 37% of the peaks rather than 50%. We could prove this mathematically, or by instruments, but it's simpler to add our dc input to the final, 675i watts, and the audio power supplied by the modulator, or about 33 watts, which gives us an average input under modulation of just over 100 watts, Thus we see that our input, and therefore output, is about 33^ higher (100 versus 75) when using plate modulation instead of screen modulation with the same tube.

Turning next to ease and economy of construction, we find that a 6SL7 speech amplifier and 6SN7 modulator will handle our 6146 quite easily. Other components required are small and few, so on a 2 x 4 minibox we can build, or purchase, a suitable screen modulator for about $12+00* If our transmitter cost us $50.00, we pay 82c per watt of input,

A plate modulator for the same rig will use 2 6L6 or larger tubes, plus speech amplifiers.

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