Modern is

the VFO7

Howard S. Pyle "YB"W70E 3-134 74th Avenue, S.E. Mercer Island, Washington

Tuned-plate, tuned-grid transmitter of the 30s+ Grid coil on left, plate coil on right.

Nkwcomeirs to the ham ranks take the "variable frequency oscillator" or "VFO" as it is popularly termed, for granted. If they have entered the field via the Novice route, a VFO is of course, 'forbidden fruit' until such time as they qualify for a higher class of license. Surprisingly enough, a large number of relatively uo\d timers" among the General class licensees accept the VFO as a development of fairly recent years, Let's see how recent!

Right after World War I, when the ban was lifted from amateur operation (October 1919), great interest was exhibited in the vacuum tube as a generator of radio frequency oscillations not only by hams but by commercial companies and the military services as well. Experimental work in various laboratories during the period of hostilities had rather conclusively demonstrated that great possibilities were evident here. While vacuum tubes had previously been used to a limited extent, chiefly by Dr. Lee deForest, their inventor, as radio frequency generators, they were not popularly accepted either in amateur circles or by commercial operating interests; "spark transmitters predominated in the wireless/radio communication field, How could you possibly work any DX with 5 or 10 watts of power from a little lamp when you had trouble covering three to five hundred miles with a quarter or half kilowatt "rock-crusher"?

A tunable oscillator of the (ate

A tunable oscillator of the (ate

Probably the greatest single incentive which brought the turn to vacuum tubes for transmission was the development and production for the U. S- Navy by the Western Electric Company working in conjunction with the Bell Telephone Laboratories, of the CW-936 RADIO TELEGRAPH AND TELEPHONE TRANSMITTER / RECEIVER, Nominally rated at ten watts, it employed a VT-2 oxide-coated filament type of vacuum tube as the radio frequency oscillator. A simitar tube acted as a power amplifier. Oxide filament tubes of smaller physical size and rating designated as the VT-1, were employed in the receiver portion of this compact unit- rhe CW-936, initially designed for the small WWJ subchasers, performed so surprisingly well that they soon found their way aboard destroyers and not long thereafter were included in the radio equipment complement of more major war ves-

sels. Hundreds of hams among the thousands who had enlisted in naval service, found opportunity to use and evaluate these little rigs. Much thoughtful pondering resulted: "H-m-m; ten watts? Only 35U volts of direct current at a few milliamperes and yet we work hundreds, even thousands of miles? Maybe it's because we are at sea on an unbroken expanse

Side view of the 200 meter transmitter. The grid coil is mounted Inside the edge-wound plate inductance.

of salt water but too, maybe they've got something there! If we ever get out of this navy and back on the air, Jet's try it!"

And they did! The transition period was relatively slow; many "die-hards" who had only heard of the marvelous performance of the tiny vacuum tube , . . they had not been assigned where they could actually observe this - . * still swore by 'spark7 and stuck to their guns, Just the same, more and more of the peculiar, high-pitched whistles began to appear in the regenerative receivers of that day. The remarkable thing was that such stations, many a thousand or more miles distant, were consistently heard stating that they were using only 5, 73/£, 10 . . . only a very fewT were reporting as much as fifty watts of power! Remember too, that, the frequency or 1 wavelengths' as it was then referred lof was in the neighborhood of 200 meters or 1500 kilocycles, admittedly far less effective than the much higher frequencies which we use toil ay. And, the use of vacuum tubes seemed to offer a solu tion to a whale of a lot of problems which had been plaguing the development of the radio telephone 1 It began to look like it wouldn't be too long before the hams could converse by means of the human voice, as well as the more conventional keyed characters of the radio telegraph code!

That did it! Somewhat of a landslide started, Hams by the score were investing in transmitting vacuum tubes and appropriate power supply components, both of which were beginning to appear on the open market. Tubes were available in several wattage ratings; most popular at the outset was the UV-202 which was, if my memory isn't faulty, rated at 1% watts. In most cases, the ham woefully overloaded them and they ran closer to 15 or 20 watts in many rigs even if their metal plates did turn a cherry red or better! But they worked! 't was not long before the UV-203 tube, with a rating of fifty watts appeared.

100 watt 200 meter transmitter circa 1924, The wavelength was adjusted by the two variable condensers.

100 watt 200 meter transmitter circa 1924, The wavelength was adjusted by the two variable condensers.

;1hey were costly , . . initially $30.00 each and some ambitious rig builders required two; they were just as subject to burn-out and breakage as the relatively less expensive smaller tubes, but a number of hams gobbled them up, perhaps paralleled two of them as a iiigh power oscillator' or used one as an os-

collator, the other as an 'amplifier' with higher voltage and current input*

And what did the ham use for an oscillator circuit? Current magazines were literally saturated with construction articles and schematics covering vacuum tube transmitters* Circuits were devised, altered and 'improved' practically overnight. It made your head swim; what to use? Invariably an 'oscillator' tube was employed; it was the actual radio frequency generator and the real heart of the rig. Hartley and Colpitis oscillator circuits were the most popular among the early experimenters. Basic equipment was the tube, an inductance coil, variable condenser and a small handful of fixed condensers and resistors; maybe an rf choke or two. Often the inductance for even a five watt rig was wound of Vi" copper tubing or even larger I It just didn't seem reasonable that a transmitter would work with smaller diameter conducting material when we remembered that our spark rigs invariably called for such copper "pipe" or heavy copper ribbon for the "oscillation transformer"! Nook at the oscillator coil in your rig today; probably wound with #20 or #22 wire; the final amplifier coil in even a half kilowatt modern rig, seldom uses anything larger than #10 copper wire!

Many of these initial tube transmitters consisted merely of an oscillator feeding directly into an antenna; no buffer, no intermediate or final amplifiers; strictly a one tube job (ignoring the rectifier lube or the 'slop-jar' rectifier, of course!). And, by simply moving clips on the oscillator coil and adjusting the variable condenser, any frequency within the L/C limits of the circuit components, could be rapidly tuned! What was that other than a 'variable frequency oscillator1 . . . a VFO?

Many, many more circuits were tried, accepted or discarded. For a long time, what was known as the "tuned-plate tuned-grid" circuit was highly in favor. In effect, it was merely the conventional oscillator circuit of the time with a tuned plate circuit, to which

Improved Mounting Feet

Although commercial rubber "bumpers'* or protective mounting feet are manufactured for installation on electronic equipment, they are often difficult to locate and the proper size may not be immediately available.

Automotive supply stores stock replacement rubber hydraulic brake cups in a variety of sizes and prices on these items are reasonable, ('he photograph shows the original unit and its installation in a metal plate. The groove at the top securely holds the foot in the mounting hole. The rubber tip which projects into the cup is cut off with a pair of diagonal pliers.

These mounting feet are a distinct improvement over those commercially available. The suction cup action will hold the equipment had been added additional L/C components in the grid circuit, making that also tuneable. Whether this was any gain over the simpler oscillator circuit alone or not, is somewhat of a moot question today; at any rate they worked good ... I used one for a couple of years.

The commercial companies as well as the military communication branches were not asleep either. The amateur was pointing the way with his impressive increase in signal exchange distance and the exceptional sharpness of the emission permitting dozens more stations to operate within the frequency sector formerly dominated by one spark signal! As with the amateur, newly designed commercial equipment incorporated a "tuneable oscillator" to which the power amplifier and the antenna could be resonated throughout the frequency range of the equipment. What were these other than 'VFOV basically?

Amateur as well as commercial and military equipment has been through a program of continued progress and improvement ever since Marconi startled the world with his reception of the letter "S" across the Atlantic Ocean in late 1901. Prior to World War 1, development of 'spark* equipment from the simple spark coil of Marconi, to 500 cycle, quenched spark gap equipment, took place. Subsequent to WW-I, with the impetus which it gave to vacuum tube transmission, developments have continued at even greater , . . almost fantastic . . . speed. The use of vacuum tubes are even now threatened to a considerable extent by a relatively newcomer . . . the transistor. Remember though, that just like the progress of spark transmission which, right up to the last, required high voltage, large components and a spark discharge gap to create rf oscillations, the vacuum tube first required an oscillator to create rf generation, just as it does today. The earliest vacuum tube oscillators of record were capable of frequency variation . . . the forerunner of our present VFO! Do you still think a VFO is something "new"? 73

securely in place on any reasonably smooth surface*


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