| A 1930's Transmitter | |||||||||||||||
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In 1930, the station transmitter was most likely built not bought! Amazon and Credit Cards were not invented as yet and as the US enetered the Golden Age of Radio Broadcasting there was a large volume of receiver manufacturing taking place. But aside from the US Government or Commercial Communications Companies, transmitters for ham use were for the most part homebrewed! We must not forget that these were the Depression years in the USA and therefore not a lot of extra pocket change was available --so simple homebrew rigs were very much the way to get on the air! Often the transmitters were single tube rigs often built on a wooden board. It was true "al fresco" -- also a shock hazard because high voltage was in the open with very little protection for the operator. There were many popular tubes used in these transmitters including the 10, 27, and noteworthy the 45 was very popular as well as the 2A3. I once built a 45 self excited Hartley Oscillator from an article in Ham Radio Magazine authored none other than by Bill Orr, W6SAI (SK). Bill worked for Eimac and was well published in QST, CQ and Ham Radio. The tank network on this rig was made out of copper tubing and the 45 could produce maybe 5 watts -- it also had a 2.5 VAC heater. I seem to recall this was about 1976 when the article was published. A video of a Type 45 Tube transmitter is shown here from N2UOV This video pretty well sums up what I will menton about using such a rig including issues with mechanical vibration, hand capacitance and keeping your hands out of the rig! I did get my 45 transmitter working and my greatest joy was to connect a dummy load to the link output and then taking a #2 wooden pencil that had been sharpened and thence bringing the pointed tip near the tank circuit -- you could see a blue arc about 1 inch long. It also created a lot of RF hash -- but that was the early beginnings of the "Cool Juliano Blue". So what kind of transmitter would you build in 1930 and what kind would you build today? There were a lot of self excited oscillators in 1930; but the frequency moved all over the place from mechanical vibration as you pounded brass or from hand capacitance, then extending that to the antenna moving in the breeze which changed the loading on the final tank. While it does limit you, crystal control today seems like a good choice especially if you could have 5 or 6 frequencies. Recently many repackaged 40 Meter transmitting crystals have been showing up on eBay. I would look for frequencies such as 7025, 7030, 7040, 7050, 7115 and 7120. This would give you quite a bit of flexibility and perhaps the chance to work some DX if you also included 7010. BUT be careful if you are not an extra class license as operation below 7025 can be problematic if you are not so licensed. Early 1930's tranmitters ran relatively low power like in the 5 to 20 watt range --hey it was only one tube. So today for a very different reason I am looking to build a "toob" transmitter in the 1 to 2 watt range. Recently my friend N2CQR had huge success running only 100 Millwatts from a single transistor -- at 10 dB gain over his rig it should be possible to make contacts running 1 to 2 watts. The reasoning behind the low power is that the tubes I have selected run low plate voltage like 135 VDC max and their Pout is accordingly limited to a max of about two watts. The several tubes I have selected also run low voltage (2 VDC) on the filaments. So the power supply does not have to be supper heavy duty and while there is a shock hazard present -- 100 or so volts is a lot different than 800 VDC. In the 1935 Radio Amateur's Handbook, a prolific left coast homebrewer, Frank Jones, published a project called A Beginners Transmitter. It was crystal controlled and developed a whopping 10 watts of pure RF. (Well maybe not so pure.) It was the ICOM 7300 of its day for the beginning ham! In this simple diagram a Dual Triode is used as a push pull crystal oscillator -- it even had the power supply schematic to boot. Note the coupling to the antenna which could be either a single wire or a Zepp antenna. Now in 1935 the FCC regulations were not a stringent as they are today so some sort of Low Pass or Band Pass Filtering is mandatory on the output side should such a transmitter be used on today's ham bands! This hummer at ten watts and crystal control is not subject to the vibration and hand capacitance issues. It is also keyed in the Cathode and there is some wave shaping through the 500 Ohm 10W resistor in the Cathode loop.You could even meter the circuit. Many variants of this rig were built using other tubes like a 6SN7 and even 12AX7's. There are issues however to watch including that the plate supply produces 400 VDC and while it may be 10 watts output the watts input (at 50% efficiency) are more like 20 watts. The Type 53 and 6A6 are both High Mu Twin Power Triodes. At 350 VDC on the plate and 60 Ma of plate current the tube as a Class C Oscillator can produce 15 watts. Easily you could see 100 ma of Cathode current with 400 VDC on the plate -- now we know why the Cathode resistor is rated at 10 watts! Oh another way you can tell about power handling capability -- the heater supply! The Type 53 tube needs 2.5 Volts at 2 Amps. That globe will be more than warm to the touch -- toasty is more like it. Thus my 1930's project will be based on the"Jones Model" with several modifications including the use of a tube that has a modest plate supply like around 100 VDC and the filament supply will be 2 VDC at 200 ma. I have chosen to operate this rig on 40 Meters and the output will be passed through a band pass filter (lifted from the Bitx40). This should keep us honest with the FCC.
I intend to link this main page to several other pages so that you won't grow a beard (or for YL's, hair under your armpits) while it loads. The basic scheme is to cover the initial noodling, thence proceeding to the nuts and bolts of the contruction including the design decisions that were made. That will be followed up with the wire up process and then the final test and on the air testing. Join the journey.
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