Are very old transmitter designs usable? - HomeBrew Forum at eHam.net

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Are very old transmitter designs usable?	Reply
by KD4AL on October 11, 2005	Mail this to a friend!
I have a great book called "Golden Classics of Yesteryear" by Dave Ingram K4TWJ. It contains a lot of receiver and transmitter designs from the pre-WWII era, those years before transmitting crystals came into use. They have relatvely few components and look very straightforward. Are these designs are spectrally pure enough for today's bands? I'd hate to fire up an old design and splatter harmonics, etc. all over the bands. I'm not talking "spark". These are things like a "Genuine Push Pull TNT Transmitter" or "The Super Fantastic 204A Classic." Anyone with experience with these designs? I'd like to build one to use on Straight Key Night. Bill KD4AL

RE: Are very old transmitter designs usable?	Reply
by WA9SVD on October 11, 2005	Mail this to a friend!
It will depend upon the individual project. While design PRINCIPLES haven't changed, the performance requirements have! That means transmitter circuits may require additional filtering (either added to the original circuit, or as an in-line, outboard filter) to meet current spectral purity and harmonic suppression requirements. You also want to be careful of receiver circuits. Some of the old circuits used the "superregenerative" concept, which while quite sensitive, often created wideband interference, which could easily be propagated into a receiving antenna and thus become an "incidental (or at least unintentional) radiator." Super regens are not normally used these days because of their interference potential.

RE: Are very old transmitter designs usable?	Reply
by WB2WIK on October 12, 2005	Mail this to a friend!
Before crystals?? That's really a stretch. I wouldn't use any simple transmitter that wasn't crystal-controlled or otherwise frequency controlled by a pure, stable source (a good VFO that's fully warmed up should be fine). The crystal's easier. If you don't exceed the crystal dissipating rating, it's hard to screw up. Using a simple single-stage power crystal oscillator (such as my old, original Novice cathode-keyed 6V6 was), you can probably even meet today's spectral purity requirements. Or come very close... WB2WIK/6

RE: Are very old transmitter designs usable?	Reply
by WB6BYU on October 12, 2005	Mail this to a friend!
One of the biggest problems will be frequency stability. Coil dimensions and capacitor values change with temperature, and the tube dissipation will tend to warm the surrounding parts over time as you use the transmitter. Also, even the large copper tubing coils are prone to vibration (which can frequency-modulate the signal) unless they are well secured. While it is possible to make a 1-tube VFO transmitter that is reasonably stable, this often takes a lot of mechanical design of the parts that won't show in a schematic diagram. Also, in single stage transmitters the antenna is connected to the VFO stage, and wires blowing in the wind will also affect the transmitted frequency to some extent. Actually I'd encourage you to try this if you are interested, especially on the lower frequencies where stability tends to be less of a problem. If you keep the power low and listen CRITICALLY to the signal quality, and test it thoroughly on a dummy load before putting it on the air, you probably can get the drift down to where it is tolerable (though you still may drift out of the passband of a narrow CW filter.)

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