Discussion in 'Amplitude Modulation - AM Fans' started by W7UUU, Feb 7, 2019.
Love that Studebaker. For some reason it makes me think of contemporary Cuba.
I don't think I'll ever use them, but I'd bet there's someone out there who would like to have them.
I remember a CQ article from the late 1950s with a very humorous article about 304TL tubes, at that time in vast surplus supply. The article was long before my time but I had tons of old CQ magazines when I was a 13-YO Novice in 1974 and loved to read those old stories. I had NO IDEA what they were but loved reading the stories.
The article was about using the 304TL as its own neutralizing capacitor.
Here's what I know about the merits of each mode (on HF):
- Easy to implement (transmitter)
- Can be received on almost any receiver
- Easy to tune in
- Easy to listen to when QRM is low
- S-meter, ANL and AVC easy to implement
- Takes up much less spectrum
- More "talk power"
- No heterodynes
- High power is much less expensive (with new parts)
- Equipment is generally lighter and smaller (transceivers)
There were a few hams using SSB in the 1930s, and the mode really began to take off in the late 1940s. But SSB didn't really displace AM on HF until the mid-1960s or so.
This long delay was due to the fact that SSB gear was more complex and more demanding.
Almost any CW transmitter could be made to produce AM by just adding a modulator, which was really just an audio amplifier. Many popular "Novice" rigs had screen modulators built-in, others could do AM with an external plate modulator. WW2 surplus made AM easy too. Receivers could be pretty broad and drifty yet still do a good job on AM. Zero-beating didn't have to be super-exact, get within a few hundred Hz and you're good.
SSB required a lot more. Even the simplest SSB transmitter required some complexity and careful adjustment. Receivers had to be much more stable, and most conventional designs didn't have AGC or S-meters that worked with the BFO on. To get the real benefits of SSB, you needed a receiver designed for the mode. Zero beating required some real skill, and drift had to be kept very low. Lots more adjustments and measurements.
And SSB audio is....well.....SSB audio. No matter what you do, it's still SSB audio.
IMHO, what turned the tide were two developments: the SSB transceiver and the table-top grounded-grid linear amplifier.
An SSB setup was about as expensive as an equivalent AM setup - particularly since SSB parts weren't found in surplus.
Then someone got the idea of the HF SSB transceiver that shared many of the expensive parts. This wouldn't work with the phasing method, butw ith the new SSB-bandwidth crystal and mechanical filters that showed up in the 1950s, it became possible to use many of the same parts for both tx and rx. Collins had proven the tunable-first-IF concept with the 75A-x receivers, starting about 1947, so there was a track record of that technology.
Using the same parts for both paths saved money, but it did a lot more: it simplified operation and reduced size/weight. No more zero beating - just tune the receiver until the received signal sounded "normal", and the transmitter was automatically zeroed. Tuneup was drastically simplified - the same knob tuned both the low-level tx stages and the receiver front end, leaving only the pi-net to adjust.
Also, AGC and S-meters that would work with SSB had been developed. SSB became an "arm-chair" mode.
Look at the Collins KWM-2 - Yes it cost a fortune in its time, but it was a 100 watt all-HF-band SSB transceiver that was smaller and lighter than most good receivers of the time. It could be easily taken mobile and portable. It would be at home in the den or living room, and could be tucked away in a small space.
Or consider Heathkit. In the late 1950s their offering was the TX-1/RX-1 pair, which were excellent but big, heavy and expensive. The HX-10 was even more so. Then, 5 years later, there's the SB-100, which costs less and is much smaller and lighter, yet put out the same 100 watts.
The final nail was the table-top linear amplifier with SS power supply and grounded-grid tubes.
To go high power on AM, the usual route was plate modulation, due to the low efficiency of AM linear amplifiers and grid/screen/cathode modulation. Plate modulation requires an audio amplifier capable of at least half the DC power input to the final - and more is a good idea, to allow some headroom. High power AM was not cheap, and usually homebrew or converted surplus. In fact, AFAIK, there were only two legal-limit AM HF transmitters manufactured for the amateur market: the Collins KW-1 (about 150 made, many of which were sold to The Government, not hams) and the Johnson Desk Kilowatt (fewer than 450 made). The Desk Kilowatt cost $1600 (without the desk, exciter, or receiver) and the KW-1 was in the "if you have to ask, you can't afford it" category. And remember, those were 1950s prices.
SSB and some new technologies changed all that. The grounded-grid triode linear amplifier could be driven by the typical "100 watt" SSB transceiver and would produce high power in a small package at a low price. Amplifiers like the Heathkit SB-200 and SB-220, the Drake L-4 and L-4B, and some others, cost much less than AM rigs of less power. Homebrewing a linear amplifier was a simpler project, and there were lots of examples in the publications of the times.
The SSB transceiver and linear amplifier made high-power HF 'phone available to hams at much lower prices (in both time and money) than ever before, and required less space and desk strength. The ham shack could move out of the basement and into the den or even the living room!
Of course not everyone was happy with the revolution. Equipment that had once commanded high prices in the used market suddenly became white elephants that only a few wanted. Hams who had only been licensed a short time and who didn't know a driver transformer from a clothes dryer were on the air running high power SSB like they owned the place. The comfortable ol' buzzard AM roundtable was crowded by fast-taking sidebanders with VOX and "nets".......
All ancient history now.
73 de Jim, N2EY
Borgi (K9YQQ) might be able to use them, he had a homebrew he built that runs 3 of them, ot it was planned that way but he had some duds and might use different modulators.
When I was a kid in the early 60s...Christmas meant a visit to Gram & Gramp. And, of course, that meant a visit to my Uncle's ham shack as well. He had a regular sked with my Great Uncle in California, and that meant a holiday QSO which we all got to participate in. Uncle Rog ran a DX-40 and an SX-96. Don't know what Uncle George was running out in Cali but he had a fine signal here in Minnesota. Pretty sure the contacts were made on either 10 or 20, since Unc had a tribander up in the air. The little Heath proved you could make cross-country contacts on AM with just a handful of watts.
I would bet that the QSOs took place on 15-meters. During the early 1960s, 15-meters was open between the mid-America region and the West Coast on a regular basis and low power, like the Heath DX-40, worked very well. The 20-meter band was the realm of the high powered AM operators and 10-meters could be very spotty although definitely a LOT better than these days.
From northwestern Indiana (about 60-miles from downtown Chicago, 12-miles from Lake Michigan, and 8-miles from the Michigan border), I worked a lot of California stations with a WRL Globe Chief 90A with the SM-90 screen modulator.
I have the complete 3 (or was it 4?) part series from 1934 R/9 magazine, describing what SSB is, how it works, and culminating in a step-by-step construction article for a 75m filter type exciter. The balanced modulator generated DSB in the upper audio frequency range (around 10 or 15 kHz IIRC) and employed re-purposed interstage audio transformers as inductors for the sideband filter. The SSB signal was then up-converted to a fixed intermediate frequency, which was then heterodyned to the ham band using a VFO or crystal oscillator.
I have often wondered if anyone actually followed the instructions in that article and built one of those. That would be a real "vintage" find if an unmolested one showed up in an estate sale or someone's junk collection. Unlikely, since hams tended to part out homebrew projects once they were no longer in use, particularly during the Great Depression and immediately following WWII before surplus parts started showing up in abundance.
Despite most opinions to the contrary, frequency stability would not have been an insurmountable problem at the time the article was written. That's about the time the National HRO receiver came out, and those had superb frequency stability for their day. In the early 1970s, I used a 1935 model with the original German Silver dial and 2.5v tubes, and after about a 30 minute warm-up period, I could park it on an afternoon 20m SSB net and monitor for over an hour without touching the dial, while working on a bench project. It would have been relatively easy to build a VFO with similar stability or else use crystal control for the heterodyne oscillator in the transmitter. Besides, phasing type crystal filters in high-end receivers were in use for CW during that era, which require nearly the same degree of frequency stability as SSB.