francini@argus.DEC (This Space Available for Rent or Lease) (10/19/85)
-- This line is offered as a sacrifice to line-eaters everywhere. -- While I was at the library not too long ago, I found an Audel's 'Advanced Electricity' handbook that was first printed in 1939. I thought I would share some tidbits about what the state of the art was like in audio & other areas of electronics at the time. TELEPHONY: The step-by-step and 'panel' systems for automatic call routing were being installed in the larger cities. In most smaller exchanges, calls were still routed by operators. There were two sets of operators in exchanges in those days--'A' operators and 'B' operators. The 'A' operators were the ones who responded when you picked up the phone receiver. They would originate your call. The 'B' operators handled calls coming in from other exchanges. If you were calling someone in your own exchange, the 'a' operator would handle it completely. If you were calling another exchange, your 'A' operator would ring up the destination exchange's 'B' operator and he/she would finish the connection. Even if an exchange had an automatic switching system, toll calls still required the intervention of an operator. TUBES: Of course, the vacuum tube was king. Tubes back then were large affairs, much bigger than the small ones that were most common before tubes were supplanted by transistors. These were the kind which had an actual bakelite base where the pins were attached. The glass envelope was much fatter at the top than at the junction with the base. Most had a hump on their tops. RADIO: Most (if not all) radio back then was AM, with a smattering of SSB and CW. [FM was not available until the 50's.] The front-ends were most often of the regenerative design. Superhetrodyne circuits were also used, but less frequently, and the IF's for those radios were all over the place from 260 kilocycles (to use the old term) to 600 kc. Most radios received more than one band -- either shortwave or longwave in addition to the 'standard broadcast band'. (By the way, the 'standard broadcast band' wasn't so clearly defined as it is now. Depending on the specific radio, the low end boundary ranged from 500 to 590 kc while the upper end ranged from 1550 to 1700 kc. Many radios were designed to be AC/DC, because there were many parts of the country that used 120 volts DC as their standard power supply. [An interesting aside: If you live in or have lived in an old house (say dating from the teens or 20s), and if it has any of its original switching, you may have noticed that the switches have quite a loud CLICK to them when they are toggled. This is because they are AC/DC switches, and are designed to make or break the connection as quickly as possible to avoid arcing when using DC. Most modern switches are the 'silent' variety, because they have no need for a fast spring-assisted action.] AMPLIFICATION: A telling symbol of what the state of the art was for sound reproduction at that time was a chart of distortion vs. frequency for a 'good' amplifier section of a radio. It showed a curve which began with a value of 5% distorion at 500 Hz, fell to a minimum of 2% at 3000 Hz, and rose to around 8% at 10,000 Hz. Quite definitely 'no-fi' sound quality. Most amplification designs were Class A or AB, with 'push-pull' or Class B circuits used in more expensive models. One of the most interesting things about the circuits then in use was the fact that the loudspeakers did NOT use permanent magnets. Instead, they used the B+ supply to energize what was called the 'field coil' in the speaker. This created the constant magnetic field necessary for the voice coil to operate. It also could provide a ready-made source for large quantities of hum in AC-operated radios if the ripple filtering on the power supply was deficient. CAR AUDIO: Many cars had or were available with radios, but the all-in-the-dash radio was many years away. The tuning and volume controls on the dash were connected to a box elsewhere containing the actual electronics. Radio manufactures had to develop special tubes with heaters that could function properly under the extremes imposed by automotive operation as well as run on the 6-volt DC battery used in all cars then. Cars usually had to have 2 antennas: the usual one on the cowl or fender, plus another fastened along the underbody of the car to pick up the ground wave of the signal. The units used a weird IF frequency - usually 260 kHz - for the broadcast band. I could go on and on, but I'm sure I've caused all but the heartiest of readers to move the next article by now. I just thought these observations on things back then would be interesting and would cause people to think a bit on how far we have come since then. Nostalgically, John Francini ...decvax!decwrl!dec-rhea!dec-argus!francini
rfg@hound.UUCP (R.GRANTGES) (10/20/85)
[] Are you sure your 1939 Audels wasn't a reprint of a 1929 Audels? - There were lots of miniature (battery radio) tubes in 1939. - Lots of amps were better than that. - There was FM broadcasting at least around NYC. - The superhetrodyne was king, not the regenerZator. - One piece car radios <were> common. There was one in our '41 Plymouth, and one in our '36 Studebaker. I don't want to brag, but I ought to know as I put in a lot of time, back then. Years and years, in fact. Why, I was 9 years old in 1939. I must admit to only having my first paying radio shop job in 1947. -- "It's the thought, if any, that counts!" Dick Grantges hound!rfg