Editor's note: Here is another neat guest tech article from my friend and colleague, Jerry Steele, Applications Manager, ON Semiconductor. A bit of nostalgia and tech info combined with an ‘engineering disaster’.
In this blog, I would like to discuss tube circuits. The scope of this topic includes early tuned-RF receivers where you had to tune each of up to 3 stages individually. Sometimes things that aren’t mistakes, but rather disadvantages turned into assets, are included in this topic. A good example was the difficulty of filtering the DC from early tube power supplies; on the other hand, they used electromagnetic speakers that had a magnet coil that made a handy filter choke. Problem was, this induced hum into the sound that was cured by deliberately introducing an opposing hum into the audio amplifier. No discussion of tubes would be complete without mentioning Earl Muntz, father of the Muntz TV’s and the legends surrounding his attempts to build inexpensive electronics.
In the Beginning
Let’s treat this history of electronic engineering disasters chronologically, and doing so just naturally will lead us a discussion of the tube era. Some might argue that tubes, in themselves, qualify as an engineering disaster, what with heaters that consumed more power than modern notebook computers, susceptibility to microphonics, and a finite lifetime. But all this must be tempered with a positive view of tubes. A really good example of a positive view of tubes is the variation in specification from tube to tube. Any engineer using discrete semiconductors knows that specification variation of nearly +/-50% (that spec is being kind, some JFET’s vary 5:1) from device to device with semiconductors is closer to the norm, rather than the exception. It is claimed that in the latter days of serious tube manufacturing in the 1960’s, some tubes exhibited less than 10% variation in specs from tube to tube. Far more predictable a device than any transistor.
Another wonderful asset of tubes is their ability to survive electrical abuse. You could routinely overvoltage and overcurrent tubes only to have them come back for more. In fact, in certain applications, you could see the effects of high stress on glass encapsulated tubes by the red glow of their plates. This was the norm in equipment such as radio transmitters. As a rule, consumer equipment didn’t operate tubes such that the plates turned red, but I remember watching the plates of a guitar amplifier redden during a loud, lengthy, practice session.
Early Radio Technology
The superheterodyne radio is a technology that we take for granted. It’s not even obvious to us that something as tiny as a cell phone may contain a highly sophisticated double conversion superheterodyne with phase lock loop synthesizer tuning. Prior to the advent of the superheterodyne, radio reception was a simple matter of tuning an amplifier to the station frequency, amplifying, and detecting. Or at least it was a simple matter until you required more stages of amplification, which was often a requirement in remote areas.
One early radio topology was the tuned RF amplifier. The need for amplification is obvious, it allows the picking out of weaker signals. But the additional benefit of more tuned stages was better selectivity, the ability to reject stations at frequencies other than the one selected, especially stations on adjacent frequencies. The problem with the Tuned Radio Frequency (TRF) circuit of Figure 1 is that three tuned stages required three tuning capacitors. Early TRF receivers actually would have had a separate tuning cap for each stage with up to 3 tuning dials. Finding a station was a tedious process of setting each dial near the correct setting, then successively setting each one for maximum gain. Subsequent development of the TRF receiver did manage to gang all three of these capacitors together. The variable capacitor on V2 was for neutralization: it counteracted the effect of the internal grid to plate capacitance permitting greater gain before oscillation occurred.
The Saga of “Madman” Earl Muntz
No description of tube era electronic engineering disasters would be complete without a discussion of madman Earl Muntz. More a creative marketer than any real engineer, after all he became wealthy and successful selling used cars, Earl elected to get into the TV business in the 50’s. Now by the time he decided to enter, the TV business already had some maturity. Yet Earl understood the marketing requirement of distinctiveness but elected to exploit it in what is often a dangerous way in the business world: offer a product based on price. From a classical standpoint, no new upstart is likely to succeed in the face of corporate giants with such a strategy, because the giants have an economy of scale that allows them to fight back. But Earl had a brilliant “out-of-the-box” plan.
TV’s up to that point were always designed to address the requirement of receiving a signal as well as possible regardless of location. This meant getting as good as possible performance in fringe and rural areas, because modern (by that time) AGC circuits would take care of reducing gain in strong signal areas. As a consequence, TV’s were designed with RF amplifiers ahead of their superhet converters, and typically 3 or more IF stages. An IF stage was not a trivial matter, requiring a vacuum tube, a handful of resistors and capacitors, and an expensive coupling transformer.
Earl felt that one of the biggest markets for TV was the inner city, typified by a Manhattan high rise sometimes in eyeshot of the transmitting tower. The previous TV designs of the time were providing something for those customers that actually went unused in the form of the extra gain stages. So, Earl set forth to design sets where he cut back on the RF and IF amplifiers to a minimal level. Earl made another choice for economy, and in the best marketing tradition of “if you can’t fix it, feature it”, claimed his sets did not need the fine-tuning control that all other sets had. In truth, the strong signal strength is what usually was his saving grace, but even then, some sets periodically needed a serviceman to come and tweak the tuning (AFC techniques were well known by that time, but even up-market TVs couldn’t afford AFC circuits; there’s no way Muntz’s could, and this AFC is not to be confused with the AFC circuits all TVs require in some form or another for vertical and horizontal sync).
Furthermore, he had engineers design simple circuits of questionable quality. Wherever he could delete a part, he did. For instance, while virtually every set made at the time had a two-stage vertical deflection circuit, he had the competition scratching their heads on his one-stage, one-tube design.
And went on to successfully market these sets to customers who were blissfully unaware (and would never have cared) that their set would only operate in stronger signal areas.
But there’s more to Earl. Earl was such a corporate spendthrift, that it was rumored that he would walk up to engineers bench testing their circuits. After a little discussion regarding the circuit and its various components, Earl would question the engineers on whether or not certain components were truly needed. Earl always carried around a pair of wire cutters, and would often pull them out or clip one lead of a component whose usefulness he might doubt. If the circuit continued to function, the engineer would be under the gun to truly demonstrate the need for the clipped part. In yet another rumor, Muntz would buy another manufacturers TV, remove all the parts he could and still have it operate.