I have seen the volume filament control on other early radios. It worked nicely on strong local stations; you could burn just enough battery power to get a decent sound to your headphones or perhaps an early loudspeaker. And with the lower gain, meant later stages were not overloaed from a strong signal. However if you were trying to pull in a far away station, and getting the most gain out of the tubes, you would shorten your battery life.
Holy smokes, it was simple. You have three consecutive transformer-coupled RF amps, each one tuned in synchronism with the other two by virtue of the mechanically ganged tuning capacitor. Then the envelope detector, nothing more than a low pass filter. Then two series connected audio amp stages. All transformer coupled, class A, no feedback, no separate voltage and current amp stage in the audio amp.
The first audio output has a high-pass filter, to cut down the squealing no doubt. They didn't like anything above 5 KHz audio in those days. And the really odd thing is the volume control. It operates on the voltage appplied to the *heaters* of the RF amps. How weird is that? As opposed to being a voltage divider just before the audio amp. I wonder if that was prevalent back then?
What strikes me most is how mechanical these old radios were. From the mechanical tuning capacitor, to the tubes, to the volume control. And yes, those wood cases seemed to be meticulously built.
But on the other hand, one reason electronics were this way is because they had no options. If they had plastics, they wouldn't have used wood. If they had solid state electronics, they never would have bothered with these touchy and expensive electro-mechanical tubes. But they are fun to watch as they glow.
Those mechanical tuning capacitors could be problematic. We had an old radio where that tuning capacitor became vulnerable to the slightest vibration, emitting loud thumps, and was scratchy too. Tubes changed characteristics as they aged, or just as they warmed up with each use, were an egregious waste of energy, and were so bulky that the electronic design always had to be compromised to keep the number of tubes required in check. And so forth. These old electronics are fun to look at, but we have also come a long way.
In spite of the common wisdom, solid state electronic products can be far, far, far more long lasting than these old radios or phonographs. I'm still using an amp I built in 1980, for instance. Hard to do that with these old tube jobs. The tubes would need changing, plus the heat they emitted caused other components to fail as well.
As we unveil EE Times’ 2015 Silicon 60 list, journalist & Silicon 60 researcher Peter Clarke hosts a conversation on startups in the electronics industry. Panelists Dan Armbrust (investment firm Silicon Catalyst), Andrew Kau (venture capital firm Walden International), and Stan Boland (successful serial entrepreneur, former CEO of Neul, Icera) join in the live debate.