London, England (Circa 1950's): As a youngster in the UK in the 1950's, I repaired neighbours' AM radios. Now in Canada for many years, one of my retirement hobbies is restoring vintage tube radios, hence this piece...
A long time ago in a different era, radios used glass vacuum tubes (valves, in the UK) to amplify the RF signal picked up by the antenna (aerial, in the UK), detect the audio modulation and amplify the audio frequencies for the loudspeaker. Commercial sets came in table top and floor standing versions, many in wood cabinet furniture that, by and large, was quite well finished, although most were made of veneered cheap wood and some cheap table sets were in plastic boxes. There were also battery portable sets. One of my hobbies is restoring vintage tube radios, hence this piece...!
For the non-cogniscenti (not for radio experts!) here's how vintage radios worked... and still do! I'll use just enough detail to get the concept across, hopefully without undue simplification. We'll stick to AM (Amplitude Modulated), BC (Broadcast Band) radios which were the majority from the 1930's to the 1950's. Commercial FM (Frequency Modulated) radios came later, even though the early ones creep into the "vintage radio" definition (I have several), but I won't discuss them here.
The more or less standard schematic ("circuit", to some!) for BC band radios of this era is the so-called superhetrodyne, or "superhet" for short. In this design the radio frequency (RF) signal from the antenna (in the range 550 KHz to 1600 KHz) is mixed in a converter tube with a local oscillator running 455 KHz above the RF frequency to give a constant 455 KHz intermediate frequency (the "IF") - called the "convertor" stage (it's sometimes called a "first detector", a misnomer in my opinion), typically a heptode tube The AM moduation envelope on the original RF signal is maintained. Conversion allows the vast majority of the pre-demodulation amplification to be done at the 455 KHz IF, a task done quite easily. Earlier "tuned radio frequency" radios just amplified the RF by means of a variably-tuned RF amplifier but it was not easy to track the tuning accurately, nor to get the stable high RF gain required. In a superhet, tuning of the RF and oscillator coils is by a ganged, mechanically variable "condenser" (capacitor), carefully designed to track across the BC band to maintain the constant 455 kHz IF frequency difference between the two circuits (RF antenna circuit and higher frequency local oscillator.)
The fixed-tuned IF amplfier, generally one pentode tube, is followed by a diode detector, actually a half-wave rectifier. It separates the audio modulation from the now large 455 KHz IF signal. This diode is traditionally part of a combination "double-diode-triode" tube where the first diode is the detector (IF rectifier), the second diode is unused or just paralleled with the first. Sometimes the second diode is used as a special Automatic Volume Control (AVC) rectifier providing DC bias feedback to the RF and IF stages, but this function is also provided by the first diode on most radios since a byproduct of detection is a DC voltage proportional to the magnitude of the IF signal level. The triode section of the "double diode triode" is an audio frequency (AF) amplfier stage feeding the output stage, the latter being a power amplifier designed to drive a loudspeaker. In a "mains set" another tube rectifies the AC house current to the quite high DC voltage required to run the tubes (called "B+" in the US and Canada and "HT+" in the UK.) There are two main types of power supply: one is the transformerless AC/DC type, typically called an "AA5" ("All American 5 Tube", i.e. Converter, IF amplifier, Detector/AF amplifier, Output, Rectifier); the other power supply, generally for higher quality radios, is for AC mains only. This type has a mains transformer to step up the mains voltage and, generally, a double-diode full-wave rectifier tube for an ultimately smoother, higher-voltage DC supply.
There are many variations of the above vintage AM "superhet", e.g. "radiograms" that include a turntable for 78 rpm and 33 rpm records, a push-pull output stage for higher audio output power, an RF stage that amplifies the signal before the convertor stage, multi-band radios from "long wave" (down to around 200 KHz) up to "short wave" (up to 30 MHz, or so), Communications receivers (with higher quality circuits, both in design and implementation) and Military radios (called "boat anchors" by the collecting fraternity due to the massively heavy construction) - all these are AM types. Other variants include an "eye tube" to aid tuning (this is a miniature cathode ray tube where the AVC voltage controls a green, V-shaped shadow on the phosphor), also push-button tuning for station preselection and multiple loudspeakers.
That's about it... If you are interested in following up this fascinating subject(to some of us!), go to the Internet and search on "vintage radios" - there is a wealth of information out there.
Why did I write this? Well, it's very simple. As a 1960's engineering graduate, I learned my electronics on tubes before I learned about transistors. As a youngster, I repaired neighbours' AM radios (actually, that lead me into EE as a career.) Now, nearly 50 years later, it's a rewarding hobby. I tend to seek out the "basket case" vintage radios, partly to save them from the landfill but also because they're cheaper! I restore most of them to full working condition - electronics and wood cabinet - but I've had one or two failures! Most parts are still available if you know where to look for them. Surprisingly, vintage radio vacuum tubes are still quite easy to obtain for a few dollars each, in some cases not much more than the original dollar price which, in dollar terms (but not in gold!), is far lower than the original cost. Of course, some of these tubes are well used but I have been surprised by their longevity - I have found only a few dead tubes in the hundreds that I have tested. Other tubes have been in storage, unused for decades - these are called "NOS" for "New Old Stock" - nice to find these.
Vintage radios are fun...! Watch the tubes glow, twiddle the tuning knob, see the "eye tube" move, polish the cabinet and listen to "old time radio" whenever you get the urge! (Green Hornet, The Shadow...)
Cheers,
Roger Jones, P.Eng, Life-Senior Member IEEE
Member, Ontario Vintage Radio Association
Thornhill (near Toronto), Ontario, Canada
Some definitions:
Vacuum Tubes
Heptode: a seven element tube with a cathode, a plate (anode) and five grids.
Pentode: a five element tube with a cathode, a plate (anode) and three grids.
Triode: a three element tube with a cathode, a plate (anode) and one grid.
Diode: a two element tube with just a cathode ands a plate (anode) (no grids)
Other:
AM: Amplitude Modulation
FM: Frequency Modulation
RF: Radio Frequency
IF: Intermediate Frequency
AF: Audio Frequency
B+ or HT+: the high voltage applied to the plate (anode) circuits of vacuum tubes.
Disclaimer: many details of vintage radio have been left out in the interest of a short, introductory article - the choice of what to leave in and what to leave out is entirely mine.
Comments (0)
