Some months ago a surprise package turned up in my mailbox. An anonymous donor had sent me three wonderful, old germanium transistors. The 2N170 and a pair of 2N107's (all date-coded December 1956) were still in their original plastic tube packaging.
A thin booklet, Type 2N107 Transistor Circuits for Experimenters [and] Amateurs, was also included. The circuits shown in the booklet appear to be aimed at the rank beginner: audio amplifiers, code practice oscillators and the like. However, the "Simple Radio Receiver" caught my eye (lower right-hand corner below).
This was exactly the sort of project that captivated me in my early youth. Growing up in St. Louis, Missouri in the late 1960's, I enjoyed listening to baseball and ice hockey games on KMOX, and pop music on KXOK, using a wide variety of simple homemade AM radio receivers. Although there's nothing much of interest to me on the broadcast AM radio dial these days, I have a nostaligia for the simple circuits that marked my first steps in amateur radio. By the way, this same circuit also appeared in the first (1957) through the seventh (1964) editions of the General Electric Transistor Manual.
While I'm not now interested in listening to broadcast AM radio, a direct-conversion receiver suitable for copying CW only requires the addition of a beat-frequency oscillator (BFO) to the above receiver circuit. With sufficient BFO drive the germanium diode can be made to function as a commutating (switching) mixer. On the other hand, a single diode mixer has several notable disadvantages; perhaps the worst is the relative lack of AM signal rejection. This type of mixer also creates more spurious frequency energy and the port-to-port isolation is quite inferior as compared to more advanced designs.
Nevertheless, the more I thought about it the keener I became to find out how well a lad might have done with such a basic radio in the late 1950's...on 20m perhaps? Fortunately, all it took was a pleasant evening to find out.
The circuitry on the left-hand side of the proto-board in the above photo carries a crystal-controlled (14.059kHz) beat-frequency oscillator made from a Philco surface barrier transistor (1959 date-code). This oscillator generates 13mw of RF output power. A miniature 50 Ohm potentiometer acts as the oscillator load and BFO input signal power splitter.
Yes, that's a cat's whisker and galena detector working as the switching mixer in the above photograph! Alternate mixer diodes are shown in the foreground. The left-hand diode is a classic, large glass 1N34A made by Sylvania in the early to mid-1950's. To the right is a red plastic-packaged 1N66 (date-coded March 1957) diode made by Raytheon. This was Raytheon's equivalent to Sylvania's hot selling 1N34A.
Here's the circuit that I'm using (BFO details are not shown).
C3, C4: 10nF
C5: 1uF (although it's not necessary I used a non-polarized capacitor)
R1: 270k Ohms (optimum value depends upon the transistor used)
L1: ~1uH, slug-tuned, 14.5t, tap 6t from ground-end, 9 x 16mm)
L2: 470uH molded RF choke
HP: high-impedance magnetic
D1: 1N34 or most any point-contact diode arrangement
Q1: 2N107, PNP germanium transistor (mine is date-coded Feb. 1957)
Signals were heard as soon as I powered-up my new receiver. I began using the Sylvania 1N34A as my mixer diode. When things became quiet on the QRP calling frequency I substituted my RF bench signal generator for the crystal-controlled BFO, in order to roam around the band. After logging a dozen or so DX calls I switched first to the Raytheon 1N66, and finally to the cat's whisker and galena. The galena appeared to be at least as sensitive as my vintage commercial diodes in this application.
The catch of the night came with the cat's whisker and galena installed when I copied F6HFX running 5w from the southwest of France! A station in Florida that Pierre was working had a fairly rough copy, judging from the number of repeats requested. The Floridian lost him several times before throwing in the towel. Pierre's signal was also coming and going here, but with spells of decent copy in between.
Another station heard with the galena was G3HGE. Tom had a huge signal here for well over an hour. It was a real pleasure to copy him sending with his old bug. Some of you may recall his company, TW Radio; a history of which may be found by clicking here.
A few of the DX stations heard include: OZ0TX, PA3CJP, 4O8A (big sig), OK2AN, YT2ISM, OK4RQ, G3VMW, DM4IM, DL0KWH, LY5A, CR7ACS, and DF8GI (579 on the galena). Altogether, I filled two sheets of notebook paper with the callsigns heard that evening. I heard a number of weaker domestic stations calling CQ (with no answer) near the QRP calling frequency; presumably some of which were running 5w or less.
A few more technical details...
The impedance looking into the base of the 2N107 AF amplifier is 2740 Ohms. Ic = 388uA, Ib = 10.7uA, Vc = 1.49Vdc, voltage gain = 44.3dB, power gain = 36.1dB
The injected BFO signal level is ~1Vpp at the diode. The 14MHz BFO "backwave" radiation measures 4uW at the antenna terminal.
Some images taken from the 1957 Radio Shack Catalog are shown below. Included are listings for the Sylvania 1N34A and Raytheon 1N66 diodes, as well as the General Electric 2N107 transistor. The 2N107 might have been a bargin in 1957 at $0.99, but adjusted for inflation the equivalent cost today would be $7.95. Even "low-end" electronic components were relatively dear in those days!
The 2N43, 43A, 44, 45 and 2N107 transistors were graded products from the very same fabrication line. In fact, what was to become the 2N107 had been discarded as rejects until the "bean counters" at GE could be convinced that electronic hobbyists might find them useful.
This transistor line was introduced in September of 1953; the first alloy junction devices ever produced. John Saby was the lead developer of these historical transistors, however, the alloy junction process itself had been invented at GE in 1950 by Hall and Dunlap for use in rectifiers.
Given the semiconductor surfaces were not passivated, or otherwise protected from the environment, GE found it necessary to evacuate their early transistor envelopes to nearly vacuum tube levels. This explains the metal "pinch" found at the top of these early "top hat" transistors.
Finally, the datasheet power gain listed for the 2N107 is 38dB. I'm tickled to find that my 2N107 is still producing 36dB some 54 years later. Having now myself reached the age of 54, I only wish that my original "specs" would have held up so well!