EMERSON 843 "TRANSISTOR III"

3-TUBE / 3-TRANSISTOR HYBRID RADIO (1956)

For a brief period of time in the mid-1950's a unique type of radio was offered for sale to consumers who were only just beginning to learn about that promising new invention called the transistor. The high cost of the first transistors kept them out of most common electronic products such as radios. Instead they were limited to higher profit margin products such as hearing aids. Portable tube radios had been around for several years, but as manufacturers designed innovative new transistor circuits which capitalized on the device's long life and low power consumption, they ran into the road block of its limited frequency response. The result was a compromise which used tubes in the radio frequency stages where they provided superior performance and transistors in the audio stages where they significantly reduced the power consumption from the batteries. It is for these reasons that hybrid receivers have been described as the "missing link" in the evolution of radio design.

Emerson made several models of hybrid AM receivers. Most, like the 838 and 856, were smaller coat pocket sized units. The 838 was based on the earlier model 747 four tube set whose circuit was later modified to incorporate transistors as described in the article below. The 838 used three tubes and two transistors while the later 856 used two tubes and three transistors. The model 843 went one step further by using three tubes and three transistors.

First hand information about hybrid sets is relatively scarce, but one excellent resource is the 1956 McGraw-Hill text book by Milton Kiver entitled "Transistors in Radio and Television". On page 179 he includes an interesting and informative discussion of hybrid circuits. Mr. Kiver's description is much better than anything I could write so I am reproducing his material here.

COMBINED TUBE-TRANSISTOR RECEIVER

   An interesting application of transistors occurs in portable receivers in which vacuum tubes are employed in the converter, i-f amplifier, and second-detector a-v-c a-f amplifier while a pair of transistors, biased for class B push-pull operation, furnish the output power in place of an output tube. The advantage of this arrangement is that it frequently increases the output power yet the over-all power drain on the battery supply is reduced substantially. In a typical four tube battery receiver, approximately 75 per cent of the relatively expensive B battery power and 40 per cent of the less expensive A battery power are consumed by the output stage (assuming a 3V4 output amplifier). When transistors are employed, the full power requirement is supplied by A batteries alone.
    An illustration of what the partial conversion to transistors can mean is given by the Emerson models 747 and 838 portable receivers. The model 747, shown in Fig. 23, uses all vacuum tubes. The output tube, a 1AG4, has very limited power capabilities, and it was chosen because the model 747 receiver is exceedingly compact and only miniature batteries could be accommodated. With this line-up, the current drain on the A batteries was 160 ma. In the transistorized version of this receiver (Fig. 24), the 1AG4 tube was removed and a pair of matched 2N108 PNP transistors were substituted. The drain on the B battery was reduced by 25 per cent, and the total drain on the A batteries brought down to little more than 50 ma. Furthermore, with the output transistors operating in class B, the maximum power output was trebled.

   The savings in power requirements would have been greater if the transistors had been used to replace a 3V4 output tube, although under these circumstances the power output increase would not have been so startling.
    The advantages gained by using this hybrid type of receiver is that tubes can be used in the more critical stages, those at the front end of the receiver, where presently available transistors still cannot equal tube performance, yet permit transistors to be employed in those stages where their lower current requirements offer definite savings in power. Of course, in this transition from vacuum tube to transistor, certain changes in design are required. For example, a miniature transformer was needed between the audio amplifier and the transistors in Fig. 24. In addition, the output transformer had to be changed.

   The small 45-volt B battery in Fig. 24 is now used solely by the vacuum tubes. The A battery value has been raised to 4 volts so that it can be used by the transistors. Because of this change in A battery (from 1.5 to 4 volts), the filaments of the three vacuum tubes were connected in series. (Previously they were in parallel.)
    In this chapter we have inspected a number of radio receivers which were partially or totally transistorized. No startling circuit differences were occasioned by the use of transistors in lieu of vacuum tubes; the sequence of stages still followed the same order, and similar circuitry was employed. However, the operating conditions of transistors do differ in many respects from vacuum tubes, and because of these, a number of minor circuit modifications were required.

It is interesting to note that the two audio output transistors in the model 838 were 2N108's (a CBS type number) rather than the more common RCA 2N109's which I have actually seen in the model 843.

The Emerson 843 also differs from most hybrids in the category of size. While most hybrids were designed as coat pocket portables to capitalize on smallness, the 843 was obviously a table model. Another difference was that the coat pocket models used tiny hearing aid type tubes while the 843 used three standard 7 pin miniature tubes which absolutely dwarfed the three socketed transistors. The model 843 seems to stand alone among hybrids as a larger table model set... measuring 9-1/2" x 7-1/4" x 3-1/2". It was also a bit unusual among table model tube sets in that it had no provisions for using normal AC house current as an alternative to batteries.

The three tubes are branded Emerson, but EIA codes confirm that the two with gold markings were made by Sylvania (312), and one with silver markings was made by RCA (274). The tube date codes range from 609 to 613.

Here is a comparison of the relative size of typical 1950's tubes and transistors. Starting on the left is an Emerson 815006 transistor. It also bears the RCA EIA manufacturer code of 274. Next is a Raytheon CK549DX tube from a Zenith Royal pocket size hearing aid. Next is a Motorola 5672 tube salvaged from an old VHF business radio. It has an EIA code of 280 indicating that it was actually made by Raytheon. Another interesting characteristic of the middle two tubes is that they are not round, but slightly flattened with the pins arranged in a straight line. Last is an RCA 1R5 tube of the same type used in the Emerson 843.

Above is an internal view of the chassis. It is actually mounted "upside down" in the radio's leather case so that the tuning capacitor faces up through a hole in the top. The volume control at the lower left aligns with a similar opening for its knob. The label below illustrates the chassis position. Date codes abound on components in this radio. The i.f. transformers are 552, the audio transformers are 605 and 607, the speaker is 604, and various electrolytics are dated 601.

In normal use the tubes and transistors were actually hanging in their sockets and the two batteries ("A" and "B") rested in the bottom of the case. The August, 1957 issue of Popular Science contained a buyers guide of both transistor and tube type portable radios. Among the tube models reviewed were two hybrids, the Emerson 843 and 856. One of the specifications included in the article was the battery cost to operate each radio for 1000 hours. As one would expect, the all-transistor models excelled in this category. The 4 transistor model 868 "Miracle Wand" (reviewed elsewhere in my web pages) was estimated to cost only 17 cents for 1000 hours. Among the tube only models the cost ranged from $4.00 to $12.75 for 1000 hours. The Emerson 843 turned in a respectable compromise cost of $2.07. If nothing else, the hybrid design really did offer a clear advantage when it came to extending battery life.

Above is closer view of the 3 audio transistors. They are all labeled "Emerson" but the EIA manufacturers code is 274 which identifies them as RCA. Interestingly, none of the transistors was heat sinked.

In the view above tubes and transistors are adjacent to each other. A tube socket is located next to the left arrow and a transistor socket is next to the right arrow.

The transistor date codes are all 6A. This format translates as follows: the number is the last digit of the year and the letter represents the month of the year from A through L, thus 6 = 1956 and A = January. The Emerson part numbers are 815006 (one device) and 815007 (a matched pair of devices). Judging from the date code and physical appearance, these transistors are all probably equivalent to the RCA 2N109. The 2N109 was introduced in 1956 so these must be some of the earliest production units.

SPECIFICATION SHEET

I have been fortunate to acquire an original dealer advertising sheet for the Emerson 843 which contains some useful information. The printing date is June, 1956 and it has a hand written price of $48.00 on the front side. The reverse contains a flowery, and in some cases exagerated, description of the radio's many features for the prospective buyer.

Here is a scanned version of the text:

Transistors are so tiny they must be seen to be believed. Transistors
are so sturdy they won't break... last for life. Rich leatherette
cabinet with carrying handle. Ruggedized and durable.
EMERSON TRANSISTOR III PORTABLE RADIO MODEL 843
Specifications
Frequency Range - 540 to 1620 Kilocycles.
Self-Powered Superheterodyne - Improved superheterodyne circuit
incorporating advanced Emerson engineered electronic features for long
life operation on self contained batteries.
Sealed Dynamic Speaker with Alnico Permanent Magnet - For greater power
without distortion - full reproduction of voice and instruments -
balanced tone quality.
Enclosed Ferriloop Antenna - Amazing new Ferrite bar loop antenna for
greater sensitivity and selectivity.
Push-Pull Class "B" Transistor Output Plus Transistor Driver - Delivers
greater power output with less distortion and improved tone quality.
Inverse Feed-Back Circuit - Provides undistorted flat frequency
response.
Full-Vision Dial - With large, easy to read numbers.
Combination Switch and Volume Control - Thumbwheel operated.
High Signal-to-Noise Ratio - For amplification of desired signals and
elimination of interference.
Automatic Volume Control - Maintains uniformity of volume with unvarying
smoothness for distant and near broadcasts.
High-Q, Iron Core, I.F. Coils - For increased selectivity and
sensitivity.
On-Off Indicator - Indicates on or off condition at a glance.
Tubes - 1-1R5 Converter, 1-1U4 I.F. Amplifier, 1-1U5 Diode Detector, AVC
and 1st Audio Amp.
Transistors - 1-815006 Audio Driver, 2-815007 matched pair Push-Pull
Class "B" Audio Output.
BATTERY REQUIREMENTS
1- 6 Volt "A" Battery, Eveready #744, or equivalent.
1- 67 1/2 Volt "B" Battery, Eveready #467 or equivalent.
Low Battery Drain - Circuit designed for economical battery operation.
Dimensions - 9 - 1/2 inches wide, 7 - 1/4 inches high, 3 - 1/2 inches deep.

FACTORY SERVICE NOTES

One unusual thing I have discovered about this set is that it is not cross referenced in the Sam's Photofact service literature index even though most of their other models are covered. This is true even in an index published in the 1950's. It may be that in some cases Emerson simply didn't grant permission for service information to be published by others. A reader of this web page was kind enought to share scanned images of the original Emerson service notes for the model 843. As shown below it is dated January, 1956 so that helps pinpoint when this radio was initially produced.

The official Emerson shematic is shown below. The only other other place I have seen this is in the Lou Garner book "Pinpoint Transistor Troubles in Twelve Minutes", but that reproduction is not very clear due to its small size.

Final thoughts

Any example of a hybrid radio is highly desirable as a collectible since they represent a milestone product. The fact that they were only made for a period of roughly two or three years also makes them somewhat rare. The Emerson 843 seems to be something of a "high water mark" in the sense that it was probably one of the the largest and last hybrids to be made.

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