Vii luta gic

IRaidl iio by PETER LANKSHEAR

The Browning-Drake receiver One of the best remembered radio names from the 1920's is 'Browning-Drake', a receiver which combined simplicity with what for its time was a rate performance. While most of its contempo- raries had production lives of little more than a year, the Browning-Drake design was popular for much of the decade. As with the IBM personal computer'. it. recently, there were also more 'clones' made by others than the official versions... By the outbreak of World War I, valve cluding them in tuned circuits coupling The alternative method, and of course receiver technology had advanced to the the valves. However the tuned RF ampli- the ultimate solution to many difficulties stage where stable detection and low fre- fier then ran into another problem. Tri- was the superheterodyne, attributed by quency amplification were possible. ode valves have sufficient inter-ekctrode Americans to work done in 1918 by Ma- However there were limitations to the capacitance that with tuned circuits con- jor Edwin Armstrong of the US Army. sensitivity and selectivity of the grid leak nected to both anode and grid, there is While much credit is due to Armstrong, it detectors that had become standard. sufficient energy transferred internally is now clear that the original concept of The newly discovered regeneration back to the grid to cause them to become the superhet was an international effort, helped, but it became clear that the only vigorous oscillators. with much of the early work being done way to improve receiver sensitivity was Initially there were two solutions. One in France. First though, there were many to increase the strength of signals pre- was to follow audio practice technical and commercial obstacles to be sented to the . Large aerials and and couple a series of valves by means of overcome and it was a good 10 years efficient tuning coils helped here, but the untuned broadband transformers. This before these problems, and RCA's licens- need now was for satisfactory pre-detec- method had some success, but with the ing restrictions, were overcome suffi- tion amplification of signals. Although valves available at the time, very limited ciently for the superhet to be anything the radio spectrum at that time was con- stage gains were possible. like widely used. fined to frequencies below 1MHz, the One example was the radio frequency valves of the period could provide amplifier made by Marconi's Wireless Naval problems very little RF amplification before be- Telegraph Company, which had needed Meanwhile, also in 1918, the US Navy coming unstable — especially at higher no fewer than six stages of amplification had commissioned their consultant Pro- frequencies, where signals were weakest. using the baseless high frequency type fessor L.A. Hazeltine to design a new The major problem in achieving useful V24 valve. The only selectivity was pro- receiver, the SE1420. This celebrated re- RF amplification was the bypassing of vided by an aerial tuner. Although it ceiver, much prized today by collectors, signals by unavoidable capacitances. Of worked, this method of reception was was required to tune from 40kHz to course, the best solution was to make use clearly very cumbersome and expensive, 1.2MHz. The SE1420 (SE stood for of these unwanted capacitances by in- and quite unsuitable for home use. Steam Engineering!) provided the choice

THE IP-501 Receiver shown in the accompanying , IP-501 I illustration is a compact unit containing the radio frequency and detecting circuits in a single case. AMITICM114 Normal wavelength range: 300 to 7.500 meters. Thisreceiver is equipped with six binding posts (normally short-circuited for 300 to 7,500 meter reception) to which loading coils may be attached for the reception of wavelengths up to 21.000 SEC. meters. The proper loading coils are, Primary. 50; elthITAL lary. 100, Tickler, 30 millihenries. DICTIlteTon The receiver is similar in mechanical design to the IP-500, with the untuned circuit omitted. The ca- pacity coupling between primary and secondary cir- cuits is eliminated in this type by heavy sheet copper boxes separately enclosing the two circuits. The panel is of Bakelite-dilecto. The coils are bank-wound inductances, of high frequency cable wound on threaded Rakelite-dilecto tubes, impreg- nated and baked.

RECEIVER, IP-501, INCLUDING HIGH GRADE CRYSTAL DETECTOR $550.00 OVerall 81nlenfii.,n, 20 in. it 11 in. 9 in. Shipping weight: 55 lb.. IftltoCO Fig.2 (above): Copied from a 1922 RCA catalog, this shows the IP-501 which was the civilian version of the SE1420. Some of these receivers were in merchant marine service well into the 1930's. Note the weight and price — both huge Basic circuit of SE-1420C for a single valve receiver. 111111104,40 (does not show shielding , binding posts ,switches , etc. ) Fig.1 (left): The circuit for the US Navy's SE1420. The 'coup shield' was developed into neutralising by Prof. Hazeltine.

114 ELECTRONICS Australia, November 1994

of a crystal or a regenerative valve detec- (a) Neutrodyne (or Hazeltine) (b) Rice System of tor, and provided variable coupling be- System of Neutralization Neutralization tween the aerial tuning and detector tuning circuits. It was essential that there be no capaci- tive coupling between antenna and detec- pk_ tor circuits as, during reception of weak lower frequency signals, this had been found to experience serious interference (c) Modified Neutrodyne (d) Inverted Neutrodyne from nearby 500kHz transmissions. CN To eliminate this capacitive coupling, the antenna tuning coil was first isolated from the secondary circuit by enclosing each in a separate compartment of heavy copper sheet. But a coupling coil, in se- ries with the secondary, was needed in the (e) Bridge Circuit of Ballantine compartment containing the antenna coil and Hull to provide the required degree of induc- tive coupling, and the residual capaci- tance between these coils left a certain amount of undesired capacitive coupling 11 11111111 which tuning could not eliminate. Radio frequency choke-- t5rna// capacity To cancel this capacitance, Professor (10-20,q,u IDS) Hazeltine placed near the aerial coil an open-ended winding, shown in the dia- Fig.3: A group of neutralising circuits copied from Terman's 'Radio Engineering'. gram of Fig.1 and called a 'coupling Each system couples a signal in phase opposition back to the grid. The method shield', so proportioned that the un- shown in (c) was used for the Browning-Drake receiver. wanted coupling to the aerial coil was that his method of eliminating capacitive generative detector was not essential — a cancelled. As we shall see, this simple coupling used in the SE1420 receiver definite advantage to non-technical users. solution was later to have an important could be adapted to neutralise the grid- influence on broadcast receiver design. to-anode capacitance of a tuned triode Do not install! Meanwhile, broadcasting had emerged amplifier. Late in 1922, he designed a Another advantage of the TRF was that — and with it the demand for easy to use successful and stable radio frequency am- RCA held the regeneration patents, and receivers. Early American broadcast re- plifier, incorporating this neutralisation. demanded royalties for its use. Some ceivers usually consisted of a grid-leak Hazeltine named his new receiver the manufacturers had found a way round detector and one or two audio stages. `Neutrodyne' TRF. Incorporating two, this difficulty by not actually installing Regeneration was frequently used to in- and sometimes three neutralised RF regeneration, but providing terminals crease detector sensitivity, but many do- stages, a grid-leak detector and two trans- with connecting instructions for a feed- mestic users were insufficiently skilled former coupled audio stages, it was to be- back winding — but with a warning for in its use, causing re-radiation and creat- come the classic standard American the owner NOT to do it! ing havoc for other listeners. receiver format of the mid-1920's. Receivers using the Neutrodyne prin- With the superhet still underdeveloped With the degree of amplification possi- ciple were still liable for royalties, pay- and tightly licensed by RCA, and the un- ble with the multi-stage Neutrodyne, a re- able to the Hazeltine Corporation — tuned amplifier an impractical complica- tion, a method of stable RF amplification was needed. Professor Hazeltine realised 4cMFR°F4°4'r:W" •

8 8 z,\i;bei2le, o\o‘-' LZ.

Fig.4: A diagram from a 1924 'Radio Engineering' article, which shows a sectional view of the primary (L1) and secondary (L2) windings of the Regenaformer. Note the position of the compact primary inside the main Home built Browning-Drake receivers were to be found in all manner of cabinets. coil former. This table top chest with lift-up lid was a popular pattern.

ELECTRONICS Australia, November 1994 115 VINTAGE RADIO which, incidentally, became very wealthy. Many receiver makers, includ- ing major organisations like Atwater Kent, resented this and circumvented the problem by introducing resistive losses in the RF amplifier grid leads, or by load- ing down the grids of the RF stages by applying positive grid bias. One receiver that used this rather cheap and nasty method was the Stewart Warner model 300, featured on the cover of my book Discovering Vintage Radio. However, Fig.5: An early circuit intended to demonstrate the effectiveness of the there was little question that a well-de- Regenaformer. Glen Browning favoured the small 199 valve for the RF stage, signed neutralised tuned RF amplifier because of its low inter-electrode capacitance. gave the best results. ceivers became unnecessary with the ad- work verified that the loss of gain was vent about 1928 of the screen grid valve, due to unavoidable residual capacitive Simple idea but continued to be used with transmit-; couplings, between the primary and sec- The basic idea of neutralisation is sim- ting . ondary windings of the RF coils. In the ple enough. It can be regarded as a form conventional pattern of coil, the primary of negative feedback. The capacitive Harvard researchers was wound over the lower end of the coupling from anode to grid inside the In practice, neutralised RF amplifier tuned winding. valve is cancelled by externally feeding stages did not deliver the amount of gain In their research work, Browning and back to the grid an equal amount of sig- theoretically possible. During 1923, at Drake found that there was a consider- nal — but in the opposite phase. the suggestion of the radio editor of the able reduction in capacitance between Hazeltine's phase reversal was ob- Christian Science Monitor, two men at the two if the primary was made physi- tained by the orientation of the primary Harvard University collaborated to study cally as small as possible. winding of the following coupling coil. the problem. They settled on the method shown in Several other similar methods of neutral- One was Frederick H. Drake, a senior Fig.4, using a compact coil wound ran- isation, including systems attributed to student, who had made a mathematical domly with a relatively small number of Ballantyne, Hull, Scott-Taggart and Har- study of the neutralised TRF; the other turns of fine wire, on a grooved ring tley were also used, and all used a small was Glen H. Browning, a noted Research mounted inside the main coil former. It variable or semi-variable to ad- Fellow, whom Drake persuaded to help was claimed that this method achieved just the coupling. Neutralisation in re- him in confirming his theories. Their 90% of the maximum theoretical gain.

Fig.6: NatiOnst's ad for the kitset of parts. The circuit was originally drawn for the 'Christian Science Monitor', which popularised the Browning-Drake receiver. Note that the Regenaformer, could be bought separately, and exact details of the audio snip were left to the builder,.

116 ELECTRONICS Australia, November 1994 Link to National practically universal feature for receiv- ers intended for loudspeaker operation. To confirm their theories, late in 1924 Late in 1925, Glen Browning formed Browning demonstrated a 'breadboard' the Browning-Drake Corporation, to receiver tuner using a single neutralised manufacture complete receivers as well tuned RF amplifier and a regenerative as marketing kitsets. Whereas for most detector. The radio press were enthusias- receiver makers the somewhat low-fidel- tic, giving the Browning-Drake tuner ity two stage audio amplifier was stand- plenty of favourable publicity, for it re- ard, Browning-Drake built receivers ally did perform as promised. used a three stage resistance and imped- Smaller and cheaper than the conven- ance coupled amplifier, with an attendant tional Neutrodyne, and with the im- improvement in audio quality. proved gain of the RF amplifier plus the regenerative detector, this was a exactly Continued popularity what the home construction fraternity were looking for. The isolation provided Unlike most contemporary receivers, by the RF amplifier prevented radiation which were in fashion for a year or two at of any interference from the regenera- the most, the Browning-Drake continued tion. The performance of the Browning- to be popular with enthusiasts for the ers. Featured in our January 1990 col- Drake could match the Neutrodyne and if remainder of the decade. 'Official' kit- umn, the 1929 Pilot Super Wasp popular- sold as a kitset, no royalties were payable sets using National components were ised this configuration for shortwave to either Hazeltine or RCA. available from B-D until at least 1928. As late as 1930, full constructional in- receivers and it was not until the mid By what turned out to be a fortunate 1930's that the communications superhet coincidence, Harvard is located in Cam- structions were still being published in became available. bridge, Massachusetts — where there hobby magazines and annuals — by The regenerative TRF with a single RF was also an enterprising firm, the Na- which time, mains powered versions stage actually saw service during World tional Company, making such diverse were being offered. War II as the R1082 receiver, which, with products as power plant equipment, There were even instructions for coverage from 111kHz to 15MHz, was household products and 'high tech' toys. building a 'screen grid' Browning- part of the T9 RAF aircraft equipment. National had recently commenced Drake, in which the triode RF amplifier was replaced by a screen grid valve. Next month, we have some ideas and making variable to cater for suggestions for building a Browning- the new fad of radio, and were very re- This development completely missed Drake receiver. ceptive when Messrs Browning and the point that the definitive feature of the Browning-Drake was the Regena- Drake made approaches about supplying Component supplier components for their tuner. These were former — which, with its special low put out as an attractively boxed kit capacitance primary winding, was un- I am often asked for addresses of sup- which, as shown in the advertisement re- necessary with a screen grid valve be- pliers of components and materials suit- produced in Fig.6, included two tuning cause it needed no neutralisation. able for valve radio repairs. capacitors, two dials, an aerial coil, and The Browning-Drake was the forerun- Brian Smith's Wireless Workshop, of the `Regenaformer'. ner of receivers using a single or 12 Mansfield Street, Rockhampton Qld 4700 now issues on request a small cata- The price of $22 was a bit high, but pentode RF stage, regenerative detector National's products were of first class and audio amplifier — a format which log, which should be of considerable in- proved to be remarkably durable, espe- terest to vintage enthusiasts involved in quality. National went on to make distin- ❖ guished equipment for amateur and pro- cially for amateurs and shortwave listen- restoration. fessional use, including eventually the legendary HRO, probably the best known of all communication receivers. Not a specific circuit It must be emphasised that the distin- guishing feature of the Browning-Drake receiver was not a specific circuit, but the Regenaformer with its special primary winding. This unit, mounted at the rear of the detector tuning capacitor, was an as- sembly comprising the detector tuning coil, the compact RF coupling coil and the rotatable tickler regeneration control. The Regenaformer could be purchased separately for $7.50. The prototype was constructed with- out a panel, on a `breadboard'. Na- tional's kitset diagram left precise details of the audio amplifier to the builder, but recommended two trans- former-coupled stages, at that time a

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