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 amplifier practice technical and commercial obstacles to be sented to the detector. 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 triode 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 RADIO RECEIVER, 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 triodes.