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Vintage Radio Browning-Drake Model 6A from 1927 By Dennis Jackson I’m fascinated by early radio sets, especially regenerative sets from the 1920s. BrowningDrake is a US company that made innovative radios in the 1920s, and this set is one of their later models, which evolved over the years to become solid performers. Shown here is the model 6A with a large Utah horn speaker that came with it. W hen listening to the conversations of older relatives who had made their own wireless receivers during the 1920s, I remember being impressed by the names Browning-­ Drake (B-D) and Radiokes. Browning-Drake made tuned radio frequency (TRF) wireless sets while Radiokes made tuning coil sets. There were none better, according to my father and some of his brothers, who had lived solid physical lives working as farmers and builders and judged things on their merits. Radiokes were a Sydney-based company that manufactured boxed sets of tuning coils for various receiver configurations, plus other desirable components that were mainly aimed at the 94 Silicon Chip amateur constructor. The story of Browning-Drake is well documented. In August 1923, Glen Browning and Fredrick Drake were students at Harvard University in the USA. They were asked to explain theoretical losses in the wireless receivers of the times. Accordingly, after a long detailed mathematical study, they concluded that the major losses were due to unwanted capacitive coupling between the primary and secondary windings and within the windings of the tuning coils between RF stages. The regenaformer Their solution was to develop the “regenaformer” transformer. Australia's electronics magazine The secondary consisted of 74 turns of enamelled copper wire wound on a 75mm Bakelite former that was spiral threaded so the windings were spaced one-half of a wire diameter apart. The primary consisted of 24 turns of 30 AWG (10thou/0.25mm diameter) silk-covered fine wire loosely wound in a slot cut into a ring and placed firmly inside the tube, level with the Earthy end of the secondary. The aim was to reduce capacitive coupling. The feedback or tickler coil is wound on a 60mm former placed in the other end of the secondary former that is free to be rotated 180°. This feedback winding used 20 turns of 26 AWG (16 thou/0.4mm diameter) wire to provide a controllable amount of siliconchip.com.au Opening the front of the model 6A’s case reveals five control knobs and primary tuner. From left-to-right, the knobs control power, variable capacitor C2, station selection, valve filament voltage and feedback coil in the regenaformer. feedback regeneration from the plate of the detector to the grid of the first audio valve. Browning and Drake’s main contribution to the regenaformer was the placement of the primary winding within a slot fitted at the end of the secondary, with a view to reducing unwanted capacitive coupling causing RF losses. Hazeltine’s balancing circuit was used to minimise plateto-grid capacitive effects within the first RF valve. Howard Armstrong had developed the concept of regeneration. To get around his patents, the complete regenaformer, its associated tuning capacitor and tuned aerial coil were initially sold as a boxed kit to people building their own radio. B-D receivers were popular with amateur constructors as they could wind their own regenaformer and the other parts were generally standard items. Interestingly, the variable tuning capacitor used in the first RF stage had a higher capacitance (400pF) than that used to tune the regenaformer (300pF). Maybe this was to compensate for aerial loading. I have noticed a tendency for stations to crowd the lower end of the tuning range on other B-D sets. Complete factory-built B-D receivers were available by the mid-1920s, and I had the good fortune to acquire a B-D model 6A from the USA around 2008 after I saw it advertised on eBay. I rather impulsively placed a bid for $250, which was knocked back due to not meeting the undisclosed reserve. I also had to consider the freight cost of around $200 at the time. I was a bit peeved by missing out on what would have been a once-in-a-­ lifetime chance and set about doing the next best thing, as many an old-time siliconchip.com.au amateur would have done, by building my own. I saved the photos used in the advertisement and whatever other information I could find. I had almost completed the RF section, ready for testing, when an email arrived. The seller had a rethink, and as I was still the highest bidder, I was given a second opportunity. I lost no time paying up through eBay, and the model 6A duly arrived through the back gate to preserve matrimonial bliss. The mid-west USA had been subjected to severe blizzards, and the unfortunate seller needed to buy shakes (wooden shingles) to repair his roof. I upped my payment a bit for the goodwill, and he added his big Utah horn speaker into the bargain. The 1927 model 6A The six-valve model 6A was a complete rethink compared to its basic five-valve predecessor, the model 5R from late 1926. It is a table set built of solid timber, probably poplar, which is light, soft, workable and stains well for an attractive finish. The double doors in front open to display the timber control panel. By 1926, dedicated output valves were becoming available such as the 71A and the CX112A, the latter used in this set. These gave a modest but welcome rise in sound output compared to using more general-purpose valves in the output stage. Still, listeners would have to wait several more years before the moving coil speaker (like we use today) provided a broader range to the audio spectrum. It was common for these pioneering wireless receivers to have all the same types of valves in the line-up. UX201s, UX199s or the Phillips B405, B409 and A609 were the main types. This set uses a bit of a mixture; the UV199 and UX201A were from General Electric (GE), the 200A was a generic type made by several manufacturers, and These coils and associated tuning capacitor make up an original B-D regenaformer sold as a boxed kit (not the one used in the 6A). The rotatable coil which controls feedback regeneration is on top. The secondary is the larger coil while the primary is wound on a slotted former and slid inside the main tube at the bottom, Earthy end. Australia's electronics magazine April 2023  95 Resistance-capacitive coupling is used between all stages, except the first RF amplification stage, which uses an RF choke and coupling capacitor to direct RF to the next stage, the regenaformer. The whole assembly is built on a flat aluminium chassis. That was an expensive metal back then, reflected in the US$85 retail price for the set. Circuit details A close-up of the regenaformer section and the detector valve V2 of the Browning-Drake model 6A. the CX112A and CX340 were made by Cunningham Inc, New Jersey, USA. The first knob to the left is the on/ off switch which disconnects the valve filaments from the A battery. The second is the ‘sensitiser’, claimed to pull in far distant stations, according to one advertisement. This controls a variable capacitor of about 100pF (C2; see Fig.1), which is in parallel with the first RF tuning capacitor (C1) and really adjusts tracking between both ganged tuning capacitors C1 and C5. The third lower centre knob provides single-point tuning, making station selection user-friendly, which was not common with mid-1920s receivers. The fourth to the right is the usual wire-wound rheostat controlling the valve filament voltage, which is adjusted as the ‘A’ battery voltage drops with usage. On the far right is the control for the rotation of the feedback coil within the regenaformer. The model 6A was a well-thoughtout design. It has other cutting-edge innovations for the time, such as the completely separate shielding of the first and second RF sections plus the rear audio sub-assembly, and the use of resistance-capacitive coupling between stages. I could not find a circuit diagram for this set, so I drew my own, shown in Fig.1. B-D receivers used similar first and second RF stages. A UX199 valve was used in the first stage because it was easier to neutralise due to its lower internal capacitance. Hazeltine neutralisation was implemented using C3 at a few picofarads. Medium-size variable capacitor C2 is in parallel with the large ganged variable tuning capacitor C1 and tuning coil L1; its purpose is to adjust tracking as ganged tuning capacitors C1 and C5 tune across the dial. RF choke L5 in the plate circuit blocks RF from the B+ 90V line to redirect through C4, an Aerovox 500pF capacitor, and through L2, the primary of the regenaformer. L2 is wound on a thin former that has been glued and fitted under the Earthy end of the larger tuned winding, L3. L4 is the rotatable feedback coil providing regeneration. The grid leak detector is made using V2, a 200A triode, together with R1 Fig.1: the circuit diagram for the model 6A radio. As there wasn’t any existing circuit online for this radio one was drawn up by tracing and testing the components by hand. 96 Silicon Chip Australia's electronics magazine siliconchip.com.au Viewing the model 6A chassis from above shows the RF and AF shielding partitions. The first RF amplifier is right, while the regenaformer-detector section is left. Four audio valves are shown below (the rear of the set). and C6. R2 blocks RF from the detector B+ 45V battery tap, while R4 and R6 block the audio signal from the B+ 90V to be redirected through coupling capacitors C9 and C10. R3, R5 & R7 are part of the negative grid biasing circuit of the four audio valves. The configuration of C7, C8 & L6 is a bit unusual. That section appears to block and bypass RF from the grid of V3, the first of four audio valves mounted on the sub-assembly to the rear of the shielding cans. There are no audio coupling transformers; instead, resistance-­capacitive coupling is used throughout. The audio sub-assembly is a separate, closely packed unit that was difficult to access while tracing the circuit. Two ‘equalisers’ (made by Amperex) are used to limit the current drawn by the valve filaments (providing a measure of protection similar to an NTC thermistor). After some probing, I determined that the plates of V4 & V5 are connected together, as are both grids; so V4 & V5 are in parallel. Valves are usually connected in series to provide more voltage gain. So my first thought upon seeing this is that they needed more current drive than a single triode could provide. After reassembly, I removed V5 to see what difference it made. There was no difference in the sound output, nor was there any difference when I replaced V5 and removed V4. So the siliconchip.com.au need for the extra valve is a bit of a puzzle. Perhaps some CX340s had weaker drive than others, and this was a ‘crutch’ to allow them to get away with using the weaker valves. Or maybe there is another reason... A bit of a puzzle It had taken almost a century, but the designers were finally caught out. Why weren’t the four audio valves operated in series? I am not sure. All TRF receivers of this general type I have known have had not more than three audio stages, and I can only suggest that adding more could have caused instability. I have another neatly-constructed, home-built TRF set that had an extra valve paralleled experimentally to the audio output valve (both UX201s). Still, from my experience, that does not improve the sound output. Firstly, the human ear has a logarithmic sensitivity; doubling the sound output power would give only The RF section of the model 6A, which incorporates a UV199 valve (V1) and the variable capacitors C1 (ganged tuning), C2 (centre), tuning trimmer and valve balancing trimmer. Australia's electronics magazine April 2023  97 a small increase in the maximum perceived volume level. Secondly, there could be an impedance mismatch to the speaker load when two valves are used in parallel. Further thoughts Unusually, this B-D model 6A will operate well with reduced volume with the first RF valve (a UV199) removed. I have sometimes pondered the actual gains achieved by placing the primary winding in a close-wound slot fitted under the Earthy end of the secondary. I have three examples of factory-­ made regenaformers, and all seem to have the primary turns wound sideby-side on a separate thin former slid inside the main tube at the opposite end to the rotating feedback winding. In each case, all turns are close-wound with fine wire. The problem is that the former cannot easily be removed to check the effect on performance without damaging the unit. This is the technology of 100 years ago and is now part of the history of vintage radio. Battery TRF sets had a short lifespan before becoming redundant by the end of the 1920s due to advances in valve technology and the rise of the superheterodyne set. Conclusion If I were an adult living around 1927 and were given the choice of any of the TRF battery-powered receivers from that period that I have in my collection, I would choose my Browning-Drake The audio side of the model 6A contains valves V3-V6. Clips within the two subpanels hold removable resistors. From left-to-right the valves are: UX201A, CX340, CX340 and CX112A. model 6A. Connected to its original Utah horn speaker, it gives a good sound level from the two remaining AM broadcasters in Hobart. It is lightweight, reasonably easy to set up and tune in once you get the knack, and it is very stable in operation. I now know why those pioneering old-timers working in the bush would get excited when they were talking about their Browning-Drake wireless. Would I remove that extra paralleled audio valve to conserve battery current. Maybe not, would I have known? To power my set, I use the Universal Battery Eliminator designed by Peter Lanksheer from Invercargill, NZ and published in Electronics Australia, March 1990. That design has proven invaluable in powering my battery receivers. After I work out the connections for a particular set, I wire it up to an eight-way connector from Jaycar that matches a connector in the supply lead from the battery eliminator. That enables me to use the one Battery Eliminator for multiple radios, with quick and correct connections to each SC wireless set. U Cable Tester S B Test just about any USB cable! USB-A (2.0/3.2) USB-B (2.0/3.2) USB-C Mini-B Micro-B (2.0/3.2) Reports faults with individual cable ends, short circuits, open circuits, voltage drops and cable resistance etc November & December 2021 issue siliconchip.com.au/Series/374 DIY kit for $110 SC5966 – siliconchip.com.au/Shop/20/5966 Everything included except the case and batteries. Postage is $10 within Australia, see our website for overseas & express post rates 98 Silicon Chip Australia's electronics magazine siliconchip.com.au