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Behemoth: Restoration of an RCA Victor Model 15K-1 – Gerry O'hara

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Behemoth: Restoration of an RCA Victor Model 15K-1 – Gerry O'hara A Mid-1930’s ‘Magic’ Behemoth: Restoration of an RCA Victor Model 15K-1 – Gerry O’Hara Background I recently completed the refurbishment of a Marconi CSR-5 receiver for a friend. Shortly before work on that receiver was completed, he asked if I would be able to restore an RCA Victor 15K-1 receiver as my next project. Quite a different ‘beast’ from the CSR-5, a WWII Canadian communications receiver built for the Canadian Navy, whereas the RCA Victor 15K-1 is a high-end domestic console style set dating from the 1936/37 model year. The cabinet was in poor condition (photo, right), and in need of stripping/re-finishing, but the chassis appeared complete and in reasonable shape from the photos I was sent in advance. To save bringing the large, heavy cabinet over to Victoria from the BC Mainland, and as I don’t have the facility to refinish large cabinets at my house at the moment, it was agreed that I would restore the chassis and the cabinet would be restored by a mutual friend at the SPARC Museum. The RCA ‘K’ series Radios and the ‘Magic Brain’ RCA Victor introduced receivers with a separate RF sub- chassis, marketed as the ‘Magic Brain’, in the mid-1930’s, initially with their models 128, 224, C11-1 and others: “Inside RCA Victor all-wave sets is an uncanny governing unit ... Human in its thinking, we compare it to the human brain. You choose the broadcast - from no matter where in the whole world. Then, watchman-like, it keeps out undesired radio signals. It concentrates on that one and makes it four times stronger. Each tone has higher-fidelity ... in a quality of reception heretofore unequalled" For 1936, RCA Victor upgraded their ‘Magic Brain’ to use three of the newly-introduced metal tube types. The original ‘Magic Brain’, introduced the previous model year, used just two tubes - glass envelope types 6D6 (pentode RF amplifier) and 6A7 (pentagrid converter). The new version used separate (metal envelope) tubes for the Mixer (6L7), Local Oscillator (6J7), and RF Restoring an RCA Model 15K-1 Gerry O’Hara amplifier (6K7): “Just as the original Magic Brain set new standards of shortwave reception, so does the new, improved Magic Brain achieve still higher standards...". The use of separate Local Oscillator and Mixer tubes was common practice by many other manufacturers at that time, particularly for their higher-end ‘all-wave’ receivers, where the separated Mixer and Local Oscillator functions resulted in generally improved performance, especially for shortwave coverage. It was this version of the ‘Magic Brain’ that was employed in the 15K-1 model. An article in ‘Radio Age’1, examining what was thought to be a prototype RCA ‘23K’ chassis (never in production), identified that RCA were further developing the ‘Magic Brian’ concept to include automatic frequency control (AFC) on the Broadcast Band. RCA also introduced the ‘Magic Eye’ (type 6E5) for the 1936 season. This was used on several of their higher-end console models and some table models. The ‘Magic Eye’ was purported to be an aid to tuning by providing a visual indication of how well a station was tuned-in and its relative signal strength. In realty, of course, this was mainly a marketing ‘gimmick’ (as was the ‘Magic Brain’ title for that matter), as it somehow brought the radio ‘to life’ by engaging another of the user’s senses – and it ‘moved’ while you turned the tuning knob – so cool! “It’s alive!...” Radios with ‘eye tubes’, as ‘Magic Eyes’ are commonly referred to in North America, are still desirable – providing the ‘eye’ is bright and working well2 - somehow that eerie green glow that changes with the signal is still ‘magic’ even after 84 years!3 Oh, and not forgetting the (not quite) ‘all-metal’ tubes! – another gimmick? or a worthwhile technical advance over their glass-envelope counterparts? – well, RCA’s marketing guys certainly thought so: "Metal Tube radios are quieter, more sensitive, better toned, superior in every way". These started to be introduced in 1935 by RCA with great fanfare over their robustness compared with their glass- envelope counterparts, along with the inherently “almost perfect electrostatic shield” due to the grounded metal 1 ‘The Should-Have-Been RCA Model 23K: Analysis of a 23-Tube Prototype Receiver’ by Leigh Bassett (Ch 1 and Ch 2) 2 Eye tubes ‘wear out’ relatively quickly – meaning that they become dim with age as the phosphor coating on the target becomes less active over time (usually the tube is still operating but the target is so dim it cannot be seen in normal lighting conditions). Certain types of eye tube are becoming scarce these days, including one of the most commonly-used, the 6U5/6G5, which includes a remote cut-off triode amplifier section (this extended the input voltage range of the tube). The 15K-1 used the less-common 6E5 tube, which had a sharp cut-off triode section 3 Check out this website for different display patterns and more info about eye tubes 2 Restoring an RCA Model 15K-1 Gerry O’Hara envelope, thus dispensing with the need for those pesky external metal shields for RF, IF and small- signal AF tube applications. After around 85 years, the jury may still be out on that question, but on balance, they probably were a technological advance, though perhaps a bit of a ‘dead-end’? Interestingly, the later ‘miniature’ tubes reverted to glass envelopes again – likely due to ease of manufacture and associated cost considerations, and it is reported that some apparently ‘all-metal’ tubes had internal glass envelopes from the late-1930’s, or even double- concentric metal envelopes4. A 1942 video of these tubes being made can be viewed here (Part 1) and here (Part 2). The first metal tubes were the 5Z4 rectifier, 6A8 pentagrid converter, 6C5 and 6D5 triodes, 6F5 hi-mu triode, 6F6 power pentode, 6H6 dual diode, 6J7 sharp cut-off pentode, 6K7 remote cut-off pentode (called a ‘super control’ pentode in the RCA announcement bulletin), and the 6L7 pentagrid mixer. My favourite metal tube is the diminutive 6H6 dual diode (photo, above) – its just so cute! - and the 15K-1 chassis sports two of them! The 15K-1 The RCA Victor Model 15K-1 was a ‘top of the range’ console radio marketed in the 1936/37 model year, which would lighten your wallet to the tune of around $2005. The cabinet styling was maybe a bit more imposing than the less-expensive RCA 13K, 10K, 8K, 7K and 6K siblings, but somewhat more modest than the earlier ‘C’ series models. Perhaps only the radio-phonograph combination sets, such as the 15U model from the same ‘golden years of radio’, were more physically imposing?… The circuit design also resembles the lower-end models, though with several enhancements (‘bells and whistles’), mainly aimed at improving the tonal quality and power output of the reproduced audio, eg. complex tone control arrangements, push-pull 6L6 output stage, and large (12”) loudspeaker, along with other tweaks, such as more sophisticated eye tube circuitry. The schematic is shown in Appendix 1. 4 A careful dissection of a metal tube (a 6J5 triode) was undertaken by the British National Valve Museum here, where no internal glass tube was found, though glass was used as part of the construction. However, other tube types, or tubes from other manufacturers, could have different construction details 5 Allowing for inflation, this is around $3,700 in 2020 dollars 3 Restoring an RCA Model 15K-1 Gerry O’Hara The 15K-1, as its model number suggests, is a 15 tube single conversion superhet that, for all the marketing ‘hype’ is of fairly conventional design for high-end domestic radios of the time, comprising: a RF amplifier stage (6K7), Mixer (6L7), Local Oscillator (6J7), two stages of IF amplification at 460KHz (2 x 6K7), AGC/detector (6H6), audio pre-amp (6C5), audio driver (6C5), push-pull output (2 x 6L6) - capable of 30W output (20W ‘undistorted’), tuning eye amplifier (6K7), tuning eye rectifier (6H6), tuning eye (6E5), and dual rectifiers (2 x 5Z4) - the latter needed to supply the higher than usual HT current. The receiver tunes from 150KHz to 60MHz in five bands, comprising: - ‘Long Wave’ (Band X): 150 – 410KHz; - ‘Standard Broadcast’ (Band A): 530 – 1,800KHz; - ‘Medium Wave’ (Band B): 1.8 – 6.4MHz; - ‘Short Wave’ (Band C): 6.4 – 23MHz; and - ‘Ultra Short Wave’ (Band D): 23 – 60MHz. A concentric knob dual-ratio tuning mechanism is incorporated, providing 20:1 or 100:1 ratios, which provide good tuning control even on the ‘Ultra Short Wave’ band. As noted above, the RF circuits comprising RF amplifier, Local Oscillator and Mixer stages, are all contained on a detachable sub-chassis, marketed as the ‘Magic Brain’. Although this is quite a neat design, both mechanically and electronically, there really is no justification for either the term ‘Magic’ or ‘Brain’ as it comprises conventional 1930’s technology and certainly does not ‘think’ – it does not even have pre-set station selection capability which was the next ‘big thing’ in domestic set development. Those 1930’s RCA marketing guys were something else! Some noteworthy features of the circuit include: - A separate and completely screened RF sub-chassis integrated with the tuning dial and band- change mechanisms (diagram above, right, from the Service Manual); - A claimed novel method of coil switching that puts some coils to double use and leaves two of the ‘Ultra Short Wave’ coils in circuit at all times, thus simplifying band switching arrangements and enhancing reliability; - Separate IF transformer and amplifier/rectifier circuit for the tuning eye; - A ‘Speech-Music’ switched
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