PHILIPS 634A

NOTES	This is one of the Philips "Superinductance" range of T.R.F. sets, housed in a large mahogany cabinet with a bakelite loudspeaker surround. It was unveiled at the Eleventh Radio Show at Olympia (in London) in 1933 at a time when most manufacturers had embraced the superior superhetrodyne design. For some reason Philips still placed their faith in T.R.F.'s and this set illustrates the great lengths they had to go to in order to compete. Although the term "superinductance" is merely a marketing name, the tuning coils are indeed special. The coils are wound using Litz wire wound on large 2-inch glass formers housed in substantial copper screening cans in order to reduce the losses and hence improve sensitivity (and selectivity) of the receiver. However, the "Q" of a tuned circuit varies with frequency and when covering the medium wave band the frequency variation is roughly 3:1. In a typical superhetrodyne there is only one variable tuned circuit so the variation of 'Q' would scarcely be noticed. However the Philips 634A contains a total of four variable tuned circuits and thus the overall variation across a waveband would be considerable. To conteract this variation, Philips ganged a potentiometer (via a phosphor-bronze belt) to the tuning control which varied the bias on the H.F. valves, altering the sensitivity in the same way as A.G.C. in a "normal" set. At the time, A.G.C. was actually referred to as "A.V.C." (or "Automatic Volume Control") and was generally only impelemented in superhetrodyne receivers. However this T.R.F. also includes A.V.C. although it can't have been that effective if they also needed that additional potentiometer. Perhaps the inclusion of A.V.C. was just a marketing ploy, the public expecting A.V.C. to be a standard feature of all new sets so Philips had to included it, whether it worked or not ! Philips were the original developers of the Pentode valve, so it comes as a bit of a suprise to find that both of the RF valves in this set (S4VB's) are tetrodes. However the strangest valve is the SD4 which is a combined diode plus tetrode used as the detector and first audio stage ; I can't believe this valve saw much commercial use and must be quite rare today (well, I've never seen one before). At least the audio output valve is a pentode (PM24A), allbeit a directly heated one. The madness continues with the silly idea of mounting the majority of wax capacitors inside a sealed block. Appart from the obvious lack of regard for servicability, this must mean that the capacitors are not in their optimum physical location. Careful wiring layout in a high gain T.R.F. circuit is crucial in maintaining stability, so having to use longer wires to reach the block of capacitors seems like a bad move to me. It takes a few minutes to work out where the waveband switch is ; its actually operated by pulling the tuning control (outwards for long wave). This also operates a little masking plate that covers the wavelength markings of inactive waveband. But all in all, the Philips 634A is a lovely looking set and you certainly can't accuse it of being just another "4-valve-plus-rectifier" design. The set rose to fame some 50 years after its introduction when it appeared in a nostalgic 1980's advert for the Ovaltine hot drink.
SERVICE DATA	There is a copy of the "Trader" sheet number 7 on CD#1.
CURRENT STATE	The cabinet has been repolished sympathetically and the set has been overhauld. Shame about the small hole in the speaker cloth though. Having never heard a 30's T.R.F. in action it is difficult to comment on the performance of the set. However the set does seem to be on the verge of oscillating when tuned exactly into a strong local station ; perhaps the set is being over-driven as the A.V.C. doesn't seem to be particularly effective. However the set is reasonably sensitive. Not suprisingly, after almost 70 years the pull-out station name sheet that should be housed underneath the set is missing.
WHERE FOUND	"On The Air" for 400 drinking vouchers. Can't think of a better use for my redundancy money, can you ?
RELATED ITEMS	Period advertisment from "The Music Seller and Radio Music Trader" from 1933. advert.jpg [259K]

Rear view, dominated by the huge copper cans
that conceal the "super" inductors.