The Philips 735a is a rare 4-valve plus rectifier receiver using the unique EFM1 valve as an AF Amplifier and Tuning Indicator (Magic Eye)
The outwardly similar Philips 855x has an additional rf stage. There is a Mullard set with a different cabinet and chassis, but very similar circuit.
The works. As usual, Philips were unconventional and unnecessarily complicated with two valves mounted horizontally.
Trader Sheet 438 shows the circuit.
The tuning scale (as found and not cleaned up). The first six keys are for specific stations; the right three to select a waveband
The station names strip isn't illuminated; the (cardboard) station names are inserted behind springy brass.
Upper is a tone control; lower lever is the selectivity switch (why do it the simple way?).
Lever to the left = maximum selectivity.
the unique EFM1 valve acts as an AF Amplifier and Tuning Indicator (Magic Eye) - the hardened rubber shroud is lying on the back of the chassis (maybe someone forgot to reinstall it?).
Typical Philips complicated pointer drive - the single dial light lights up the glass magnifying pointer
Mare's Nest of the separate power supply - not looking forward to tackling this as the hardened rubber insulation on the wires is sure to flake off.
Thankfully no hum-buck capacitor on this set (these used to short out and take out the mains transformer on Philips sets)
The set has a good back cover, however the station adjuster tool (which should fit in the hole top left) is missing.
This is a typical stupid Philips design from the late 30s! When I first got this radio in 2002, I spent hours trying to extract the chassis from the cabinet. After finally realising that on this set, it is necessary to remove the wooden cabinet sleeve from the radio chassis in a complete reversal of normal procedure, the restoration work proved easy enough.
So here we have a "Linodyne" pushbutton stack with a pointer which is optically projected onto the rear of the scaleplate. Works well. Until the bulb fails.
The tuning gang is effectively a giant multi-section beehive trimmer which traverses axially by means of a hefty plunger. Here is one tuning gang which does not rotate at all, just to buck the trend. RF tuned circuits are bandpass coupled, complete with tracking notch filter made necessary because of the low IF frequency, 128kHz. Oh, and there are wirewound preset RF trimmer capacitors. Mustn't ever forget those confusing things... but performance is decidedly good because of them.
Shortwave works well on this set. The EF9 IF amplifier provides AGC delay from its suppressor grid, an unusual arrangement which works well in practice. The front panel is a very massive bakelite moulding. So heavy in fact, that this radio will topple forwards onto its face at the slightest provocation.
Detection & output come from a EBL1, but this sensible valve is forced to work around an EFM1 which performs the magic eye and 1st Audio voltage amplifier functions. Not to mention generating feed-forward as well as feed-backwards AGC.
Early examples of the EFM1 had a bulb that was so heavily inked that little light reached the operator, even when new. (The EFM1 that is, not the operator!)
There are 2 positions of IF selectivity, and the usual complicated Philips tone control. The awful CT8 side contact valves need cleaning occasionally to avoid intermittency.
I view the 735A as justifiably rare."
"Type EFM1 is strange. It led a short production life, being made only by the Philips companies and Tungsram. It drove its beam deflection electrodes from the screen grid of an integral AGC controlled audio pentode.
The dynamic range of the display was slightly lower than a 6U5/6G5, and far below what was achieved only 2 years later by the EM4 which was the precursor to the famous EM34.
The EFM1 was made in Germany in re-based form as the EFM11, and in Italy as type WE18. All are nowadays firmly in the "collectors" category. Chris. full EFM1 data
From The National Valve Museum site:
"The idea of using the screen grid of a signal amplifier valve to amplify the AVC bias lived on for a couple of years in a strange valve, Type EFM1, introduced by Philips-Mullard.
The EFM1 is an oddity. It combines a tuning indicator with an audio variable μ pentode. The indicator is based on the EM1 tuning indicator. This valve could save on valves in a receiver but the design was a compromise and did not replace the more conventional indicators.
This valve had a magic eye section set in the end of an otherwise conventional pentode amplifier, with the control electrode of the magic eye internally connected to g2 of the pentode. Since the magic eye is visually important to the frontal appearance of the set this usually meant that (at least) one of the set valves had to mounted awkwardly off chassis with flying leads.
To this day I do not understand why anyone should think this a good idea. Apparently, most set manufacturers did not understand why, either. The EFM1 made a brief appearance in some Philips sets but these, and the valve itself, are now collectors' items."
There is a large inspection panel underneath "giving access to most components". Being a Philips, I take this assertion with a pinch of salt.
To dismantle the set be aware that the wooden cabinet slides off the front of the set. Unbolt the chassis and power supply from the base and then remove eight screws holding the front moulding to the wooden cabinet.