Tubes are great in radios and amplifiers as long as you have plenty of space in your kitchen or living room, not forgetting a nearby AC power outlet. Most people would say that’s because of “them tubes’ inefficiency, you know, the heater current and all that”. True, a medium sized 1950s tabletop radio consumes anything between 25 and 100 watts of AC power for just 1 to 5 watts of audio power to enjoy or bother the neighbours, which makes it a tad difficult to use in a picnic location or on the beach. The trouble was with the large amount of energy required to make the filaments in the tubes heat up to temperatures where sufficient emission is obtained to establish an electron flow. Small signal triode and pentode tubes easily waste more power due to the filament than to anode current.
Not long after WW2 Philips, a leading tube manufacturer in the Netherlands, in their famous Natuurkundig Laboratorium (‘NatLab’, in English: Physics Laboratory) developed and perfected a series of miniature low-power tubes employing direct heating instead of indirect, specifically for use in portable radios. These tubes identified by the first letter D in their type code (rather than E for 6.3 V indirect heating) operate at a filament (heater) voltage of just 1.4 VDC and 50 mA current consumption (typical), with the cathode doubling as the heater. Four of these small tubes could be connected in series for running off a standard car battery, which was 6.2 volts at that time, not 12 V. The anode (‘plate’) voltage was typically between 45 and 90 volts, which was considered low at the time and easy to furnish by a rechargeable battery. US equivalents also appeared like the 3S4 (DL92).
Clearly the Miniwatt D series of ‘battery receiving tubes’ opened the way to portable radio design. Towards the late 1940s Philips started to release its first portable radios, most of these models were housed in rather dull looking Bakelite cases and offered MW, LW and SW reception only. Expensive at the time and for the well to do only, these radios added considerably to the success of the little tubes of the ‘cold and dark’ variety. In 1951, a piece of equipment based on D tubes was launched for the professional market: the backpack PMR type SDR314.
Meanwhile a 72-page book introducing the Miniwatt D series to engineers rather than scientists was published around 1951. It became famous probably because of the solid theory, individual presentation of tubes and nice application examples of radios you could build at home with a complement of these D tubes:
- DK92 self-oscillating heptode mixer;
- DF91 RF pentode;
- DAF91 diode/AF pentode demodulator
- DL92 and DL94 AF power pentodes;
- DM70 and DM71 tuning indicators or ‘magic eyes’.
Moving on in time, towards 1955 the first official VHF FM broadcasts were aired, in Europe, initially in Germany. Not surprisingly, Philips’ first portable FM radio featuring the brand new 87-100 MHz FM band got designed there. It was called ‘Colette’, following a widespread craze started around 1950 in the USA and lasting well into the 1970s to add the suffix “–ette” to product names, like DryCleanerette, kitchenette, Sedanette, Echolette, Corvette, Mobylette, Autoette, Wagonette, sandalette, The Ikettes). In this case, the marketing people at Philips Germany did not want or need the ‘endearing diminutive’ but still rode the wave by using French girls’ names for their range of portable radios. Technical staff and radio & TV repair men on the other hand generally stuck to Philips’ established type code system. Colette technically was model LD562AB (later changed to L5D62AB) where
- L = radio, portable;
- 5 = price class indicator (0–9);
- D = manufactured in Germany (X =Belgium/Netherlands);
- 6 = year in decade (195x);
- 2 = model;
- A = AC powered;
- B = battery powered.
Colette with her prestigious FM band coverage proved hard to get, probably due to her price tag of 398 Dutch guilders (roughly two month’s wages for a factory worker). Not surprisingly, lesser priced “sisters without FM” were also available called Annette, Babette, Evette and Georgette. Not meaning any offence to these young ladies, they were pitched in the ‘3’ and ‘4’ price classes. In Holland, Philips’ home country, a number of earlier and almost identical radios got named after small boats like Jol (dinghy), Klipper (clipper), Flying Dutchman, Regenboog (Rainbow), Valk (a medium size open sailing boat), and Boeier (a Frisian vessel). As opposed to the German division the Dutch did not actually put these names on the radio grille — they only appeared in sales brochures.
Colette (‘Klipper’) is the only model with stylish dual-function knobs on the tuning scale, rather than cheaper plastic thumbweel controls beside it. The radio contains ten D tubes, one diode and two selenium rectifier bridges. In portable operation, the heater voltage is supplied by a 1.2 V ‘Deac’ rechargeable battery with a capacity of 6 Ah. The anode voltage comes from by a 90 V battery. The Deac is a nickel cadmium battery and notorious for its spillage of heavily corrosive substances that attack the inside of the radio, slowly rendering Klipper a wreck over 25 years of neglect. After opening the back cover of a tube radio like the Colette (Klipper), to look at the toxic remains of a forgotten Deac is a depressing sight. With some drawbacks the Deac can be omitted and its function taken over by one or two D size (LR20) 1.5 V batteries and a large electrolytic capacitor across them. Dry cells were optional originally and a dual holder is provided within the radio case. The anode battery is a different problem and today usually takes the form of a switch-mode step-up voltage inverter with proper screening to prevent radio interference. Various designs float around on the web as well as kits on Ebay, some visually perfected, wrapped in an Eveready carton you can’t distinguish from real. Colette also has an internal AC power supply section for the 90 V plate voltages as well as (primitive) Deac charging, where the Deac doubles as a smoothing device — with directly heated tubes you do not want AC on the cathodes. A complete charging cycle is stated to take 14 hours at 0.6 A. The radio can also charge up while playing. My Colette plays on AC power only, it has no Deac or 90 V battery. The sound, particularly on FM, tends to surprise people pleasantly probably because they associate a portable radio of this age with tinny sound. By contrast, the sound from Colette is warm and quite full, with a good dose of loudness thrown in at lower volume settings. The biggest surprise is the amount of bass you get from the measly 400 milliwatts of AF output power.
The radio can be operated in ‘economy’ mode by pulling out the volume control. This switches off half of each of the two filaments of the DL96 output tubes, with the obvious effect of saving battery capacity at the cost of some AF output power.
Collette has a design quirk. While you would expect the DM71 magic eye to act as a stylish tuning indicator, in reality it’s just a green on/off light. The circuit diagram shows what’s going on. The DM71 actually functions as a phase splitter for the DL96 balanced output amplifier. This is probably a workaround for the DAF96 (B7) supplying insufficient drive to the balanced DL96s, which in turn is the result of low signal yield from the FM and AM detectors.
As for period design features you can mention to the Antiques Road Show presenter, the retractable antennas with plastic protective end covers can be aligned at any angle between 0 and 180 degrees to optimise FM reception. They form an open dipole and if the signal is still too weak you can connect an external antenna via a ribbon cable. The round holes in the side panels allow a car radio antenna and a car battery (6 volts!) to be connected. Later models offered more connectivity but USB is not provided as standard. You can tell Colette is off duty or on the way to a picnic or beach rave by the closed lid in front of the tuning scale.
My Colette is in good condition overall with just some scuffs at the front near the underside. The soft rounded corners, light ochre case (once green?) with taupe red hard plastic parts and the gold grille and frame immediately identifies it as 1950s. No repairs were necessary to make this beauty come alive again after 30 years on a dusty attic, except replacing both DAF96s and tidying the battery compartment.