Philips Portable Radio

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.

Continue Reading

Grid Dip Meter

Only a handful of components are needed to build an instrument which is indispensable in any RF workshop: the grid dip meter or ‘dipper’. The main function of this clever piece of test equipment is to determine the resonance frequency of unknown tuned circuits within a certain range (typically, 1.5 to 80 MHz). Apart from this, the dipper doubles as an RF signal generator and an absorption frequency meter. Some amateurs even use it to repair radios or tune short-wave antennas! The operation of the grid dip meter is based on the principle of energy absorption. An oscillator produces RF energy through an inductor. When this inductor is brought in the vicinity of another inductor with the same resonance frequency, the latter ‘draws’ energy via inductive coupling. Because the oscillator is equipped with an RF output level indicator, the energy loss is easily detected as a ‘dip’ when the instrument is tuned, and hits upon the resonance frequency of the unknown inductor. The name ‘grid dipper’ is historic, and a remnant of the days when this instrument was built using valves (electrically, the nearest equivalent of valve grid is the gate of a FET). The oscillator in the grid dipper may be modulated with a fixed tone to make its signal easily identifiable on a short-wave tuning scale (where chaos may reign). Basic Circuit and Operation The heart of the circuit is an RF oscillator. The oscillator is usually a Colpitts design. In other words, the tuned circuit has a capacitive tap. Other oscillators, for example, the Hartley oscillator, use a tap on the inductive element. A capacitive tap is made by connecting two capacitors in series, and then in parallel with an inductor. The junction of the two capacitors forms the capacitive tap. The capacitive part of the tuned circuit is formed by two series- connected halves of variable-capacitance diode (‘varicap’). Each coil gives a different frequency range, and is plugged into a socket. The tuning in each range is accomplished by varicap. The adjustable voltage required for the tuning of the varicap is supplied by a potentiometer. So, turning this pot tunes the grid dipper across the relevant frequency range.

Continue Reading

Energy Storing Devices

Unfortunately, the sun may not shine just when you need electrical energy. The reverse is also true: energy may not always be required when the sun supplies plenty of it! In addition to the solar cell array, a stand-alone system requires another important component: an electrical energy storage device. The first device that comes to mind for this function is the battery, which is available in many different shapes and structures. Apart from special battery types, including chloride-zinc, iron-sulphide, lithium, nickel-iron, silver-zinc and sodium-sulphur, a number of which are still under development, familiar types such as the lead (gel) acid, NiCd and NiMH batteries are widely used for this purpose. These batteries feature a high and fairly constant capacity, nearly loss-free current acceptance and delivery, and extended durability despite many charging/discharging cycles. Lastly, they are almost maintenance free. The lead plates of special batteries for solar systems have selenium or calcium doping instead of antimony as used in car batteries. These special batteries are marked by high cycle repeatability, excellent charge efficiency, low self-discharging, high immunity against deep discharging and overcharging, and, unfortunately, a high price! Of course, there are exceptions. Some online stores sell top products for low prices. Check before you buy! Just as with solar cells and modules, batteries may be connected in parallel or in series. When doing so, it is essential to use batteries of the same type, with the same capacity, nominal voltage and charge condition. You can also connect the batteries in parallel and in series (few batteries in series and then few such lines in parallel). But this increases voltage and current that is needed to charge them. Reverse Current Protection The reverse current protection diode prevents the battery from discharging itself via the solar cell. Irrespective of its operating principle, the control circuit avoids overcharging of the battery. A deep-discharging protection should be integrated in all cases.

Continue Reading

From Solar Cells to Solar Panels

A single crystalline solar cell supplies a no-load voltage of about 0.6 V, independent of its size. A cell made from amorphous silicon produces a slightly higher voltage of about 0.8 V. Under normal circumstances, i.e., assuming a normal cell size of 10×19 cm, the power output is relatively low at 1.2 to 1.4 Watt. Consequently, cells have to be joined into solar panels (or modules) before usable currents and/or voltages become available. As with batteries in a torchlight, cells are connected in series to obtain a higher output voltage. Conventional solar modules supply a no-load voltage of between 15 V and 22 V, which indicates that they consist of up to 40 series-connected solar cells. To raise their output voltage, solar cells may be connected in series. Similarly, parallel connection may be used to raise the output current. In many modules, cells are connected in parallel as well as in series. The size of the cell surface determines the maximum output current, which is usually indicated as the short-circuit current (i.e., at an output voltage of 0 V). Available versions range from small amorphous cells with an output current capacity in the micro-amps range, right up to square-metre size modules from monocrystalline silicon with an output short-circuit current rating of more than 5 A. Several identical modules may be connected in parallel to obtain a higher output current. The output voltage then equals that of a single cell. Finally, it is also possible to resort to a combined parallel-serial configuration. Strictly speaking, 20 cells connected in series should be sufficient to charge a 12 V battery. In practice, however, a solid margin should be designed into such a system. Unfortunately, the output voltage of a solar cell is not constant. In fact, it drops with increasing temperature, and decreasing brightness of sunlight. This effect is far more pronounced with polycrystalline cells than with monocrystalline types. Because of this, the voltage characteristics of the relevant cells or modules should be studied before a solar power system is planned and built. To achieve the highest possible output power, the cell or module should be operated at the so-called maximum power point, MPP, at which the electrical output power reaches its maximum. The MPP shifts with light intensity and cell temperature. Inside a module, the individual cells are connected in such a way that the lower part of a solar cell is always connected to the upper part of another cell. Professional modules constitute a symmetrical glass assembly with a layer structure: melting adhesive foil, solar cells, melting adhesive foil, glass. High-end frames consist of stainless V4A steel.

Continue Reading