The finalised design has a generous gain Audio Amplifier that can even drive a loudspeaker via a transformer and actually needs a volume control on stronger stations!
The finished Hergé radio (with reaction for volume)
The theory is that using a loop aerial as the tuning coil (with a feedback coil on it too) will remove the need for a separate aerial and earth. Also less local electrical noise will be received compare with a whip or long wire aerial.
The aerial loop / tuning coil has 5 coils wound side by side. When you put 2 x 0.6mH inductors in series you get 1.2mH, put them in parallel you get 0.3mH. But if they are very close the two inductors become ONE inductor with twice the turns. Inductance of a simple coil is proportional to N2 so you get 2.4mH if two coils wound with each strand side by side are in series. If they are in parallel you get the same inductance, but it’s as if you used fatter wire or “Litz” wire, the “Q” is increased. In this case I have 4 coils for tuning and the 5th is the feedback or tickler coil. Putting four in series should be 2.4mH, fine for LW. two pairs in parallel/Series is 0.6mH, fine for MW. All four in parallel is maybe 0.3mH, and should do about 1.7MHz to 4MHz. Or 3.5MHz to 6MHz if you have a smaller tuning capacitor.
Note the first coil is wound with about 4.5mm spacing and each subsequent coil has each turn beside the previous, with about 0.8mm to 1mm pitch (about 0.9mm average). Each coil is about 16 turns (the number of turns depends on the overall arm size, the prototype loop is 41cm tall and 41cm wide using about 7.6cm width of a 9cm wide baton ).
The circuit concept dates from the 1920s! Only a 1.5V AA cell for “filament” (LT = Low Tension) and a few PP3 batteries for HT (High tension).
With an external amplifier it will work with just 2 x PP3 batteries (16V to 19V HT). With a “crystal earpiece” very like those in the 1940s to 1960s (available today in Maplin for €3) and 27V to 90V HT it is audible with no extra amplifier. You’d need a strong station and good ears for 27V (3 x PP3) but it’s quite good at 6 x PP3 (about 45 to 50V). At 8 x Alkaline PP3s it’s quite loud!
With suitable C13, C14 and coils it will work on LW, MW or SW.
How it works
With a Pentode rather than Triode operation the advantage is that you don’t need an RFC (Radio Frequency choke) on the Anode. The screen grid is essentially acting as a psuedo RF Anode and the actual anode as AF Anode.
Regeneration is essentially “Q” multiplication by using the screen grid for controlled positive feed back to cancel the losses of the coil (which lowers Q). The Q becomes very high just before oscillation. The Pentode characteristics are such that the valve is also a mixer, so at almost oscillation the carrier mixes and creates baseband and twice frequency components (What people called Homodyne/Syncrodyne, though these are regarded as two diffferent modes by some writers). Since the Anode is decoupled at RF (220K and 470pF), only the modulation is left on the anode.
I can only test MW at night as there are no MW stations near. But Five Live and “Absolute Radio” 1215 very clear from Twilight.
R1 RTE 252 LW during day with very faint BBC R4 and slight “growls” on other LW frequencies.
Anyway, no diode detector needed.
Normally there is a series resistor bypassed with capacitor to bias the grid more negative. At the 18V to 22V HT (PP3s sitting on the filament supply) and grid biased to 0.25V below “f-” (the 12 Ohm resistor between -LT and f-) the bias is OK. Adding such a “grid leak” scheme makes it tend to oscillate at low audio frequencies.
Also there are usually a couple of RFCs (Radio frequency chokes) with problems of self resonance. The “reaction” or “tickler” coil feedback is often adjusted with a variable capacitor for historic reasons, now a 50K Ohm pot is much easier to get. A 100K may be a little better.
The Aerial is often coupled via a 3rd winding. If the aerial is very long or 50 Ohms, then a 3rd winding with about 1/3 or 1/4 the number of turns of the “tickler” or feedback winding may be needed. A Short whip is very high impedance and the series coil helps match it and reduce its tendency to affect the tuning. I used a “resistor” sized 1mH coil. You can wind an air core one.
Next to try is a tuned loop so no separate aerial whip is needed. It will have two windings and replace the coils. If the main winding is two parallel windings of 35 turns on about 18″ (45cm), I think switching from parallel to series will quadruple the inductance an thus the one loop will work for LW & MW.
The colours are the “standard” large ferrite rod aerial colours. If winding your own make the “tickler” or feedback coil about 1/4 turns of main coil and beside it on ferrite, or on top at “earthy end” on air-cored. If there is silence at all positions of pot, reverse connections on the feedback winding. It should get noisier as pot slider moved from HT resistor. If near a station it will squeal if turned to high. If turned full it will be totally silent again as the circuit simply oscillating. So adjust pot over full travel before deciding the feedback wires need reversed.
1.0V to 1.35V is fine on the filament. These are not like D-series battery valves, which are designed for direct connection to Carbon-Zinc, these are designed for NiCd batteries. Hence 0.9V to 1.4V limits in data sheet. The extra 0.25V across filament resistor really optimises the grid bias at these low HT volts.
Battery valves historically used nominal 22.5V, 45V, 67.5V and 90V HT packs and sometimes 6 x 1.5V “D” size Zinc Carbon cells in parallel for “LT”. Performance was best with 90V. The 22.5V was very poor and only used in hearing aids. Some packs combined the LT & HT in one box. Though the batteries where expensive, they lasted much longer than a modern DAB Radio. The large cabinets (often wood) and 6″ to 8″ loudspeakers meant that late model Battery portables sounded far better than the early 1960s transistor radios that killed them off.
Approximately 170KHz to 270KHz on LW and 550KHz to 1200MHz on MW.
4 x AA Alkaline nominal6V LT @ 21.5mA (about 120 hours), 4 x C cells would be better match to HT life
4 x PP3 Alkaline nominal 36V HT @ 700uA (about 250 hours)
4 x 1j18b valves. (Cost under 3 Euro total excluding postage)
No external aerial or earth required.
Note that higher voltages are dangerous, especially above 50V. There is also risk of fire and burns or even explosions if NiMH are used to make higher voltage packs.
The latter half 1950s Battery portable radio sets used a loop aerial in the lid. They used 125mA LT 1.5V and about 6.5mA HT from 90V pack. Line up DK96, DF96, DAF96 and DL96. Usually LW & MW. The earlier 1950s and late 1940s miniature valve sets used DF91 which is twice the filament current. They had more variation of valves and design and up to 250mA LT 1.5V current.
The “Hergé” uses 24mA from 4 x AA cells (6V) and about 1mA from 4 x PP3 (36V)
The chassis is a piece of plywood (Hardboard or MDF will do too) mounted on battens deep enough to house PP3 battery. If you want C or D cell instead of AA cells for the “filament” supply (LT = Low Tension), then use taller battens.
My base is 26 x 17cm, but a bit wider would be fine.
The front panel is easiest to do neat if 2mm plastic sheet. But Aluminium or MDF can be used. Perspex / Acrylic is difficult to work without cracks. A disk type motorised tile cutter will cut Perspex or thin Aluminium sheets. My front panel is 12cm x 26cm x 2mm.
The coil / aerial is supported on two bits of panelling wood 41cm x 8cm x 9mm with the tongue and groove split off. This makes the coil “diameter” vary from 30cm to 42cm
The “cross” is supported a few cm above the base by a 3rd piece of panel board.
The 3 x 1.5V AA cell (with two dummy cells if only one valve) and the 4 x PP3 batteries are mounted on a plastic or wooden board screwed to the bottom. (was 8, but four is fine)
The valve is mounted on screw terminal block, though you can use veroboard/stripboard.
You can see the slow motion drive variable capacitor mounted to base. The front panel is scored with a line at height of centre of the shaft. A small screw driver blade was poked through and the hole enlarged by twisting a scissor blade till it was required size. You can use a vertical drill. Then the other controls and socket mounted at same height.
For convenience of re-wiring and for those inexperienced in soldering you can cut the excess plastic off sides and one end of screw terminals and fit them on the Pot and Tuning capacitor.
You can see the PP3 batteries only need one clip, cut in half:
The next page in the series will discuss the circuit revisions and design of the coil.
The radio is working on MW and LW, though in Limerick MW only at night.
It’s a bit faint, and WWII versions such as “sweetheart” that used an external aerial (more signal) used three valves. So there is an addon amplifier option.
A four pole two way toggle switch is mounted on the “aerial frame”. The main tuning coil is two coils wound in parallel, that are wired in parallel (more Q) for MW and in series for LW (x4 Inductance). The green wire at the edge is 5 turns for the “Tickler” or feedback coil across the “regenerative feedback” control pot. The waveband switch also adds parallel capacitor on LW.
Front panel Tuning scale
Print at a suitable size for your front panel. Of course many of these stations don’t exist any more. Athone on LW is actually Summerhill, Near Trim, Co. Meath, Ireland. Home on LW is R4.