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Part of the Ladybird radio circuit

T.R.F. Radios Employing The "Regeneration" Technique For Extra Sensitivity And Better Selectivity

The ZN414, MK484, ZN415 and ZN416 radios described on the "TRF Radios Part 1" page are so called straight TRF radios.  With a straight TRF radio the signal is passed straight through the circuit in stages, the RF or Radio Frequency stage, the Detection or Demodulation stage and the AF Audio Frequency amplification stage.  Such radios use no reaction or regeneration whereby a precise amount of feedback is used to boost the sensitivity of the circuit.  A popular design pre-dating the ZN' circuits was the H.A.C. (Heard All Continents) circuit.  The H.A.C. uses three transistors, two of modern NPN types for audio amplification and a rather more special FET (Field Effect Transistor) for the RF front end.   The H.A.C. Triple-T is described below.

My first real TRF radio was not the HAC, however, but a home made radio described in the Ladybird book "Making A transistor Radio":


"Making A Transistor Radio" by Rev George Dobbs  (G3RJV)

I was introduced to the construction of TRF radios as a child with the Ladybird book 'Making A Transistor Radio' written by Rev. G.C. Dobbs and published in 1972.  The book describes how to make a reasonably simple but very effective radio receiver.  Starting with a crystal set and then progressing in stages up to a powerful 3 transistor TRF radio that uses a technique called Regeneration (also referred to as Reaction).

This little set, when completed, used the first Radio Frequency transistor, the Mullard OC45, three times by implementing the regeneration method. Regeneration is positive feedback , similar to the howl-around produced when putting a public address microphone too close to the loudspeaker.  In a radio circuit the amount of this feedback has to be very carefully controlled and set to just the right level, just before it begins to howl.  In this state the radio is at its most sensitive and is really very selective. 
Mullard OC71 & OC45 Transistor source:

From Doug Wallace:  "I have a small supply of OC45 & OC71,which I am happy to share should anyone wish to build this excellent radio. Please contact me via this website. Regards, Doug Wallace".

Selectivity is the radio's ability to tune into one station sharply without hearing the sounds of other stations in the background.  A crystal set, by contrast, tends to suffer with quite poor selectivity and is also very insensitive, requiring a really good strong signal from a large wire aerial to operate.  A TRF radio, on the other hand, will often operate quite happily with just the signal picked up from an internal ferrite rod aerial, with no long wire aerial being necessary.

The Ladybird book design uses the old Mullard OC71 and OC45 germanium PNP transistors which are unfortunately no longer made, but some supplies of these germanium transistors can still be found if you are willing to search.  I still have my Ladybird set that I originally built in the 1970's and it works a treat, in fact I use it to listen to the football in the garage!

If you have difficulty in obtaining the germanium PNP Mullard OC71 transistor I am informed that it can be substituted with an AF125 which is an equivalent  device.  I think that the AF125 or the older AF114 or AF115 or AF116 could be used too.

The OC45 transistor seems to be rather more widely available today than the OC71, but I have not yet found an equivalent for this device, although one must exist. 

I am quite sure that with some research that modern silicon NPN transistors could be used in this circuit with some adjustment to component values and the reversal of the polarised components, such as the electrolytic capacitors, since with NPN transistors the battery will have to be connected with the negative side to ground, i.e. the opposite way around to the original circuit.

MDS975 reader, Denny Lin, usefully adds these comments about semicondutor availability:
"If you are planning to rebuild this radio, you could try some NTE transistors and diodes. Apparently the NTE102A can be used to substitute the OC45 and OC71 transistors (according to the NTE substitution web page at  this web address:;  The characteristics of the NTE102A seem to suggest that it's a good replacement for the OC71 and not for OC45; the suggestion of AF116 yielded an equivalent transistor of NTE160, which seems to be a good replacement for the OC45 transistor (since it's a germanium oscillator). The OA81 diodes may be replaced with NTE109. Radio Shack sells an Eagle LT700 equivalent (1K primary and 8 Ohm Secondary) transformer labelled 273-1380."


The Ladybird book describes a novel solderless technique for construction, using a wooden 'breadboard' and brass screws and screw cups to trap the leads of the components.  I have since rebuilt the radio into a smaller case and soldered the components to tag-strip for better and more reliable performance.

THE PLANS: See The Complete Original Plans by Rev. George Dobbs G3RJV here >

Here are some illustrations of the radio:

Breadboad layout
The Circuit Board Layout

First Stage - the simple crystal set radio
First Stage completed - the simple crystal set radio

This simple Crystal Set would be greatly improved with the addition of an aerial trimmer capacitor of about 100pF connected between Screw A and the Aerial Lead to reduce the damping on the tuned circuit (ferrite rod coil and 0.0005uF tuning capacitor) and also a resistor and a capacitor wired in parallel across the crystal earphone - Screw B and Screw 2. The resistor can be of a value between between 22k and 47k Ohm. The capacitor can be a simple disc ceramic with a value of about 0.001uF - often marked as 102. See capacitor and resistor charts >

Completed circuit board
Photo showing the first two transistor stages of the radio. 

The OC45 transistor, on the left, is the radio frequency and regeneration stage.  The OC71 transistor, on the right, is the first stage of audio amplification.  This is followed by a second stage of amplification, which boosts the audio output enough to drive a loudspeaker - shown below below:

Left and above: These photographs show the third transistor which is used in the final stage of audio amplification, again using an OC71 transistor.  The output impedance of the circuit is too high to directly couple to a 3 - 8 Ohm loudspeaker, so an Eagle LT700 step-down transformer is used to couple the speaker to the circuit.

These photographs show the 'Breadboard' method of construction using No.6 brass wood screws and screw-cups and a length of 1/2 inch thick by 4 inch wide planed softwood.   The wires and component leads are trapped underneath the brass screw-cups to make a connection at each junction point.  This is a good system for a beginner using solderless connections, but a tag-strip and soldered joints makes for much better connections and improved performance - so it's well worth practising a good soldering technique!


The coil for this circuit is wound on a piece of 10mm diameter ferrite rod which is 75mm long in the illustration above, but I have used a rod that is 150mm long, which improves the amount of signal picked up and the directional characteristics.  The coil is 50 turns of 36 s.w.g. enamelled copper wire close-wound, with a tap at 5 turns which is the regeneration (feedback) tap.  The 0.01uF capacitor is connected to the end of the five turn winding (the regeneration amount being controlled by the 10pF variable capacitor) as can be seen in the circuit diagram below:

Circuit diagram
Above:  The complete working circuit digram.  The radio performs very well indeed on the medium wave band, but the regeneration control has to be very carefully set for each station to extract best performance.  Tune in a station and adjust the 10pF variable capacitor until the radio falls into oscillation due to too much feedback (i.e. starts screeching), then reduce the amount of regeneration until the screeching just stops.  At this point the set will be at its most sensitive and its most selective.

The 10pF variable capacitor was originally specified as a trimmer, but I have modified this and used a 15 pF variable air-spaced 'tuning' type capacitor that is mounted through the front panel to facilitate much easier adjustment of regeneration by means of a knob on the front panel, alongside the tuning and volume controls.

The Radio Frequency Choke (R.F.C.) is about 4.7mH (milli-Henries) and any choke near this value should be okay.

The OC45 transistor amplifies the tuned radio signal once, and then again by use of feedback.  The diodes are used to convert the radio frequencies to audio frequencies (detection) which the transistor then also amplifies.

The two OC71 transistors add further audio amplification which will be enough to drive a loudspeaker via the Eagle LT700 transformer which converts a high impedance output from the final transistor (about 2k Ohms) to the low impedance of the 3" loudspeaker, which should be between 3 and 8 Ohms.  the 10k log potentiometer is the volume control.

Breadboad layout
The Completed Ladybird Radio

Rehoused TRF radio        TRF Radio
Above: My own re-housed TRF radio, with components soldered together onto a tag strip.  The left-hand knob is the volume control and on-off switch, the centre knob the regeneration control and the right-hand knob is for tuning.

A Ladybird book -


Rev Gorge Dobbs (G3RJV)

In August 2005 the author of the Ladybird book "Making A Transistor Radio", Rev Gorge Dobbs (G3RJV) himself,  wrote to us here at MDS975:

"Hello Mike,
Lovely to see a reference to the book.  That was a long time ago!  Found your site when browsing for regen' receivers.  I am thinking of yet again looking at them in my monthly Practical Wireless column.  Good to see the HAC - I remember them well!
Have fun.
My best wishes,
George, G3RJV"

The Ladybird Transistor Radio by Dave Bullimore

Hi Mike,

I discovered your excellent site a little while ago, and I was very interested to see the Ladybird radio mentioned. I built one of these as a teenager back in the early ‘80s, and loved it. Sadly, it didn’t survive very long as I needed the parts for other projects, and ever since I have wanted to build another. Finding the original parts are very difficult to obtain, I decided to challenge myself to see if it were possible to make a version of the Ladybird radio using modern silicon transistors.

For the project to remain valid, I decided that the original Ladybird circuit should remain unchanged, and only the bare minimum of component value changes allowed. In the end, I found it only necessary to alter the values of five resistors, and change the semiconductors. I also used a modern tuning cap.

The radio works well, but as I didn’t have a germanium version to compare it too, I decided to build one of those as well!! I have noticed some interesting comparisons between the silicon and germanium versions:

The germanium radio needs a decent length aerial to produce any amount of volume, whilst the silicon one works much better with just an earth connected.

The silicon audio amplifier is rather unstable, and tends to break into oscillation at higher volume settings, especially without the earth connected. I found a 0.01uF capacitor connected directly across the primary of the LT700 makes the amplifier much more stable. On the germanium version this made very little difference.

The silicon radio seems to prefer a slightly lower battery voltage, say about 8V. I used an 8.4V NiMH PP3 battery rather than an alkaline one.

The silicon radio takes about 20 seconds to ‘warm-up’, presumably due to the 680K resistor taking time to charge the 100uF cap.

Finally, the output OC71 on the germanium set does get rather warm, especially when there is little or no signal.

I have attached some photos, and a schematic for the ‘Silicon Ladybird’. You are more than welcome to use any of them on your site.

Keep up the good work!!

Kind regards,

Dave Bullimore.
Derby, UK.

September 2013

Ladybird Transistor Radio by Dave Bullimore
Ladybird Transistor Radio by Dave Bullimore
More photographs here >

The "Ozzie Ladybird Transistor Radio" by Shane Way

Shane Waye sent some photographs of his impressive project in June 2013. The attention to details in reproducing George Dobbs' design is remarkable:

Hello Mike,

I came across your website while sourcing components to complete my replica of the Ladybird radio.

I thought I would share some photos of my completed radio.  My intention was to attempt to reproduce the radio as closely as possible to the original version as published back in 1972. I feel that the completed radio is about 95% accurate. 

One of the first problems I quickly ran into was the requirement of 4 x 1/2 inch wood stock which is not easily available here in Australia to make the case. I had the wood specially cut to suit.  Even sourcing the number 6 slotted screws and cups was difficult.  These came from a supplier in the Old Dart (UK). The 500pf Jackson tuning capacitor was probably the single most difficult item to find.  After months of searching the globe for one, I found that my next door neighbour had one sitting in his junk box and didn't know what it was. After some gentle persuasion and careful negotiation, I managed to procure it!

Most other components are authentic including the transistors, the trimmer cap, the Eagle LT700 transformer and the Erie 0.01uF ceramic cap. All the resistors are modern carbon film, but they look very similar to the originals.  I did deviate from the original by soldering the antenna leads to the tuning cap and to a solder lug on the Erie cap.  It was just too difficult to get a reliable connection by screw contact alone.

I did make a minor blunder though. I wanted to show the pencil lines that the original uses as a layout guide, but I did not account for the diameter of the screw cups and most of the numbers and letters are covered.  The radio works very well but I do have a problem with thermal runaway on the OC71 transistors. This was easily fixed by placing 1K resistors bypassed by 100uF electrolytic capacitors in the emitter circuits.  These are not shown in the photos for the sake of authenticity.

Best regards,
Shane Waye
June 2013

The Ozzie Ladybird Transistor Radio by Shane Waye

The Ozzie Ladybird Transistor Radio by Shane Waye

The Ozzie Ladybird Transistor Radio by Shane Waye

The Ozzie Ladybird Transistor Radio by Shane Waye
The Ozzie Ladybird Transistor Radio by Shane Waye
(June 2013)

Karen Orton's Radio

Hello Mike, Just thought you might be interested to see a photo of my most recent creation - my version of the Ladybird radio. It works very well for only three transistors!

I've used a polyvaricon type tuning capacitor and the ferrite aerial was from a certain high street UK chain. I've used an OC44 which, from all accounts, is more lively than the OC45. As a consequence the reaction trimmer had to be reduced in value quite considerably using a series capacitor. I've made the reaction trimmer adjustable from the front panel but adjustment of this trimmer is not that critical.

Usually, reaction controls are pretty critical but this one isn't. Oh, and I've included a 1k resistor bypassed by 100 microFarad in the emitter of the first OC71 to prevent overheating. I found I didn't need this on the second OC71 and in fact these extra components just reduced speaker volume so I left things as original.

Best wishes, Karen
March 2014

Ladybird Radio by Karen Orton
Ladybird Radio by Karen Orton (March 2014)

Mark Easton's Ladybird Radio

Hi Mike, As promised ages ago, here are some images of my Ladybird Radio. And a list of stations I can receive at night in Cambridge. I haven’t tried to identify all the foreign language stations, but Hermal Boel’s excellent website suggests what they might be. I used my Tecsun PL310 radio with a digital readout as a reference.

Mark Easton's Ladybird Radio
Mark Easton's Ladybird Radio (February 2014)

Very pleased with the radio. It works FAR better than the ZN414 radio I have prototyped recently but there is a real art to tuning it. Many of the channels listed below suffer from poor reception, but they are listenable.

549kHz  -  Germany
603kHz  -  Gold (Littlebourne, Kent) & France Info (depending on orientation of radio)
630kHz  -  BBC Three Counties (Lewsey Farm, Luton)
657kHz  -  Spain
675kHz  -  Radio 10 (Netherlands)
693kHz  -  BBC Radio 5 Live
711kHz  -  France Info
720kHz  -  BBC Radio 4 (Crystal Palace?)
729kHz  -  BBC Essex & Radio Nacional (Spain)
738kHz  -  Radio Nacional (Spain)
747kHz  -  NPO Radio 5 (Netherlands)
756kHz  -  Deutschlandfunk (Germany)
774kHz  -  Spain
792kHz  -  Gold (Zouches Farm, Bedford)
810kHz  -  BBC Radio Scotland
828kHz  -  Gold (Lewsey Farm, Luton)
855kHz  -  Spain
864kHz  -  France
873kHz  -  BBC Radio Norfolk (West Lynn)
882kHz  -  BBC Radio Wales
909kHz  -  BBC Radio 5 Live
1026kHz -  BBC Radio Cambridge  (Tends to over load the radio as I live just a couple of miles from the transmitter)
1089kHz -  Talk Sport
1197kHz -  Absolute Radio  (Tends to over load the radio as I live just a couple of miles from the transmitter)
1269kHz -  Germany
1332kHz -  Gold (Peterborough)
1422kHz -  Germany
1440kHz – Luxembourg RTL Radio (Marnach)

Thank you to Doug Wallace for sourcing the transistors.

Best Regards,

Mark Easton. (February 2014)

Mark Easton's Ladybird Radio

Mark Easton's Ladybird Radio

Mark Easton's Ladybird Radio
Mark Easton's Ladybird Radio (February 2014)

Yoga Raj's Ladybird Radio

Detailed below is Yoga Raj's very neat and faithfully reproduced Ladybird Radio project. Yoga has gone to great lengths to source the now difficult to obtain components and has constructed a very impressive radio!

Dear Mike,

I stumbled upon your website on TRF radios about a year ago.  At that time, it reminded me about the ladybird radio that I attempted to build while I was young but never fully completed because of incomplete parts. With your suggestions, I started collecting parts (ebay) and from all around the world to build one as good as the photos in the book. Finally,a few days ago, it was working well and much to my surprise, very well without an antenna.

I made every attempt to get the exact components where possible and stuck to the original photos on the book.

The changes made were :

1)       4.7k resistor was switched for a 1k resistor after the 1st transistor stage (gives less distortion)

2)       OA81 diodes were replaced for an IN34 diode (better detection)

3)       36SWG wire for the antenna switched for a SWG 32 wire

Do have a look at the photos and your comments / questions. I live in Dunedin, New Zealand and as you see from the dial, there are many stations that I can receive without an antenna here.

George Dobbs' Ladybird Book TRF Radio - by Yoga Raj
Ladybird Books - Making A Transistor Radio by George Dobbs
Yoga Raj's very neat project.

George Dobbs' Ladybird Book TRF Radio - by Yoga Raj
Close up photograph showing the tuned circuit, detector and regeneration stage.
Note that the ferrite rod is a full 6 inches (150mm) long and not cut in half, as in
the original design. This should provide better signal pick up. The ferrite rod
is held in place by two plastic P Clips that provide a very neat and effective
method of fixing.

George Dobbs' Ladybird Book TRF Radio - by Yoga Raj
Close up photograph showing the audio amplification stages. Note the care taken with the
additional PVC insulation added to the component leads.

George Dobbs' Ladybird Book TRF Radio - by Yoga Raj
Overall view of the completed radio. Note how faithfully the leads and wires have been
formed and placed on the board between the screw cup connectors. Yoga has also connected
a small on-off switch. An alternative method might also be to use a potentiometer with
an integral switch.

George Dobbs' Ladybird Book TRF Radio - by Yoga Raj
Photograph showing the competed circuitry.

George Dobbs' Ladybird Book TRF Radio - by Yoga Raj
Photograph showing the front panel and controls of the competed radio together with a
rather impressive looking loudspeaker. As can be seen from the tuning scale on the
right hand side, an impressive number of stations can be tuned in!

Grant Searle's Ladybird Radio

Grant Searle also kindly dropped us a note and some photographs of his own faithful and extremely neatly constructed Ladybird Radio:

"Hello there.
Thanks for a very interesting (and nostalgic) web site.  I was doing a Google search for MK484 and stumbled upon your site. I was most surprised when I found your page on the Ladybird book "Making a transistor radio". I got my mum to buy this book mail-order from Ladybird when I was about 8 years old.  I made the radio many years ago and still have it. It still works!!  I have taken a couple of photos of it (it sits proudly on my shelf, along with the book) for you.

I kept to the layout shown in the book, but turned the tuning capacitor, volume control and speaker around so that the whole board could lie flat. I couldn't get the RFC at the time so I wound my own.  As you will see, the speaker has rather faded (so has some of the wires) and the shiny brass plates on the tuning capacitor have dulled a bit but otherwise has lasted time rather well (I think I replaced a couple of the components some years ago as I raided it for some spare parts and knobs when I was desperate!). Obviously, the battery is new!  If these photos are of any use to you, please feel free to use them.  What impressed me was the whole thing only takes about 3mA so the battery lasts for ages.
While looking around your site I also see (to my complete astonishment!) that you also built one of my other first radios - the matchbox radio from Everyday Electronics. My magazine has, alas, parted company (and I think I know who I lent it to and lost it...) but I have the radio knocking about. I still remember the sonic bomber project from that magazine - a plane attached to a length of wood suspended from the ceiling with an electromagnet holding a ballbearing, if I remember correctly...

Anyway, it's great to see others have similar interests and experiences.


Photograph of Grant's neat radio showing the detector, regeneration and
audio amplification stages.

Photograph showing how Grant included the loudspeaker
on to the base board.

Views of Grant Searle's excellent and completed "Ladybird Radio"

Dominic Tasker's 'Ladybird' TRF Radio with tuning scale

Here is an interesting implementation of George Dobbs' radio. Dominic Tasker kindly sent us a description of his radio along with some photographs of his version:

Hi Mike, Big fan of your site which inspired me to re-attempt to build the Ladybird transistor radio. I first attempted it in 1990 when I was 10 years old; I bought the book for 5p from a church jumble sale and spent 2 weeks of my summer holidays going round radio shops and junk shops to try get the parts. That build was partially successful as it worked but I couldn't tune it in due to using the wrong tuning capacitor!

I built it recently exactly per the book, then the circuit slowly evolved (with help from your site) and my own trial and error, I have never built a radio before and I didnt really like the the wooden plank idea. I wanted to have The original design was rather deaf and under-amplified and now I pick up many more continental stations; I'm currently listening to a German jazz station!

I aimed at trying to build this from junkbox parts, some of the parts are; tuning cap pulley came from a scrap Leak Troughline tuner same as the scale pointer, the front grille and handle was from a fidelity portable that was smashed beyond repair. The diodes came from an old carousel reel to reel tape machine. I also wanted the components to look like components not specks of dust so I went all out and used 5w resistors from Maplin lucky bags and 'old school' looking electrolytic caps. The tuning system is made up of Mechano and a salvaged tuning cord. As you can see by my picture it is adjustable in the bottom left corner (rear view) on the lockable lever pulley.

The knobs were spares out of the knob box, bottom one painted black in the middle to match the top, the perspex front and back panel were out of a skip on an industrial estate. The antenna - source unknown - has been on the shelf for years and is held in with a fuse holder which came out of the reel to reel. The Celestion 3ohm speaker came out of another reel to reel. The battery is a copy of a 1970's ever ready PP9 that I made from a scan of an original, it is a very good copy.It has a modern ever ready PP9 inside it.

Yours sincerely, Dominic Tasker. Hull, East Yorkshire.

Dominic Tasker's TRF Radio with tuning scale
Dominic Tasker's TRF Radio with tuning scale

Dominic Tasker's TRF Radio with tuning scale
Rear view of
Dominic Tasker's TRF Radio with tuning scale

Dominic Tasker's TRF Radio with tuning scale
A better view of the circuit layout

Dominic Tasker's TRF Ladybird Radio

Dominic Tasker's TRF Radio with tuning scale
Dominic Tasker's TRF Radio

Rosha and Daniel's 'Ladybird' Radio

Hi, I finally managed to make my own Ladybird Radio in Iran. I bought that book [Making A Transistor Radio] in winter 1987. My last attempt was not successful because of mistakes in the third transistor connecting and some other faults. Since I didn’t find any “OC” transistors, I used a Japanese IF Amp transistor (2SA49) instead of OC45. It was acceptable. Other transistors such ac AF117 (General Tr.)  and AF126 (FM  IF  Tr.) didn’t  work.

All of Japanese 2SB series could be used. I  had some 2SB56, 2SB175 and 2SB376.  .

Because I  couldn’t find a radio frequency choke (r.f.c.) I used a Long Wave Coil [This would be of similar inductance]. Unfortunately my 9 volt battery was dead, so I connected a 9 volt DC power supply. My the only problem is noise.

I made some radios but I feel real satisfaction this time! Last night when I was ready to switch it on, my four year old son, Daniel, said to me: “Will you give it to me if it works this time?” ...and he owned it!

Update April 2011:

I have taken some photographs, as you see the radio works with germanium diodes OA79 and IN60 - obviously silicon diodes such as 1N4148 cannot be used.

I bought a tatty PHILIPS radio for just  $2.00 with the intention of breaking it for spares, but I repaired it and replaced my 2SA49 in the Ladybird radio with a Mullard OC45 (hilips I.F. stage). But I still use Japanese one and no re-alignment was necessary.

My radio (It is better to say Daniel’s one!) has 2 trimmers and a power indicator LED. When I connect a 9 volt adapter, the third transistor (2SB376 or 2SB75) warms up - so it is better to use lower ranges such as 7.5V .Connecting a 9V battery, noise will be reduced. I admit that I didn’t spend much time making it’s box.

Thank you very much for your help and advice.

Best regards,


See the complete plans here >

Other 'Ladybird Radio' Resources:

A kit based on George Dobb's three transistor radio design, featured above, is available on ebay (as of December 2010 anyway).
As Dominic Tasker suggests, "...he's got lots of kits ready to go for those that done have time to bumble around for parts." See this link:

Henry's very useful related web page highlights some technical pitfalls and solutions to problems that may be encountered along the way. It also explains why substituting alternative "equivalent" transistors may not work. For example there seems to be no suitable alternative to the OC45 in the RF section in this particular circuit configuration. While the OC71 could be subsituted for an OC81, for example, but still needs a small modification to the emitter circuit. For example if substituting the first OC71 with an OC81 then it will be necessary to connect a 1k Ohm emitter resistor in parallel with a 22uF bypass capacitor to ground, rather than a direct connection to ground. This will stabilise the d.c. operating point while maintaining the audio gain.

HAC Triple T

HAC stands for 'Hear All Continents', and when this radio is used as a short wave radio reception that is certainly possible.  It is very sensitive and reasonably selective too.  I have built one and received Japan, America, China, Australia, Poland and Turkey, amongst many many others.  This circuit was very kindly sent to me by a Short Wave Magazine Reader, Len from Hampshire, some years ago.

As with many such circuits of its day it used plug-in coils that were made by Denco Ltd of Clacton.  The coils are no longer available, but it is perfectly possible to wind your own as I have done and obtain very good results indeed.  I have provided the coil winding details below.

Note:  The MPF102 F.E.T. can be substituted with the 2N3819 F.E.T.

HAC trf radio


internal photo


Coil Substitutes For The Denco (Clacton) Green Range Of Coils

The old HAC circuit detailed above is quite easy to construct, however the design relies on the use of the DENCO (Clacton Ltd.) Green Range of coils which are no longer available.  It is quite straightforward to wind your own coils and the diagram on the right shows how and the table below details the number of turns and what gauge of enamelled copper wire to use.

Take a short piece of 10mm diameter ferrite rod (about 50 to 100mm long) and make a paper sleeve to wrap around the rod (for easy repositioning of the coil assembly once in use).  Over this paper sleeve wind the first coil (L1) and secure the wires 2 and 5 in place using some Sellotape.  Over the top of L1 wind the second coil (L2) and secure the wires 3 and 4 in place.  Finally alongside the first windings wind the third coil (L3) again securing the wires 8 and 9 in place with a little bit of Sellotape.

When wiring the coil into the circuit ensure that the wires are connected in the correct place according to the numbers shown.  Be especially careful not to get the wires of the regeneration coil L2 swapped over as the regeneration (reaction) effect will not occur. 

With a cursory glance at the schematic diagram and the drawing of the coil opposite you might expect that wire 3 from the regeneration winding (L2) would be physically next to wire 5 of the main tuning coil (L1) however wires 3 & 4 are effectively swapped over because the feedback from the Drain MPF102 field effect transistor is out of phase with its Gate and so would be out of phase with the radio signals in L1 if applied to the 'top' of the coil near the wire 5 end of L1.  Since regeneration is effectively controlled positive feedback it must be applied In-Phase with the received radio signal.

4 - 14MHz
10 - 30 MHz

Coil details

The diagram above (top) shows how the coil is physically wound onto the ferrite rod former while the circuit  diagram shows the importance of the lead connection numbering and how the coil is wired into the tuning circuit.
The number of turns for each winding on the coil and the gauge of enamelled copper wire used are just suggestions that happen to work on my radios quite well.  Other gauges of wire may be used and the number of turns adjusted until the required range is obtained.  In any event some experimentation with the number of turns will always be required to obtain the required tuning range and proper regeneration function.

L3 is not too critical as it is just a coupling winding for the aerial.  L1 determines the range of frequencies covered by the set and the number of turns can be increased to tune lower frequencies or decreased to tune in to higher frequencies.  The most critical winding is L2 and some experimentation will almost always be required here.  You may find that you can obtain reaction across most of the band but not all of it, in which case this winding will need to be adjusted by a turn or two.  I found the Medium Wave coil the most difficult to get just right and had to adjust the position of the windings relative to each other to obtain satisfactory results    

More About Coils

In general a medium wave coil wound on a ferrite has an inductance of approximately 350 to 400 µH (microHenries) while a longwave coil will generally have an inductance of around 4 to 4.7 mH (milliHenries)


Hi Mike, Congratulations on a great web site. I remember building one of the Ladybird book circuits back in 1975 and was quite pleased with the result. This was at a time when transistors seemed expensive. Nowadays, I have amassed quite a stock of radio components, the result of being an electronics hoarder.
I am looking at building the Triple T HAC receiver and noticed your nicely finished radio. I see that in the circuit diagram there is no provision for band switching and that I assume the three ranges being selected by plugging in the relevant Denco coil. In your receiver, I see that you have a rotary switch for the three ranges. Did you use a 4-pole 3-way switch? I expect that only three of the four poles are used to thus switch in each of the three windings for each coil. Is there any chance of a simple diagram for the wave band switching and a close up of the component layout on the tagboard. Also, in the circuit diagram, the main tuning capacitor is shunted with a lower value variable capacitor. I assume this is for band spreading and that any dial calibration will need to be done with the small variable capacitor in its mid position (half meshed).
Best Regards, Symon McCabe.

Hi Symon,

Thanks for your email. Great to hear from you! You are correct, the original idea assumed the use of plug-in Denco green coils. As you'll see, I wound my own coils for three ranges; MW and two SW bands. I did indeed use as 3-way 4-pole switch. I don't have a better photograph or diagram. If you get stuck with your construction, I will see what I can do with regard to doing something better. I wont promise to do anything quickly as we're rather up to our necks in other things at the moment, but I'll try to help if necessary.

Good luck with your project. The HAC is a really good TRF radio. Enjoy! Mike.

Hi Mike, Many thanks for your reply. I am looking forward to building the HAC Triple T shortly and will let you know how it goes. I am presently working on the DRC3 shortwave receiver by Sir Douglas Hall (Radio Constructor June 1972).

Best Regards
Symon McCabe.

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Andrei Lazarescu contributes a useful link!
Hello again! I found a very good utility for electronics - the DL5SWB Ring Core Calculator:
Click here to obtain the Windows program:

Best Regards,
March 2013
Inductor design program: DL5SWB Mini_Ring_Core_Calculator


The HAC company of East Grinstead, Sussex, produced radio kits for home construction radio enthusiasts and short wave listeners.  The three transistor circuit is described above, but Jake Haskell very kindly sent MDS975 photographs of his original HAC Model "T" Twin Transistor short wave radio instruction booklet.

You can download the PDF file of the HAC Model "T" Twin Transistor Radio Kit instruction booklet  HERE

You will also need to have a PDF viewer, such as Adobe Reader, installed on your computer - but this is a simple matter.  Visit  to download Adobe Reader.



Here is the circuit for my first Short Wave t.r.f. radio.  As you will see it is very similar to the HAC described above and uses the same type of coils, but with just two transistors it will only power an earphone effectively.  I spent many hours with this set as a kid listening to short wave stations from all over the world on a crystal earphone.  Most listened to broadcasts on this simple little radio were Swiss Radio International, Radio Netherlands, Radio Sweden, Radio Canada, Radio Moscow and The Voice Of Turkey - and many, many more.   I called it the TWO TRANSISTOR WONDER - because it was!
2 transistor TRF set

2 Transistor Wonder

The very old and rather battered but trusty little Two Transistor Wonder.  The case is original but the knobs have been replaced with more modern ones. 
(Note the several holes remaining in the front panel from previous modifications).

Inside view

The 'gubbins' inside the Two Transistor Wonder
A TRF Radio by James Marvin

Heya Mike and Jules,
First off, I want to congratulate you for your site, and the interesting information that you have posted on there. I am based in the Highlands in Scotland , UK.
I first came across your site September last year and I was particularly intrigued by the H.A.C radio schematic that I found on there. This inspired me to build a much improved version of the radio, and to this day I use the radio on a regular basis to listen to International Short Wave broadcasts, particularly the Beijing hour on China Radio International and the english language programs on Radio Prague, although I recieve broadcasts from pretty much every continent of the planet.
I use a random wire antenna, which is constructed primarily from 16 SWG tinned copper, about 90 foot of it, and I am very pleased with the way the radio has performed.
Attached, is a much improved schematic of the H.A.C radio and a few photographs of the radio itself. The coils are not inculded in the schematic, as I feel that a) You give enough relevent information on the Denco coils and how to make them and b) Anyone wishing to use the schematic will probably want to decide for themselves how many bands they want and how they want them configured. I have added a simple 500 miliwatt amp to the origional schematic and have done very minor modification to the origional circuit to improve performance.
I do hope that you will post the schematic on your site, with pics of the radio, if for nothing else but to inspire other hobbyist builders into giveing the H.A.C radio a new lease of life and to give many hours of enjoyment that I have recieved from this project.
Many thanks again for providing a very interesting site and I hope to hear from you as soon as you have the time.:)
Yours sincerely
James Marvin.

James Marvin's HAC TRF Radio
James Marvin's HAC TRF Radio
James Marvin's HAC TRF Radio
James Marvin's HAC TRF Radio

No AM radio stations or transmitters in your locality or country? AM Medium Wave Transmitter from Vintage Components

Has your local medium wave broadcast station closed or been moved to VHF/FM or Digital? Don't worry. You can still build and experiment with crystal sets and TRF radios by also buying or even building a simple low power AM transmitter. So, not only can you use your crystal sets but you can also run your own radio station that can be heard in and around your home - playing the music or programmes that you want to hear!

SSTRAN AMT3000 Superb high fidelity medium wave AM transmitter kits from SSTRAN. Versions available for 10kHz spacing in the Americas (AMT3000 or AMT3000-SM) and 9kHz spacing in Europe and other areas (AMT3000-9 and AMT3000-9SM). Superb audio quality and a great and well designed little kit to build:

SSTRAN AMT3000 low power AM medium wave transmitter

Other AM transmitters available:

Spitfire & Metzo Complete, high quality ready built medium wave AM Transmitters from Vintage Components:  Vintage Components offer a choice of the high quality Spitfire and Metzo transmitters:

SPITFIRE AM Medium Wave Transmitter with 100 milliwatt RF output power:
Spitfire AM transmitter from Vintage Components

METZO AM Medium Wave Transmitter with built in compressor:
Metzo AM Transmitter fromVintage Components

AM88 LP  A basic AM transmitter kit from North County Radio.

Worried About Soldering?  Don't Be:

For some simple ideas on solderless construction techniques have a look at the Crystal Sets 2 page.  When constructing these small electronic projects it will be necessary to determine the exact value of resistors, which are colour coded, and capacitors, which sometimes have confusing numbers on them.  I have included a table for both Resistor Colour Codes and a Capacitor Conversion Table HERE.


Readers Radios >

Including The Medium Wave Mini




LINKS to Other Great Websites


Visit my Amateur Radio pages . . . .
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