TRF Radios 1, Ferranti ZN414, MK484, Matchbox Radio, ZN415, ZN416, TA7642, HAC Radio

"Hi, MDS975 folk. I bought a MK484 from Maplin and didn't have a circuit for it so I entered the chip number into Google and I was taken directly to your site. You've given better and more detailed info on this than Maplin have and I certainly will be building the TRF soon. By the way I also LOVE pussycats. Yours are really georgous! Thanks for a great site! "
Liz Costa 2E1FQN

From Dave Summer:

Hello Mike

An interesting site. I have made valve TRF radios since a boy in the 50’s. I heard about 60 amateur countries on a two valve 1.4 volt set using AM. I know many hints and kinks about making these sets work. Another good circuit is to use a valve followed by a transistor. If you use a 6.3 volt mains valve, such as the 6AK5 or 6AM6, it can work perfectly OK with 6.3V HT as well, and is very sensitive. Of course, if you use an RF stage the set is isolated from the aerial, which is more steady in frequency. I find the 1.4 valves to be poor performers and prone to microphony.

If you want the set for short wave reception, you can use it in oscillating mode for CW, SSB and AM. In the case of AM, if you use a high anode load resistor, the circuit pulls-in slightly to the carrier. But if you do not like this, use a low resistor, then it does not. In practice, these two conditions are best obtained using a pentode with its screen acting as the oscillator anode. Use either a high screen dropper resistor, or a low resistance 5k potential divider for the screen depending if you want pull-in or not.

Modern valves are very high gain, and will work down to a few volts of HT. It is necessary to avoid too much regen. A good way is to use a trimmer as the grid condenser, as a small capacitance of say 5 pF may be all that is needed. If you use a small grid condenser, you are in fact tapping the grid down the coil – it is an impedance matching action.

Always locate the grid condenser right at the grid, with a short lead; this prevents hum pick-up.

As for aerial coupling, the best way is via a small trimmer. In this way, the loading can be adjusted. A coupling coil is not so satisfactory and may introduce unwanted resonances and dead spots.

A resonant aerial is not desirable as it causes rapid changes in loading across the band. Choose a length such as a third of a wavelength.

If you want bandspread, a good method is to tap the bandspread capacitor down the coil, until it just gives the swing you want.

As far as hand-capacitance goes, this is a big problem and makes the use of traditional baseboard sets more or less useless. To avoid hand capacitance you must have the set in an enclosed metal box, and the phones lead must be decoupled.

RF chokes are a bit undesirable, but the Hartley oscillator (cathode tapped into the coil) avoids their use. Regen can usually be obtained using a resistor instead of a choke.

If you use an iron cored choke in the detector anode circuit, I believe the circuit will suffer from threshold howl.

If you use a battery valve, or you need more gain from a valve with very low HT, you can take the grid leak to a positive supply, either filament positive or HT. This increases gain but reduces overload capacity.

Finally, you cannot use a long wave ferrite rod in a set using a power audio IC, as the chip ”radiates” noise in the long wave band which will be picked up by the rod, and the circuit will howl.

Finally, finally, although not TRF, a superb simple superhet can be made using a crystal controlled mixer ECH81 etc followed by an ordinary low frequency TRF.

I hope these things are of some interest!!

Dave