Decca Debonaire Deluxe TP60 mains conversion.

[julie_m]

With germanium transistors, the difference between a survivable temporary overvoltage and a fatal overvoltage is small. So the use of a regulator helps prevent noise pulses on the mains from reaching the radio supply. You would really need separate regulators in each of the positive and negative lines. It's not a single 12 V supply, but two 6 V supplies in series, providing + and - 6V with respect to the midpoint connection.

If you build a proper split supply, you won't need the electrolytic. It is just there to provide an AC return path for the loudspeaker when using a single supply with no midpoint connection; it will charge up to half the supply voltage when the set is switched on, after it is charged then there will be no DC through the speaker.

EDIT: Crossed with everyone

[TrevorG3VLF]

The original circuit uses two 6V stabilised power supplies (batteries).

The simplest circuit has been suggested which is a big capacitor in series with the speaker. The current used will be very dependant on the volume.

[julie_m]

By my reckoning, the most the set will draw from a single supply will be just shy of half an amp (12 V / 25 Ω, plus an inevitable bit for the RF and IF stages and any losses), for a brief instant while the added capacitor charges. Then, never more than about 250 mA on the loudest sound it can make (+ or - 6 V, because the other side of the speaker is at 6 V with respect to the negative rail, / 25 Ω, plus the rest), dropping to only the idle current when the speaker is silent. This kind of amplifier uses ideally all, but in real life most of the energy supplied to it for moving the speaker cone; so the more the speaker has to move, which will depend on how loud it is playing, the more current it will draw. Your power supply is a constant-voltage device (constant-current power supplies do exist, for certain specialist purposes e.g. lighting ballasts) so will deliver as much (up to a maximum) or as little current as the load on it demands. If it is regulated or stabilised, that means the output voltage will not change with varying current demand. But the output voltage of an unregulated supply will increase above the nominal value as the current drawn decreases.

I personally wouldn't want to use an unregulated 12 V power supply with a germanium radio. At least, not without first measuring its output off-load and on-load .....

[Nickthedentist]

Yes, go regulated! It adds very little to the cost or complexity.

Would one of these do, if you replaced the 7805s with 7806s, giving +/- 6V outputs?
http://www.ebay.co.uk/itm/LM7805-LM7...IAAOSwoJZXSvui

Nick.

[sobell1980]

Many thanks for all the interest in my thread. After a bit of a heated discussion with the wife ( and it is her radio that she chose ) she feels she does not want it tampered with and modified being powered from the mains. So we have come to a compromise with re chargeables. She didn't want it to be seen being powered by a modern transformer and a mains lead coming out of the back and would rather we bought a more modern or mains powered radio than make her little set onto something it was never meant to be. So I've been out this evening and bought a battery charger and batteries. However, I would like to thank you all for your input. The replies are all valued and I've learned something anyway from the links you have posted and replies on building a power supply. Also, thanks again Lawrence for the article on push pull circuits / stages. A bit of home work for me. Many thanks.
Dave.

[Nickthedentist]

That sounds like a good solution, Dave.

You might want to consider adding a couple of fuses, as rechargeables can deliver a very high current under fault conditions, which can cause a lot of damage.

Nick.

[sobell1980]

Thanks, Nick . Will do. And again thanks for everyones input.
Dave.