Modifying the Murphy A40C power supply.

[ukcol]

Many of you will know that I am a big fan of the Murphy A40C. Probably the best set produced by any manufacturer in the late 1930s.

However the design of the power supply was not Murphy's finest hour.

If you examine the original circuit in the first picture you will see that if one of the rectifier valves fails all the load will be transferred to one half of the centre tapped H.T. secondary winding.

There is a very high risk that this large expensive transformer would be damaged by overheating in such circumstances.

I suggest rewiring the circuit to that in the second picture. If one rectifier fails the load on each half of the H.T. winding will remain equal so that the transformer is not put at risk.

I have included 1N4007 diodes in series with the rectifier valves to reduce the stress on the known to be vulnerable UU4 rectifiers.

Do not be tempted to used solid state diodes instead of valves as the H.T. will be high during the warm up period and damage may result.

When you replace C81 and C82 make sure you use high quality suitably rated components.