[Mr 1936]

Hi Katar83

I have had a look at the Murphy data sheet. As far as I can work out, the HT current is about 68 mA for each output valve and 53 mA for the rest of the set. This gives a total of 68 + 68 + 53 = 189 mA.

The sheet quotes 80 volts total across the field coils. For the radiogram this means L30 and L40. Taking 80 volts and 189 mA implies a total field resistance of 423 ohms and a dissipation of 15 watts. That's 212 ohms and 7.5 watts per speaker. The data sheet quotes 200 ohms for both L30 and L40 so that tallies.

The console model has L30 listed as 400 ohms, so they must have used a different design of loudspeaker. A resistor field alternative would be 390 ohms with a rating of at least 10 watts. I would suggest a 'metalclad' conduction cooled type, bolted to the chassis with a smear of heatsink paste underneath. The alternative would be a ceramic bodied type spaced well away from other components (its surface is designed to be hot) with a free airflow around it. Your present 260 ohms is too low, and will be increasing the HT by about 26 volts.

The HT ripple reduction will be less than for a choke, but given the other chokes still in circuit you may not notice. If 100 Hz hum is too high, try increasing the value of C62. Don't increase the reservoir C64 as this would stress the rectifiers.

Regarding speaker voice coil impedance, the radiogram used the two speakers in series and my guess is that they were both 3 or 4 ohms. Since the console speaker had a different field coil, it might well have had a different voice coil too e.g. 6 or 8 ohms, but who knows. The same output transformer is shown for radiogram and console chassis, so the output stage will 'expect' to see 6 to 8 ohms. Having said this, don't get too hung up on exact speaker impedance as its not that critical.