22-03-2017, 09:27 PM
I've started the restoration of a Double Decca radio. My example of this series of mains and battery radios employs miniature valves, the earlier model 46 used octals.
I'm told that these sets can give a good account of themselves, although my set doesn't perform all that well at the moment.
The circuit diagram of the model 46 appeared in this website in 2012:
http://golbornevintageradio.co.uk/forum/...p?tid=2817
Valve types apart, my set employs an almost identical circuit, the only significant difference is the method of biasing the output valve. The model 46 uses the usual resistor in the HT negative return to produce the grid bias.
The later set simply relies on the 7.5 volt filament supply to do the biasing for the Osram N17 output pentode. The grid is returned to chassis via the 2.2Mohm grid resistor and as the top end of the series filament supply is approximately 7.5 volts the correct bias for the N17/3S4 is assured. On account of the valve's series filament supply it could be argued that the mean bias for the valve is actually less than 7.5 volts.
Link to the Richards Radios website showing the similar model 46:
http://www.richardsradios.co.uk/ddecca46.html
I'll report back later on how the restoration of my Double Decca progresses.
Geordie McBoyne.
I'm told that these sets can give a good account of themselves, although my set doesn't perform all that well at the moment.
The circuit diagram of the model 46 appeared in this website in 2012:
http://golbornevintageradio.co.uk/forum/...p?tid=2817
Valve types apart, my set employs an almost identical circuit, the only significant difference is the method of biasing the output valve. The model 46 uses the usual resistor in the HT negative return to produce the grid bias.
The later set simply relies on the 7.5 volt filament supply to do the biasing for the Osram N17 output pentode. The grid is returned to chassis via the 2.2Mohm grid resistor and as the top end of the series filament supply is approximately 7.5 volts the correct bias for the N17/3S4 is assured. On account of the valve's series filament supply it could be argued that the mean bias for the valve is actually less than 7.5 volts.
Link to the Richards Radios website showing the similar model 46:
http://www.richardsradios.co.uk/ddecca46.html
I'll report back later on how the restoration of my Double Decca progresses.
Geordie McBoyne.
