Re: Pilot PT650 "Spacemaker" Television
The Pilot PT650 has a vey low valve count. Having only eleven valves in the heater chain means that the mains dropper is dissipating 31 watts of heat when the receiver is supplied from 240 volts.
The total voltage across the heater chain is 135V so 105 volts has to be dissipated in the form of heat, that's not good especially one you consider the frame timebase components are very close to the mains dropper resistor.
If we check out the number of valves employed in the BRC 1400 we find that this receiver also has a low valve count and the total heater chain voltage adds up to 140V, only five more than the Pilot. However, the 1400 employs a silicon diode in series with the heater chain so that it is only the negative cycles of the mains supply that provide the power. The negative going heater supply also provides the grid bias for the frame output valve.
Another good feature of having the valve heaters supplied from the negative half cycles is that it almost balances the demands of the HT supply.
The 1400 has a 128ohm resistor in series with the heater chain so if we assume a current of 0.3amp the heat dissipated by this resistor will be just over eleven watts.
DFWB.
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