Quote:
Originally Posted by turretslug
One thing to check (apart from its basic safety!!) is that the revised power arrangements don't compromise the warm-up protection associated with the delicate DM70 filament.
Colin
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Good point Colin. Well the thermistor had been removed when the transformer was grafted in so there was no protection from the instant voltage applied. I've no idea (yet) if the DM70 is good or not but I'm sure I have a spare somewhere. Anyway now that I've decided to use a capacitive dropper, the switch-on surge is not a problem. The series/parallel heater arrangement requires about 70 volts at 200mA and last night I monitored the voltage at switch-on. It ramps up over about 12 seconds to the required voltage (well it's the right voltage now since I added another 0.22uF in parallel with the existing 2.7uF) so I don't think there is any danger to the DM70. It will be worth checking the voltage across the 8 ohm shunt resistor to make sure its at or near 1.4V.