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Mike. Watterson · 23rd Jan 2019, 11:11 am

Often a shunt selenium was used to have close to the recommended 1.35V. You need to add a series resistor so voltage is about 1.4. Then add 2 x 1N4007 (not any other 1N400x) in series across filament supply as otherwise if half of O/P valve filament fails the voltage would rise too much.
1.7 - 1.4 = 0.3V. Assume 130mA (the 125mA is nominal), so 300/130 Ohms = 2.3, so use 2.7 Ohms
1.35V is philips/mullard recommended.

23rd Jan 2019, 11:11 am	#10
Mike. Watterson Heptode Join Date: Dec 2014 Location: Limerick, Ireland. Posts: 901	Re: Roberts R66 diode MR2 Often a shunt selenium was used to have close to the recommended 1.35V. You need to add a series resistor so voltage is about 1.4. Then add 2 x 1N4007 (not any other 1N400x) in series across filament supply as otherwise if half of O/P valve filament fails the voltage would rise too much. 1.7 - 1.4 = 0.3V. Assume 130mA (the 125mA is nominal), so 300/130 Ohms = 2.3, so use 2.7 Ohms 1.35V is philips/mullard recommended.