22-08-2022, 03:35 PM
(21-08-2022, 07:05 PM)boater sam Wrote: A special hearing aid valve?
No, too high power and high consumption if sample is 4 Ohms cold. Also they used Pentodes except in 1920 & 1930s
This DF70 is typical of hearing aid type
https://www.radiomuseum.org/tubes/tube_df70.html
0.625V Vf so two in series for LT. 25mA, so about 25 Ohms when hot, likely more than 13 Ohms cold.
Maybe two 22.5V HT packs like in AVO. Or even one.
Admittedly it looks a bit similar.
Transmitter or high power oscillator on a proximity fuze is possible. Also used on bombs so that it would explode before hitting the ground rather than impact making a deep pit. I suspect fuze applications more like metal detector / Therimin circuits than actual radar.
Curiously battery powered hearing aids existed before valves!
Used a carbon mic and 2,000 + 2,000 ohm earphones. But a sound insulated capsule had a single 2,000 earpiece (driven with battery & chest mounted carbon mic) and carbon microphone (driving headset via a battery) sharing a single diaphram. Probably used 3V. A pair of old phone handsets wired in series with a 6V 996 pack using bottom and top wire of our fence was a great toy telephone. Those used carbon mic and 600 Ohm earphone.
