Old Tube symbol. There is a circle around the symbol indicating the glass envelope. On the left side a connecting wire comes in from the bottom and once inside the circle starts to squiggle like a resistor symbol. It ends before reaching the top left of the circle. In the center there is an upside down U which is the filament. A leed comes from each end of the filament to come outside the circle. To the right of the filament is a rectangle indicating a square plate seen just a little off of edge on. A leed connects to the plate and comes outside the circle.
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