A preamplifier input terminal which is connected to the pick up tube by a direct current connection is connected to a control electrode of a transistor , to {TAIL ~ a terminal} of a resistor and, in accordance with the invention, by a diode to {HEAD ~ a terminal} carrying a reference voltage 
A preamplifier input terminal which is connected to the pick up tube by a direct current connection is connected to a control electrode of a transistor , to { TAIL ~ a terminal } of a resistor and , in accordance with the invention , by a diode to { HEAD ~ a terminal } carrying a reference voltage
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