. Electronic apparatus for biological research . Paraphase amplifier valve HT- Figure made exactly equal to unity by making R^ rather greater than R^. i?i and i?2 may reasonably be about 5 times the values of the anode loads of the valves, and the value of the A'-shift control is, of course, determined by the HT negative supply available and the bias required by the paraphase valve. Screen stabilization of the latter is only necessary in the highest class of apparatus and it is often possible to omit the stabilizer tube, or even to replace the pentode by a triode, here. The performance o
. Electronic apparatus for biological research . Paraphase amplifier valve HT- Figure made exactly equal to unity by making R^ rather greater than R^. i?i and i?2 may reasonably be about 5 times the values of the anode loads of the valves, and the value of the A'-shift control is, of course, determined by the HT negative supply available and the bias required by the paraphase valve. Screen stabilization of the latter is only necessary in the highest class of apparatus and it is often possible to omit the stabilizer tube, or even to replace the pentode by a triode, here. The performance of the circuit at high sweep speeds is improved by shunting R^ and R2 by equal small capaci- tors, say of 25 pF, to compensate for the input capacitance of the amplifier valve. Post deflection acceleration ()—This is a recent development with electrostatic cathode ray tubes by which high deflection sensitivity is com- bined with good brightness and focus. The scheme is to use rather low ^j, A 2 and A^ potentials so that the electrons pass slowly between the deflector plates, and then to accelerate them in a further electrostatic field. electrodes may be formed by one or more annuli of deposited graphite on the inside of the flare of the tube envelope (Figure ). These are connected to potentials considerably positive with respect to ^3 and the deflector plates. For example, the type 1601 BCCA(F) cathode ray tube has V^^ = 3-5 kV, V^^ = 7-5 kV and K^, -- 10 kV. Another, extremely elegant, scheme is to use a single electrode in the form of a deposited graphite spiral {Figure ) along which there is a potential gradient, so that the accelerating field is continuous rather than stepped. 459
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