GB401673A - Improvements in resistance coupled thermionic amplifiers - Google Patents

Abstract

401,673. Valve amplifying circuits. DUBILIER CONDENSER CO. (1925), Ltd., Victoria Road, North Acton, London. -(Assignees of Nyman, A. ; Genehurst Apartments, Dobbs Ferry, New York, U.S.A.) Feb. 12, 1932, No. 4217. Convention date, Feb. 12, 1931. [Class 40 (v).] In a direct-coupled thermionic amplifier, in which resistances are inserted between the negative supply lead and the cathodes of the valves and are so chosen that the potential difference between the grid and cathode of the second valve remains substantially constant in spite of small variations in the supply potential, condensers are associated with the feed resistances, so that these potential differences may be made equal'in phase as well as in magnitude. Fig. 1 shows a screen-grid valve 1 having two resistances R1, R2 in the cathode side of the anode circuit, from the common point 10 of which direct connection is made to the grid of a power amplifier 2. As the grid of valve 2 will be positive with respect to the negative supply lead 4 by an amount equal to the voltage drop across R1 (which for optimum coupling should equal the drop across the valve 1), the voltage drop in R3 should equal this amount plus the requisite grid bias for valve 2. The screen grid voltage for valve 1 is derived from G on potentiometer R4 and the input is applied to the control grid and tapping 7 on resistance R2 to provide control grid bias. The connection H from the cathode through C2 compensates for hum introduced by the screen-grid tapping G. Resistance R2 is shunted by condenser C1 to ensure that maximum signal voltage developed across R1 is transferred to the next stage. In order that no hum from the supply mains shall reach the output of valve 2 it is necessary that the points 10 and 12 shall divide the total impedances of their respective circuits 5-4, 3-4 in the same ratio, this being effected by shunting R3 by a variable condenser C4. The potentiometer R4 may be dispensed with, in which case the screengrid voltage is taken from the anode supply through a voltage-dropping resistance and hum is prevented in this stage by a condenser between lead 4 and a variable tapping on the coupling resistance R1, Fig. 2 (not shown). The invention may also be used with indirectly heated valves having a separate heater circuit for each valve, Fig. 3 (not shown). Specification 398,826 is referred to.