SHIELDED GRID VALVE

Radio Record, Volume I, Issue 42, 4 May 1928, Page 14

SHIELDED GRID VALVE

TUNED IMPEDANCE COUPLING CONDEMNED. — An Aancriean radio writer, in refer: ting to a new circuit, says:--"A very special feature of the radiofrequency amplifier circuit of the receiver is that it does not employ the tuned-impedanee coupiing, which has been believed necessary to the operation of shielded-grid valves. This type of coupling is, inherently, extremely broad in tuning and is far from desirable; though circuits for tuned-imped-ance coupling are given with the data sheets accompanying shielded-grid valves as a theoretical, but not necessarily a practical, means of operating these valves. This type of coupling is highly undesirable in a radio receiver which is to be sufficiently selective on inodern broadcast-receiver conditions, and it introduces circuit losses occasioned by the necessary grid-blocking condenser and grid leak which seriously impair the amplification possibilities of the shielded-grid yalve; for, unlike the practice im previous radiofrequency amplifiers, regeneration is not employed in a shielded-grid amplifier and may not well be utilised to offset: circuit losses. "Tt is apparent to engineers that the amplification obtainable from a shield-ed-grid receiver is dependent upon the excellence of the tuning cirenit (coil and condetiser) making up the R.I. amplifier stage ; aud the selectivity upon the degree of coupling of one valve to the next through the tuned circuit."