This type of tetrode was developed to correct deficiencies in the triode tube which became apparent when attempts were made to use triodes as small-signal radio-frequency amplifiers. In the triode, the control grid was next to the plate. Capacitance between these two electrodes caused instability and oscillation when both anode and grid were connected into tuned resonant circuits, as was the case in early radios, or in any application where the anode circuit presented an inductive load to the valve. Oscillation could only be avoided by using very small stage gain at frequencies above about 100 kHz, and at frequencies above 1 MHz, triodes are virtually useless in tuned amplifiers in which both anode and grid circuits are tuned to the same frequency. A second advantage of the tetrode, which also arose from reduced anode-grid capacitance, was that, when the anode voltage was in a certain range, changing its value had much less effect on the anode current than was the case with triodes. This corresponds to an increased anode slope resistance, and allowed much higher voltage gain by permitting a larger external load resistance. At the time of the introduction of screen-grid valves (around 1927) a typical small triode used for small-signal amplification had an anode slope resistance of 20 kilohms or less, and a grid-anode capacitance of 1 to 5 pF, while the corresponding figures for a typical screen grid valve were 1 megohms and 0.004 pF - in some cases much less capacitance.