 A few special-quality tubes did not have a standard equivalent, e.g.the E55L, a broadband power pentode used as the output stage of oscilloscope amplifiers and the E90CC, a double triode with a common cathode connection and seven pin base for use in cathode-coupled Flip-flops in early computers.The E91H is a special heptode with a passivated third grid designed to reduce secondary emission; this device was used as a "gate", allowing or blocking pulses applied to the first, (control) grid by changing the voltage on the third grid, in early computer circuits (similar in function to the U.S. 6AS6).