Mullard describe the ECC91 as a double triode with a common cathode for use as an RF power amplifier or oscillator. They further state that an output power of 1 Watt may be obtained from an ECC91 in a push pull oscillator at 250 MHz. This would involve an anode voltage of 150 Volts and a common grid resistor of 2000 Ohms. Each anode would dissipate 1.5 Watts.
Each anode has a flat face with side wings and is a flat bottom U in shape. The grids have been wound conventionally on copper supports by the notch and peen system. However, after winding one side has had the wire shaved off leaving a single sided grid that passes flat across the anode and cathode.
Here the spacing between cathode, grid and anode can be clearly seen. Also the cut ends of the grid wires are easily seen on the left hand grid support.
The thin glass tube envelope is 18 mm in diameter and, excluding the B7G base pins, is 46 mm tall.
References: Datasheet & 1040. Type ECC91 was first introduced in 1956.