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.
This second Mullard ECC91 exhibit is included to show the change of shape between production batches or production line or factory.
The physical construction can be seen here. The central cathode is a flat rectangular tube. The Copper grid supports are either side and each anode is a single plate.
One of the grids. The wire is fine and the pitch small.
A general view of the construction between mica supports.
The thin glass tube envelope is 18 mm in diameter and, excluding the B7G base pins, is 46 mm tall.
References: Data-sheet & 1040. Type ECC91 was first introduced in 1956.