The 811A is a power triode for RF or class B modulator service. These valve found much use in amateur transmitters.
A pair as a modulator could provide a peak maximum power of 340 Watts but would usually be run below maximum.
A pair operating in Class C as an RF amplifier at HF (up to 30 MHz) would yield around 130 Watts of output for a DC input of 225 Watts.
The box anode has flat U section channels spot welded to the sides for extra surface area for heat dissipation.
The anode face view. Note the supports are ceramic and substantial.
The pinch components and base of the anode.
The box anode with the extra heat radiating fins. The anode is blackened for best radiating efficiency. The maximum continuous anode dissipation is 45 Watts. For amateur service (intermittent use) the maximum dissipation when operating rises to 65 Watts.
A closer view of the grid wires. The pitch is small.
The classic envelope is 61 mm in diameter and, excluding the UX4 base pins, is 143 mm tall.
Reference: Datasheet. Type 811A was first introduced in 1954.