The 808 was designed as an RF power amplifier or power oscillator. It can be used at full input up to 30 MHz and with reduced input up to 130 MHz.
The filament is of thoriated tungsten and runs at 7.5 Volts and 4.0 Amps. The anode is substantial and glows cherry red at maximum input, the grid is connected to a side contact. The grid is a set of three support wires fixed to a ceramic spider, upon which the helical wire grid is wound. The maximum negative grid voltage is -400 Volts.
The filament and grid assemblies are secured to a collar clamped to the pinch stem. The anode is supported only at the top of the envelope.
The RCA logo. Also the grid connection is by tape to the clamp.
The anode appears to be graphite with a metal shell as support and with side flanges for a greater radiating area.
The tightly wound helical grid.
We have another 808 that has been broken at both the anode and grid entry points. By lifting the anode clear of the grid the filament can be seen as a thick wire helix of quite a wide pitch.
The broken valve is a General Electric 808 and shows a significantly different construction to the RCA version.
The balloon envelope is 65 mm in diameter and, excluding the UX4 base pins, is 134 mm tall.
References: Datasheet & 4061. Type 808 was first introduced in 1938.