The 6MB8 is a frame-grid pentode and hi gain triode used in colour TV receivers. The triode was designed for video-amplifier service. The pentode was used used in colour burst-amplifiers.
The triode section with screen plate behind.
The triode on the left with grid visible. The screen is in the centre and the pentode on the right.
The thin glass tube envelope is 20 mm in diameter, and excluding the B9A base pins is 47 mm tall.