Type RZ1-150 is a directly heated (4 V at 4 A) full wave rectifier and is rated at 1,000 V, 150 mA, which is no more than a type 5R4GY.
This exhibit typifies a large group of rather odd valves supplied by Mullard to the Admiralty during the 1930s. One almost wonders whether Mullard had a standing contract to supply a dozen (say) specimens of anything new and duly obliged with anything the Philips empire could dredge up!
Specimens in this class usually carry the Mullard logo in red and are often marked 'ONYX' for no reason that we understand. This is no exception. The anodes have a wide cavity with twin inverted V oxide coated ribbon filaments. The 5R4 has a sigle ribbon filament per anode.
The L2 base with the three part pins. This specimen is unused.
The wide glass tube envelope is 53 mm in diameter and, excluding the L4 base pins, is 170 mm tall.
References: Private communication & 1043. Type RZ1-150 was first introduced in 1934.