The Mullard EL84 is a classic audio pentode valve, immortalised in the 1950s 5-10 design for an audio amplifier.
The UL84 was designed with a 100 mA heater, for use in universal voltage environments and thus with limited HT available. Our exhibit carries the Mullard and the words British Made - the genuine article.
This exhibit has had a hard life and as well as the external dirt the glass has internal discolouration.
On a DC mains supply no transformer to increase the HT voltage is possible so the UL84 is designed to operate at lower anode voltages and increased anode current. By the time the UL84 was popular the reason for using a universal design was to save the cost of the safety isolating mains transformer.
The UL84 can be used for push pull operation but the normal service would have been in single ended applications such as radios and record players.
The PL84 is another variant but with a 300 mA heater. The main application was TV frame output but audio was also possible.
The anode box with side flanges for extra heat dissipation. The discolouration of the glass speaks of a hard service live.
The cathode is rectangular and the copper control grid supports are topped with heat radiating fins.
The thin glass tube envelope is 20 mm in diameter and excluding the B9A base pins the valve is 70 mm long.
References: Data-sheet. Type UL84 was first introduced in 1955. See also1955 adverts.