LP4

The LP4 is a directly heated Ferranti audio output triode linked by our sources to the PX4 family. A Ferranti advertisement in Wireless World on 8 March 1935 shows it with other valves in the range: evidence of commercial availability, not proof of the exact introduction year. The four-pin B4 base carries anode on 1, grid on 2 and filament on 3 and 4. Radio Valve Data, 1958, gives the filament as 4 V at 1 A; voltage, current and exact type must stay together when comparing equivalents lists.

The Ferranti table in Radio Valve Data, sixth edition, describes single-ended class A LP4 operation. It gives 250 V anode, −36 V grid and 48 mA anode current, with 5.5 mA/V transconductance and 870 Ω anode resistance. Bias resistor is 700 Ω, load 2500 Ω and output 3 W at 4% distortion. These values form one coherent table example; individual numbers must not be replaced by figures from a different PX4 edition or construction.

The PX4 relationship explains differing data. An Osram PX4 sheet specifies 250 V maximum anode voltage and 12 W dissipation; Marconi’s July 1956 sheet raises these limits to 300 V and 15 W. The latter describes a single PX4 in class A at 300 V and 50 mA with 3500 Ω load, giving 4.5 W at 4% distortion. Those data belong to that PX4 document and must remain separate from Ferranti’s 250 V example. Family correspondence does not guarantee identical construction or rating revision for every LP4-marked specimen.

The archived LP4 photograph shows a bright anode with welded dark fins inside a shaped bulb about 48 mm in diameter and 120 mm high excluding pins. Its associated descriptions propose Marconi-Osram origin with Ferranti rebranding: an attribution for that specimen, not certainty for all production. The 1935 advertisement shows a different physical profile. For the PX4 family, Osram also recommends referencing automatic bias to the electrical centre of the filament or heater-transformer secondary to reduce hum. With direct heating, the filament circuit also participates in audio behavior.