PX4 - A Brief History

The PX4 was a product of the English valve manufacturer the Marconi-Osram Valve Company (M-OV) of Hammersmith, London. M-OV was a division of GEC, and its products were badged as either Marconi or Osram. The PX4 started life around 1928, and remained in production until the late fifties. There was a significant upgrade in power in 1937.

In the Beginning

A Drawing of an Early PX4

The PX4 was developed as an increased power version of the earlier P410 valve. It used M-OV's dull emitter technology, with the cathode being coated using the barium vapour process.

These first PX4's had a sloping anode, which you can see in this publicity picture of a PX4. It is difficult to make out at all in a photograph of a real valve, because the barium vapour (azide) process leaves substantial and unsightly black deposits on the inside surface of the glass envelope.

An early PX4

Valves are notoriously difficult to photograph at the best of times, even more so if you have to contend with the barium deposits, so M-OV adopted the then-common practice of using technical artists to draw their publicity pictures.

Moving Forward

Another early PX4

The early PX4's used the classic balloon-shaped glass envelope, and there were three versions of this type, each with steadily increasing power and cathode current( 0.6, 0.8 and 1.0 amps).

At one of these stages, the anode was made vertical, and at the same time the cathode manufacturing process was changed from barium vapour to oxide-coated filaments. Now you could see through the envelope.

Changing Shape

An early domed-top PX4

In order to provide good mechanical support to the electrode structure, the envelope shape was changed from a balloon to a domed top, morphing from the balloon to the final shape via an intermediate 'balloon with domed top'.

The anode in these valves had a shiny finish. The top end of the electrode structure was supported by a mica disc assembly, which was located in the domed top by spring steel lugs. At the same time, the filament support was changed from glass beads and springs to coil spring tensioning from the mica disc.

A PX4 with finned anode

With the final change in envelope shape, to the straight-sided domed top, fins were added to the anode, which was still shiny. This change occurred in 1937 and resulted in the final performance figures of the PX4, see Technical Data

The Last Act

The Final PX4

Post World War II, in 1956 I believe, the final version was developed. The anode fins were dispensed with, and the anode was now oxidised grey.

Rod Burman also thinks he's seen tubular PX4's with shiny anodes but no cooling fins, and ones with black anodes as well as grey.


A KR Electronics PX4

Nowadays, PX4's are being re-manufactured by the Czech company KR Electronics of Prague. The classic balloon shape has been restored, with additional support of the electrodes to compensate for the loss of the domed-top mica location. Each valve is serial-numbered with a numbered tag attached to the electrode structure and supplied in a nice fitted box with warranty and individual test results.

In my opinion the KR PX4 equals the performance and sound quality of genuine New Old Stock Marconi-Osram PX4's.