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The EML PX4 mesh tube is upwards compatible to the 15 Watt Version of the historic PX4 by OSRAM and MARCONI. The EML PX4 has higher heater current, and also higher maximum power ratings.
The PX4 is a well balanced design by Osram and Marconi. It works best at the recommended operating points. The higher dissipation as possible for the EML is only for extra lifetime and more safety.

Features
Gold Plated Grid
Soft rubber suspended tube base
Hard metal Construction (Note1)
Extra large getters
Hand blown Glass bulb
Anti-microphonic Anode- and grid suspension
These tubes are shipped in a high quality dual box
Tube printing with 24k gold, and red color burned into the glass
Gold Plated, black ceramic socket
Description
We quote from the OSRAM data sheet: 'The PX4 is a Directly Heated Power Triode for the output stage of receivers and amplifiers, where a considerable output power is required with a maximum anode (to Grid) Voltage of 300 Volts. For this purpose the tube has exceptionally good characteristics, and is particularly suitable for operating moving coil speakers at large volume without distortion'

Application
In accordance with the historical datasheets, EML PX4 is most suitable for applications for medium output power.
There is some resemblance with the 2A3 or AD1 tube, which can also operate nicely at 250V 60mA, same as the PX4. Yet PX4 is a tube with somewhat higher impedance character, positioned somewhere in between 45 and 2A3. This makes it a very good tube for high efficiency loudspeakers, similar to what OSRAM wrote already, now almost 100 years ago.

Filament / Heater
The EML PX4 uses a heater current of 1.6 Ampere, vs. only 1 Ampere for the historical tubes. The EML heaters are specially constructed to have lowest possible hum, which is lower than historical tubes. For best results we recommend DC heating. This becomes increasingly important if a tube is used for very high efficiency loudspeakers. Please note, for all directly heated tubes, the best DC heater schematic is using the same as for AC, and apply DC voltage to it, instead of AC.

Anode dissipation
Very early types had 12 Watt Anode dissipation. Later types were upwards compatible, and had 15 Watt dissipation. These can be recognized by the working points of 15 Watt in the datasheet such as 300V 50mA and 250V / 60mA. These 12 Watt tubes and 15 Watt tubes have essentially the same tube curves, but of course you will see other recommended bias resistors in old data sheets, to achieve 12 Watt or 15 Watt dissipation.

Correct Bias
The EML PX4 is intended to be upwards compatible with original Osram / Marconi tubes, with the only difference being the higher heater current of the EML. By OSRAM, the PX4 was designed for low to medium Anode voltage, and indeed the tube works best with the Anode voltages as specified in the data sheet. As the Anode voltage is relatively low, it is recommended to use a classical auto bias circuit, as this keeps the cost and construction effort low. Unlike the historical tubes, the EML tubes have some hidden reserve as you can see from the Peak Anode Dissipation of 17 Watt. This makes it possible to bias the tube in idle at maximum 15 Watt indeed, whereas under peak signal level, the dissipation may shift up for a short time to the peak level 17 Watt.

On the other hand, you can take any PX4 amplifier designed for OSRAM tubes, and replace those by EML PX4, and bias will be the same, but care should be taken, the amplifier can deliver the higher heater current of the EML.