Not just an replacement, but an improvement!

EML PX4 Mesh

Datasheet and Design Considerations

Uploaded: 27-Jun-2017 17:32

The EML PX4 mesh tube is equivalent to the 15 Watt Version of the historic PX4 by OSRAM and MARCONI, apart from the heater current. The EML PX4 has higher heater current. Since the historical data sheet gives no numerical value for Gm and Rp under normal bias conditions, the values for Gm and Rp were extracted from the historical tube curves.


The first owner can register the tube at the Emission Labs ® website, to participate in the 5 years guarantee program.


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'

EML PX4 Mesh Output Power Vs Distortion.
Single Ended Operation, at Ua = 300V, Ia = 50mA

3 Watt
Note, distortion is the tube only. Transformer distortion adds to this.


In accordance with the historical datasheets, we say EML PX4 is most suitable for applications for medium output power. Achieving lowest distortion is possible with historical tubes, or new build tubes, by choosing the load impedance slightly above the original recommendations of OSRAM. The Output Power can well achieve 4 Watt at full signal, when going to a Load impedance of 3...4k. Already at marginally lower signal, distortion begins to drop fast.

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. The sound of the PX4 comes closer to a 45, but the output power comes closer to tubes like 2A3 or AD1. This makes it a very good tube for high efficiency loudspeakers, similar to what OSRAM wrote already, a long time ago.

Filament / Heater:

The EML PX4 uses a heater current of 1.5 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. Altough the is specified for AC or DC heating, 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 superioir DC heater schematic is the same as for AC, just a DC voltage is applied 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 amplfier can deliver the higher heater current of the EML.




Side view (shows also gold grid)

Anode curves

Standard UX4 Base


EML PX4 Mesh Filament Ratings 
Filament Voltage 
4 Volt (AC or DC) 
Tolerance on filament voltage
Filament Current 
EML PX4 Mesh Maximum Conditions
Anode Voltage 
Anode Current 
Peak Anode Dissipation
17 Watt 
Maximum Continuous Anode Dissipation
15 Watt 
Grid resistor to Ground.

250k Ohm Original OSRAM
500k Ohm possible for EML

EML PX4 Mesh Factory Test conditions
Anode Voltage 
Anode Current
Operating Suggestions. Single Ended, Class A
Bias points correspond to this OSRAM curve
Missing data reconstructed in green, from OSRAM curves
Supply Voltage
Anode Voltage
Control Grid Voltage
Anode Current
Anode Dissipation
Bias Resistor
840 Ohms
708 Ohms
Load Resistor (Ra)
Gain (Mu)
Transconductance (Gm)
Anode Impedance (Rp)
1233 Ohms
1133 Ohms
Possible Lundahl transformers

5k6 to 4-8-16Ω.

4k5 to 4-8-16Ω.

4k to 8Ω

Estimated power Output
4.2 Watt
3,8 Watt


PX4-Mesh Mechanical Data

Size including Socket
(but excluding pins)

147 x 60 mm

Weight of one tube:
140 Gram

Pin 1: Heater1
Pin 4: Heater2
Pin2: Anode
Pin3: Grid

Shipment weight for 
pair in gift box: 730 Gram

Standard UX4 Base

  • Note 1) This operating point is taken from the historical datasheet of OSRAM. This is an 'emission test', under realistic conditions. This was done by operating the tube at only 100V anode voltage, then try to pull maximum Anode current, at zero Control Grid Voltage. This is quite difficult for much used tubes. This method was further refined by the Funke company, but probably started by the AVO company . In this particular test, documented in the OSRAM data sheet, the transconductance was measured by going from 0Volt grid , to a small negative voltage, obtaining a Gm value that way.
  • Note 2) Historical datasheets praise the PX4 as a tube to achieve large output power at low distortion. This must be seen in the light of that time, the 1930's, and the main purpose of PX4 was to produce high output power. For this reason, recommended load impedance by OSRAM was relatively low. Today, requirements about distortion are higher. Of course you can use the OSRAM recommended load Impedance, but it may be a consideration to use higher load impedance, as recommended in 1930. This leads to a small loss of output power, with significantly less distortion, and better damping factor of the bass speaker, so HiFi results are better. Like OSRAM recommended 4k at 300V/50mA, today a value of 5k6 is a very good idea.

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