What is the PX4 vacuum tube?

The PX4 is a triode vacuum tube designed for power (set) applications. British DHT classic..

What is a PX4 substitute or equivalent?

Direct equivalents to the PX4 include PP5/400. These share the same pinout and operating envelope, so they can usually be swapped without circuit changes — though batch-to-batch variations between manufacturers (NOS vs current production) may shift the sonic character noticeably.

What are the key specifications of the PX4?

Amplification factor μ ≈ 5, transconductance gm ≈ 6 mA/V, plate resistance rp ≈ 830 Ω, max plate voltage 300 V, max plate dissipation 15 W, heater 4 V / 1 A.

Is the PX4 still in production?

Production status varies by year and manufacturer. The current Amperatubes catalog classifies the PX4 as Rare availability. NOS (new-old-stock) examples from RCA, Mullard, Tung-Sol, GE, Sylvania and Telefunken trade among collectors; modern production where it exists comes mostly from JJ Electronic, Electro-Harmonix, Tung-Sol (New Sensor), Genalex, and a handful of small-batch makers.

What does the PX4 sound like?

British DHT classic.. Sonic character also depends heavily on the circuit operating point, the surrounding tubes in the stage, and the output transformer when present — use the interactive loadline calculator on this page to explore the trade-offs.

PX4 Triode Vacuum Tube

Also known as PP5 · 400

Power triode delivering 15W for push-pull and SET designs

TriodeHeater 4 V · 1 AUX-4Osram 1930

Plate dissipation: 15W
Max plate voltage: 300V
Transconductance: 6mA/V

PX4

Characteristics

Datasheet values, once each. Every value cites its sheet.

HeaterDirectly heated

Specifications of PX4 — Heater
Heater voltageVh	4 V
Heater currentIh	1 A

Limiting valuesDatasheet maximum ratings — not to be exceeded

Specifications of PX4 — Limiting values
Plate voltageVa max	300 V
Plate dissipationPa max	15 W

Mechanical

Specifications of PX4 — Mechanical
Base	UX-4
Rarity	Rare
Pinout

Small signalDatasheet parameters at the reference operating point — not limits

Specifications of PX4 — Small signal
Transconductance	6 mA/V
Plate resistance	830 Ω
Amplification factor	5

Capacitances

Specifications of PX4 — Capacitances
Grid–cathode	9.3 pF
Grid–plate	13.5 pF
Plate–cathode	5.8 pF

Charactereditorial reading, not a measurement

Specifications of PX4 — Character
Gain	5%
Power	38%
Drive	99%
Linearity	95%

datasheet cited · model or editorial reading · catalogue value, no citation attached yet · nothing invented

Typical operation

Conditions published by the manufacturers, one row per condition. Nothing is computed.

Typical operation
Setup	Va	Vg1 / Rk	Ia	Pout	Source
Class A, single-endedtriode connection · cathode bias	300 V	1 kΩ	50 mA	4.5 W	OCR, to be checked Marconi · 1956 · p. 2 · transcript

Using this tube

Applications

SET output
Push-pull output
Headphone output

Generic triode power (set) stage

Hi-Fi stereoHeadphone amp

Replace or compare

Sources

By Jean-Michel Falligan DevergneReviewed May 29, 2026Active model : Norman Koren

Manufacturer datasheets

4 manufacturer datasheets cross-checked for PX44 verified

PX4 DatasheetLocal mirror · PDF

SPICE models

Revision and verification status

Source: Osram
Reviewed: 2026-05-29
Status: DataModelCalc
Model: Norman Koren

Context

The PX4 is another Marconi-Osram directly-heated power triode from the 1930s British tradition, rated for about 3.5W in single-ended operation. It shares the family resemblance of the PX25 but at lower power, with an intimate, detailed presentation that devotees consider superior to the 2A3 in triode purity. The 4V filament and UX4 base distinguish it from American types, and its low plate resistance (~1040 ohms) provides good output transformer coupling. Genuine Osram and Mazda PX4s are exceedingly rare collector items, making this a tube for the dedicated connoisseur.

History

Introduced: 1930
Manufacturer: Osram
Era: Pre-War

METRIX TESTER