EF50

Small Signal Pentode — Loctal

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Philips

↗ r-type.org ↗ TDSL ↗ Radiomuseum

Known Equivalents & Substitutes

CV1091CV1578VR91VT25063SPTZ90

Similar Tubes (Not Drop-In Compatible)

6BA6

American equivalent by function (RF pentode, similar gm) but Noval base — different socket

EF50 Mullard

British-made versions (Mullard, Cossor, Brimar) have identical specs but wartime production quality varied significantly

EF53

Philips derivative with improved characteristics — same B9G base, listed as equivalent by Brans but VR91A designation suggests slightly different specs

EF54

Higher-frequency version of EF50 family — same B9G base, verify pin compatibility

EF86

Later Philips low-noise pentode but Noval (B9A) base — completely different socket

Electrical Specifications

Absolute Maximum

Anode Voltage (max)Anode Voltage (max operating)300V

Screen Voltage (max)Screen Voltage (max operating)300V

Anode Dissipation (max)Anode Dissipation (max)3W

Screen Dissipation (max)Screen Dissipation (max)1.7W

Cathode Current (max)

Notes

The EF50 is arguably the most historically significant valve ever made. Described as 'next to the magnetron, the most important tube from World War II,' it was the key component in British Chain Home radar receivers and the Telecommunications Research Establishment's centimetric radar IF amplifiers.
Designed by Philips at Eindhoven in 1938, it pioneered the all-glass pressed-base construction (B9G/Loctal) with short internal leads for VHF stability. This manufacturing technology became the basis for all subsequent modern valve construction including the Noval base.
When Nazi Germany invaded the Netherlands in May 1940, Philips engineers smuggled the EF50 production tooling and a large stock of finished tubes out of Eindhoven to the UK, where Mullard set up mass production. Over 30 million EF50s were manufactured during the war by Mullard, Cossor, Brimar, and others.
The B9G base has a metal spigot locating key and was designed for VHF screening. Pin 9 connects to the internal metal screen/shell. Pins 3 and 4 are both screen grid connections (g2) — both should be connected for best HF performance.
The damping resistance rg1 (4 kΩ at λ=6m) is frequency-dependent — it decreases with the square of the frequency. This limits the tube's usefulness above ~200 MHz without special circuit techniques.
Triode-strapped data: no manufacturer data exists for triode connection (g2 tied to anode). Estimated from pentode parameters and EF91 analogy: mu ≈ 30–50, gm ≈ 5–6 mA/V, ra ≈ 6–8 kΩ. These are rough estimates only — the EF50 was never designed for triode-mode audio use and may exhibit poor linearity in this configuration.

Pin Layout — Loctal

1Suppressor Grid (g3)

2Anode

3Screen Grid (g2)

References

https://frank.pocnet.net/sheets/030/e/EF50.pdf