The Marconi-Osram Valve Co. Ltd. has been formed by Marconi and GEC in 1919 and a year later the name became M-O-Valve or M.-O. Valve. Production was made by GEC in Hammersmith. All valves with the bearing Marconi we list under Marconi-Osram (MO-Valve).

Marconi and Osram used following tube codes for “Broadcast receiving tubes”:

A: Indicates an experimental type of tube adapted for commercial use, but not essentially for broadcast receiver

B: Indicates a Double Triode for class “B” applications

D: indicates a Diode, either single or double electrode construction¸ also used in combination in case of multiple tube including diode and amplifying elements (like DL145).

G: Indicates gasfilled tube

GT: Indicates gasfilled triode or gasfilled relay

H: Indicates a High Amplification Factor Triode

L: Indicates a low impedance Triode

N Indicates a Output Triode

Q: Indicates a Double Pentode for “Q.P.P.” push-pull application

S: Indicates a Screen Grid Triode

U: Indicates a Power Rectifier, either for half or full wave rectification

V-in conjunction with “S” indicates a Variable Mu Screen Tetrode

W: indicates variable Mu H.F. Screen Pentode

X: Indicates a Frequency Changer

Z: Indicates a Straight H.F. Screen Pentode

In some cases, code letters may be employed to designate particular features of the type, e.g.:

      GU: Gasfilled power Rectifier

      DL: Diode + low impedance Triode

Some old tubes do not follow this nomenclature, e.g:

      PX: Formerly indicated a Power Triode (PX, etc.)

      PT: formerly indicated a Power Pentode (PT2, etc.)

      M:  formerly indicated an A.C. Main Tube (MS4B, etc.)