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ID = 19573
Great Britain (UK)
Brand: Osram-Valves GB (not MO-Valves)
Developer: Marconi's Wireless Telegraph Co. Ltd. 
Tube type:  Transmitting Triode, air cooled 
  <1925 very rare. ***
Identical to MT7A
First year 1922 ?? "The Saga of Marconi Osram Valve" by Vyse/Jessop.

Base TRANSMITTER-Base in gneral
Filament Direct
Description Thanks to Adri de Keijzer, Delft, NL who found the information and Dr. Ruediger Walz who cared for we can show you the following:

The details below were found in: "The Saga of Marconi Osram Valve", a history of valve making by Barry Vyse and George Jessop, ISBN 0-9539127-0-1 (hardback), ISBN 0-9539127-1-X (Paperback).

Page 78:
MT7A, MT7B: Very large valves operating up to 12kV with molybdenium anode yielding continuous anode dissipation of 600 watts and peak dissipation of 1kW. The MT7B became famous as the modulator valve used in the original BBC transmitter 2LO, see chapter 7.

Page 126:
The original Marconi transmitter 2LO, set up at the top of Marconi House in the Strands in 1922 was of a type that would have been recognised as a Marconi 6kW type-Q transmitter though actually, it slightly pre-dated the type-Q. It was designed by Captain H.J. Round and is described in Marconi Leaflet 1066.

Page 128:
The 2LO sub-modulator consisted of a single MT4B. The output was applied to the grids of seven MT7B power output modulator valves operating in parallel. The valves of both amplification stages operated from the same 10kV rail. The combined anode current of the MT7B's was passed through a choke which was common to the anode circuit of the main RF power amplifier. ...

From a table on page 127:
Filament voltage 15V, Filament current 10A.

MT7A or B were produced by MOV at least from 1922 and at least until 1932, the closing date of the early transmitters. It was somewhere noted, the letter M indicates that such tube was designed for Marconi. Other MOV transmiting tubes from that time only had the letter T in their type code.

Note by E.Erb: My tube seems to be made before given to MOV because it reads Marconi's Wireless Telegraph Co. Ltd. England (MWTC). But maybe this was the sales company?
Stokes, page 129: "On the British scene the first transmitting valves were developed by H.J. Round for the Marconi Co. in 1915 ... by 1922 htere were 5-kW water-cooled types in producrtion". At this same time air-cooled silica envelope types of up to 15-kW rating were being produced for naval use."
My knowledge (not on good grounds) is that by 1919 the MT1 to MT6 existed and MT7A was first produced in 1920. 
Text in other languages (may differ)
Dimensions (WHD)
incl. pins / tip
185 x 545 x 280 mm / 7.28 x 21.46 x 11.02 inch
Ernst Erb

MT7A: Data Sheet
Ernst Erb

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