2.3 - THE EVOLUTION OF THERMIONICS
The triode age comprises the period 1913/35 when many innovations were introduced to the DeForest device.  These included high efficient cahodes, suitable materials for the plates and grids, new type of glass for bulb construction and advance technics in bulb evacuation.
In 1926, B.Tellegen, working at Philips laboratory invented the pentode.  Basically, the inner elements of this valve consist of an additon grid mounted between the screen grid and the anode.  This third grid was later denoted the suppresor. 
During the thirties other families of valves appeared in the market: the output tetrodes and the beam-power valves, the catode ray tubes, the tuning valve and finally the metallic and miniature valves.
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Dr. Lee de Forest and his invention the thermionic valve, the spherical Audion which was the fundamental of Electronics. The photograph was published in the American magazine "Radio News" in May 1930.

Illustration of the first types of triodes commercialized in U.S.A., circa the Twenties, the left for the right: triode of DeForest type DV5 triode type WE 205D manufactured by the W. Electric and a Socket for setting of the valve.

American and european valves

AMERICAN AND EUROPEAN VALVES

LEGEND
ORIGIN
TYPE
MANUFACTURER
DATE

1

USA
DV-3
DEFOREST

1923

2

USA

101D

W.ELECTRIC

1924

3

USA

24-A

RCA

1929

4

FRANCE

E

SFR

1922

5

ENGLAND

AC2/HL

MAZDA VALVE

1928

6

HOLLAND

C-509

PHILIPS

1925

7

HOLLAND

KK2

PHILIPS

1933

8

USA

UX 280

RCA

1927

9

USA

UX-201-A

RCA

1921