In 1930, the valve was still using manufacturing techniques from the electric lamp, where the glass was the basic element for the construction of the bulbs to encapsulate its inners elements. However, after the WWI the continuous evolution of wireless, the transmitters were requiring higher-powered transmission valves
(b) Cross section of the metal valve made in the USA.
Metallic valve pentode type 6J7. Also known following denomination:VT-91, VR-56, CV-1074
with their higher operating temperatures and associated cooling problens. In order to facilitate the cooling, the first high-powered valves were manufactured in such away the anode in the form of a copper cylinder, fused in the bottom of the bulb, having its outer surface totally exposed.
These types of valves were called "CAT", cooled anode transmitting. In the reality, they were embryo of the first metal valves, launched in Europe in 1933 by the companies Marconi and Osram. In those types of valves, having external anode the conventional glass mounting stem were replaced by a new assembling technique where the lead-out wires were arranged in a circle and sealed through the end section of the glass portion of the bulb. These new thermionic devices were called as CATKIN valves. (a)
In the USA, the first metal valves appeared in 1934 in a joint venture between the companies RCA and GE.
(a) The CATKIN valve

They used a different manufacturing process in such away the valve inner elements were assembled in a total metal bulb that improves the valve mechanical rigidity as well as actuating as an electrical shield. (b) During the manufcaturing process the enginners used specific types of material in order all the components could have the same thermal expansion coefficent. The metal valve had also a new kind of base, provided with a guide pin in order to improve its fitting in the chassis socket. (c)
The appearance of the metal valve revolutionized the manufacturing methods by introducing tight quality standards which soon reflected in the developments of new types of communication equipments for military applications whose performance was duly tested and approved in the scenario of the WWII. (d)

(c) Several types of American metal valves:
a) Type 6H6, double diode - b) type 6SJ7, sharp cutoff pentode - c) type 6SA7, pentagrid converter - d) type 6J5, medium mu triode - e) type 6SK7, remote cutoff pentode.
(d) Metal valve RV2P800, used in communication radios of the German army.
(e) Metal Valve type MG