Babani, International Radio Tube Encyclopaedia p. 30 & 31
History of the British Radio Valve to 1940 p.145 & 186
Universal Vade-Mecum, Electron Valves and Semiconductor Devices
Taschenbuch zum Röhren-Codex 1948/49
220HPT: Telefunken Werkstattbuch
Wolfgang Bauer
Just Qvigstad
220HPT: COSSOR VALVE MANUAL 1935-6
Jose Casas-Avalos