History of the British Radio Valve to 1940 p. 145 & 167
Babani, International Radio Tube Encyclopaedia p.30 & 31
Universal Vade-Mecum, Electron Valves and Semiconductor Devices
Taschenbuch zum Röhren-Codex 1948/49
220PA: Telefunken Werkstattbuch
Wolfgang Bauer
Just Qvigstad
220PA: COSSOR VALVE MANUAL 1935-6
Jose Casas-Avalos