[Robert Gribnau]

Based on books by Philips dating from about 1939 to 1942 I deduce that around 1938 Philips thought that the EF22 (6.3 V/0.2 A), the ECH21 (6.3 V/0.33 A), the EBL21 (6.3 V/0.8 A), and the AZ1 (4V/1.1 A) series of valves would be all that was needed for consumer radios with 6.3 V heaters.

The same goes for their U-series of valves (UF21, UCH21, UBL21 and UY21).

For battery radios their range of valve types was much broader (DAC21, DBC21, DF21, DF22, DK21, DL21, DLL21, and DM21).

For 'special' applications Philips envisioned the "50" series of valves. Remarkable of this "50" series was that the valve bases are not identical:
- P-base: EC50, EL50, EL51
- Loctal 8-pin: EF51
- Loctal 9-pin: EC52, EC56, EC57, EE50, EF50, EF54, EF55, EFF50, EFF51
- Octal: DAH50
- Various: EA50, EA52, EC53, EC55

In some Philips publications of around 1940 more valves of the "50" series are described but they never made it to the production stage, most likely because of World War II. Some of the EF(F)5x valves mentioned above were developed by Mullard.