Pro Electron took the popular European coding system in use from around 1934 for valves ( tubes ) , i.e. the Mullard -- Philips tube designation .
Pro Electron essentially re-allocated several of the rarely used heater designations ( first letter of the part number ) for semiconductors .
Pro Electron took the popular European coding system in use from around 1934 for valves ( tubes ) , i.e. the Mullard -- Philips tube designation , and essentially re-allocated several of the rarely used heater designations ( first letter of the part number ) for semiconductors .