Power cars 001 to 020 and 501 to 520 had conventional power converters. All other power cars (051 to 090 and 551 to 591) have power converters that use GTO thyristors which are responsible for the distinctive "melody" when the power car is brought up to speed. Power cars with conventional power converters weigh about 80.4 t, those with GTO power converters two and a half tons less. In October 2007, Deutsche Bahn tendered refitting two power cars with IGBT power converters instead of the GTOs. After field testing, up to 36 additional power cars were converted.