[Mark1960]

74ls06 has ttl level inputs, 30v open collector outputs.

74c906 has Vcc operating range to 15v, absolute max 18v, but not really clear from that spec if the open drain output can be connected to voltage greater than Vcc. They seem to have missed that line from the spec below the max input voltage range. It has cmos inputs, so higher impedance, but also probably cmos switching levels so might not work well in a ttl level system.