I need 5V/600mA from a 12V DC power source.Main concern is the heat on linear regulators.Do not have modern Switch mode converters & needs to do with commonly available parts. I'm thinking to do with two 7805. Can it supply 5V/600mA without heatsinks?
No, each device will dissipate 2.1 Watts ( in an ideal Balanced condition - not guaranteed in this setup) and will need a Heat Sink to dissipate the heat.
Why not use a 12 Volt bulb in series with a single device, the major voltage drop being across the bulb? Of course, the bulb will glow red and dissipate heat.
The CUI VXO78-500 series is a switching regulator designed to replace a standard 78xx linear regulator which almost meets your needs. These modules supply up to 500mA and are less than two bucks at Digikey. These solved a tough problem for me, so others may be interested in them too.
600mA in 8 ohms= 4.8V. Then the input to a single 7805 regulator would be 12V - 4.8V= 7.2V and the regulator would heat with (7.2V - 5V) x 600mA= 1.32W. The regulator will be close to its maximum allowed temperatrure and might not last long.