Unsure about a circuit.

[Cliff Matthews]

You have the 2n3055 so get a $0.25 PNP like the TIP42 you can still use it (eg: http://www.instructables.com/id/5A-Variable-Linear-Power-Supply/ may be a quick example).  Lot of other examples on Google images too:
https://www.google.ca/search?biw=1600&bih=788&tbm=isch&sa=1&q=LM317+TIP42+2N3055
.. just pick one from a reputable site with EE user feedback

[davelectronic]

Yes i understand, a voltage drop of 1.5 Volts loaded is a little undesirable, i guess this is the poorer regulation by using the transistor after the regulator. I also understand using a modest pnp transistor to drive an npn transistor before the regulator. I might except the voltage drop under load. If not then use a MJ2955 before the LM317. Most of what i try is from searches of schematics. Thanks again for the help

[Zero999]

One of the disadvantages of using a PNP transistor to drive an NPN is lack of short circuit protection.

Another transistor and current limiting/sensing resistors can be added to give overcurrent protection. The attached schematic will limit the current to about 5A.

[David Hess]

Here is an alternative way to implement the current limiting from the National 1980 Voltage Regulator Handbook although it increases the dropout voltage.  The text also mentions how the regulator's thermal protection can protect the power transistor.

[Audioguru]

A 7812 regulator regulates as good as an LM317. In your circuit the output of either regulator has good voltage regulation but the emitter output of the 2N3055 emitter-follower is not regulated. The PNP transistor added to any voltage regulator allows a properly regulated output voltage.

[davelectronic]

Thanks for all your help, much appreciated.  I will go with the LM317 and a single pnp for now and see how it goes. I will load it up gradually to see how it performs. Really i only need 3 Amps at around a 50% duty cycle. But a much bigger version might be worth a try some way down the road..

[David Hess]

A 7812 regulator regulates as good as an LM317. In your circuit the output of either regulator has good voltage regulation but the emitter output of the 2N3055 emitter-follower is not regulated. The PNP transistor added to any voltage regulator allows a properly regulated output voltage.

The line and load regulation of a 317 is much better than on a 7812.  Also, the adjustment node of the 317 which is not available on a 7812 can be bypassed with a capacitor for lower noise.