If you are trying to build something like Nomad's 555 circuit, yes, it uses a PNP transistor, but the design is capable of nothing like a 6 ampere output. If you use a good, high current PNP transistor, say a TIP36 or D45H. But this circuit will not provide enough base drive to switch one of these with a 6 ampere collector current. to get good saturation (less than .3 volts collector to emitter) one of these may need a base current between 1/20th and 1/10th of the collector current (.3 to .6 amperes). The design shown has a 470 ohm resistor limiting the base current. After you discount the 1 volt base voltage and the
1.5 volt loss through the 555, this leaves 10 volts across that resistor for a base drive of 10/470=.021 amperes. This resistor would have to be lowered to about 20 ohms to get the needed base drive, and then the 555 would overheat.So the output capability of the 555 has to be increased. Adding a second PNP (say, a 3 amp rated TIP32) with its base connected directly to the 555, its collector connected directly to the negative rail, and a 15 ohm 5 watt resistor between its emitter and the big PNP's base, you get the needed base drive. Then you need to put a 10 ohm resistor between the base and emitter of the big PNP to suck the stored charge out of it when the 555 output goes high, to have a clean turn off.
All these designs are also missing the diode that should be across the motor, cathode up. Without this, the high voltage pulse you get when you interrupt the motor current is likely to damage the switching transistor. For a 6 ampere motor, this diode should be fast (turns off fast when reverse bias is applied by the transistor) and rated for at least 3 amperes, average. Candidates would be the SR503 (30 volt 5 ampere schottky) and 80SQ045 (8 ampere, 45 volt).
If you want to simplify the circuit quite a bit, replace the entire PNP and resistor network with a big P-channel mosfet (gate directly connected to the 555). Something like IRF5305 (0.06 ohms on resistance, 55 volt).