A few basic calcs can yield a lot of info when designing.......
24v ac supply from tranformer, offload dc = 24*1.414=33.96v however there are losses across the diodes...assume worst case 1v per diode that's gonna be around 4 volts..so across the cap you got around 30v assuming the transformer puts out 24v ac.......it could be higher !
So 30vdc across cap, your series pass transistor is seeing 30v at its collector with the wiper of the pot at max, your output is going to be 20v less the b-e volt drop of both transistors around 1.2v
Assuming the s/c current control circuitry works,max dissipation of series transistor is going to be max volts across c-e* s/c current.....(30-20)*1 watts, around 10w so yup BD241 looks good if thats the case
Otherwise, its total meltdown........use a fuse! lol
I said a BD711 coz I got 1, thats all..... 8) equally a TIP41A would do too.
HTH Chip