avr_fred:
Please do. I have a 20 volt 16 amp transformer......
20 volts may be a bit high. As it is, my supply uses an 18V RMS transformer which gives about 25V peak and about 20V under full load. The NPN transistors have to dump 20-13 = 7 volts at 40 amps which is a whopping 280 watts!
I may end up using a different transformer to minimize the drop while still having enough overhead to maintain regulation.
I get away with the horrible power waste because the supply is for an amateur radio (ham radio) transmitter and the full load is only applied intermittently (either CW or SSB - both of which load the supply with probably a 30 to 40% duty cycle - full continuous 40 amps would probably overheat it).
I don't have a good power resistor array to test the supply under full load... all I have is an array of eight 4 ohm power resistors wired in parallel which only gives me a 26 amp load. The supply gets quite warm, but handles the 26 amps continuously with no problems.