When I loaded my SWTPC 6800 with two 8K Seals memory boards, a 16K Smoke Signal Broadcasting memory board, Midwest Scientific Instruments 2708 EPROM board along with 2 serial ports, a parallel port and a floppy disk controller the power supply voltage would sag. The SWTPC transformer was rated for 7.5 volts at 10 amps but that was marginal. The 12 volt DC supplies would not handle a board full of 2708 EPROMs.
The first step was to upgrade the 12 volt power supplies. The power supply board could be mounted vertically and that would give you room to mount a 24 volt center tap transformer. The problem with a 24 volt transformer was that you would get around 18 volts DC instead of 12 to 14 volts. The transformer I used (Stancor TP-5) had multiple taps and I used the one that gave me 21 volts center tapped. I also replaced the diodes and capacitor on the plus 12 volt side.
To increase the + 8 VDC supply I replaced the stock SWTPC transformer with a Stancor RT-206 that was rated at 12 amps. The RT-206 is much heavier that the "10 amp" SWTPC transformer and you can use taps on the primary to adjust the voltage. I never had to worry about the +8V supply sagging again.
The only problem was the computer was much heavier now. The pictures below show the computer with a 6809 CPU board and a low power CMOS 56k memory board. I have since replaced the RT-206 with the original SWTPC transformer but still have the improved 12 volt supplies.
Click on the pictures to see a larger image.