I've been reading up and not sure what resistor to change. Ideas? I can poke around with a multimeter, but not really sure what I'm looking for . . .
Assuming you have an old(ish) supply with a TL494, you should have a resistor from +5V out and pin 1 of the TL494 there should be another resistor from pin 1 to ground, as the feedback voltage is realized with a voltage divider (except in those cases where the 5V is compared directly to the reference). the feedback resistor could be replaced with either a larger resistor or a smaller resistor and a trimmer - if you find both resistors, it's easy to calculate the new resistor value.
Apparently some have 7V+ outputs, but I may need 6.5V later.
I don't think so, but if it's the so called "7V trick" (using the 5V as the lower ref. and 12V as the upper ref.) you have heard about, don't do it!
You will have a potentially unstable supply (since your load is not grounded), the max. current is smaller than the current on the +12V line and there's more reasons... Besides, it's impossible to predict how a given supply will handle this unconventional loading.
How stable is the output voltage?
Oh well, PSUs are different beasts, dependant on manufacturer, designer, material quality etc., but raising it from 5V to 7V by means of the feedback circuit alone shouldn't change from the unmodified PSUs stability, unless you intend to load it to the fullest (in which case the inductors/transformers would need a rewind as well).
Your load (6A/45W peak) is peanuts to even the smallest PC PSU.
If you dig into a PSU with a TL494, I'll talk you through, should you need it and with an old PC-unit, it won't cost but a resistor, a bit of solder and a little time.