I have a similar question... and a partial answer. I am using an AD595 (instead of Max 6675) with RAMPS 1.4. I connected it according to the following link:
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reprap.org]
The difference will be that you have to search through firmware for the words "MAX6675". There should be comments telling you what to do. For instance, in Configuration.h, you will have to change Temp_Sensor_0 to a value of "-2". You'll also have to change the Pins.h file.
If you get it working, perhaps you can help me with the next step... getting the extruder to work with it.
I was able to read temperatures in excess of 270C. I also got it to stabilize nicely, and feed Nylon 618 by hand at 245C to 260C. Again, stable. However, when I turned on the extruder, the readings went haywire. It jumped up to 299, then dropped to 150, all in a few seconds. I know that's not the accurate temperature, and I know the thermocouple worked fine when extruding by hand... so it must be in the wiring or in Marlin. It's like the extruder steals the signal from the thermocouple, simply by the act of running it. I have to shut down and restart to get it to fix itself.
