Edit Comment Only

I have a vast collection of MP3's.
What I would like to do, is edit multiple files in one go,
but all I want to change is the comment on the Tag's

This is so I can make a comment such as The Producer of many different groups.

Example: Joe Meek produced many groups and singers who only recorded a small number of songs

What I don't want to do is change the Song or Artist.

example:

Song : Diggin' My Potato's
Artist : Heinz
 song : Telstar
Artist : Tornados
 Song : Jack The Ripper
Artist : The Savages

I want to add to all files at the same time "Produced by JOE MEEK" in the comment section

Select all the files that should get the same data.
Enter the string in the comment field in the tag panel.
Press Ctrl-S to save the modification.

Did you know that there is a dedicated field PRODUCER so that you can save the comment field for other (better?) purposes?

PS: isn't this more or less the same question as in
Add my comment to multiple files

More or Less is not exactly the same.

I have already tried saving the files after adding only the comment, It saved the comment but cleared all the other tags.
I tried your way with ctr + s It cleared all tags, even the comment.
Tried another Three times, It saves the Comment but clears the other Tags.

Then you have to describe how you do it.
Because, by default, MP3tag sets all fields in the tag panel to "keep" if you select more than one file.
Nothing gets cleared unless you tamper with these settings.

What you could also check: what kind of tag versions have you got in the files, Like APE, ID3V1, ID3V2?