I had the 6500 and now have the 7700 -- I used tons of metallic thread but never in the bobbin (I would match the metalic with a non-metalic in the bobbin). The only thread my machines regularly break is the sulky metalic -- don't know why but it may be something about how it is wound on the spool.

Unless doing "Bobbin" work I use an ordinary cotton thread in the bobbin but I always use a Schmetz Metallic thread needle, http://www.schmetzneedles.com/shop/l...php?prod_id=19 someone gave me that tip and I have never had a problem.

I have only just, last week, upgraded my elna 7200, to the elna 720, both excellent machines and in the 7 years I have had the 7200 I have never had to alter the bobbin tension. I have had to do it on some older machines for my pupils and I just mark the original setting before adjusting.

I have a Janome 6600 and have also found it to be picky about thread...and you're right, for a while I thought it only liked Guterman (or poly threads, like Sulky Polylite).

However, I discovered that it's very happy when I use Superior's Bottom Line in the bobbin (sometimes I also use it in the top).

Also, many problems were solved when I had it services (the guy did lots of things I could not due myself). At this point, it's fairly flexible about thread.

My Janome 6600 P likes Gutermann and Essential thread from Connecting Threads. Does not like Coats and Clark at all. I have been lucky enough to not have bobbin tension concerns, but think I may go ahead and get an extra bobbin case "in case".

My Janome 6600 P will use any thread I have tried. I use lots of WalMart Coats & Clark. Just have to use a brush to clean under the plate every time I change the bobbin. If lent builds up on the needle, I clean under the plate. Seems like it needs cleaning more frequently than my old 1967 Fleetwood used to require. It is okay, I love to pet my sweet "Jan".

I haven't had any problems with any thread so far in my 6600. This is not to say it's not there, but I don't remember ever reading anything in my manual that said not to touch the bobbin tension. I agree with Scissor Queen in that you have to change the needle size depending on what thread you're using.

I learned many years ago how to adjust bobbin tension and it has never failed me yet. It's pretty simple, but I would suggest you should only try the following if you're comfortable in what you're doing. I'm no expert or repair person by any means, just had about 45 - 50 years of maintenance experience on my own sewing machines.

Remove the bobbin and shuttle, insert the bobbin into the shuttle. While holding the lose thread end, do a very gentle "up-down" motion. The thread should move slightly through the tension, but not "run away" through the shuttle tension. If it doesn't move at all, it's too tight...loosen the screw a very slight amount and try again until you get that very slight give on the thread with the "up-down" motion. If it moves too much, tighten the screw a very slight amount and try again.