A contrarian method, which I have used to good effect when cooking for folks I know like less tender meat (which I know sounds odd, but, actually, many of the farm and ranch folks I know do not like 'pink' meat).
Prepare a marinade of 1/2 cup Italian salad dressing and 1/4 cup BBQ sauce. I like Sweet Baby Ray's. You could also just add some ketchup, molasses and Worcsestershire sauce to make 1/4 cup total. Place tri-tip(s) and sauce into a vacuum bag or ziploc (vacuum sucker is better) and get all of the marinade in touch with the meat. Let sit for 24 to 72 hours.
Get a direct grill hot, to medium, and grill direct until lightly charred on outside and inside is done to temperature. I still like 130F internal, but, when cooking for others, 140F is pulling temperature. Then rest, slice and bask in the glory.
me: I don't drink anymore
Yelonutz: me either, but, then again, I don't drink any less