dropzone said was I was going to say (as he usually does. He’s way smarter than me. :D) I’ve never made cheese sauce without a bechemel - I wouldn’t even know how to go about it.
2 T butter, 2T flour - melt butter, whisk in flour, cook flour taste out. Whisk in 2C Milk. Simmer and continue whisking till thick. When it’s thickened, add the cheese.
I use a bechemel sauce, and it’s always smooth. I always add dry mustard to give it a little “bite”. If I’m really feeling vicious, I add cayenne pepper and/or Worcestershire sauce.
Vicious? Around here that’s just the way it’s normally made. Vicious is when you add horseradish. 
Mustard and Velveeta are excellent things to add to your sauce if you want things to go smoothly. Mustard oil is a pretty good emulsifier and Velveeta conatians yummy triglycerides which help in the melting process.
And I think there is Worchestersire sauce, a food group of its own. With extra anchovies. Okay, skip the latter, but the stuff is Cheddar sauce in a 3lb block.
(Tossing a kiss on the cheek, and a regret that I’m married, to Missy2U!)
Yep. But AB recommended corn starch. He also says that melting cheese too fast can cause problems.