I upgraded to some aluminum cranks and decided to just use a single 50 tooth used chainring. There are 6 gears in the back. The chain tends to slip off the front when I'm in the smallest or second smallest gear in the rear.

The chain is well aligned with the middle of the cassette. The chain is stretched by about 1/16", so I don't think that's enough to be the problem. The chainring is not very worn at all.

So, the only reason for the chain slipping off that I can think of is that there is slack in the chain. Does that make any sense?

I've heard that when mountain bikers convert to a single chainring they usually put a guard on the inside and outside to stop the chain from coming off, but is this necessary when I'm just riding on roads? Should I just put the front derailleur back on to keep the chain in check?

Note: if you left the chainring on the outside position of a double crank the last cogs on the inside
opposite end of the rear cluster are at quite a chain angle

+ the used parts.. if you want to stay as a 6x1, a shorter BB will put the ring
centered between #3 and 4 in the back .

Surly's stainless steel chainrings are nice , made for single speeds, up to 50t.

You need some sort of chain retention device. A front derailleur with the limit screws locked into place works fine. You may also want to put the single ring in the inner position.

[excuse my bad eglish]
Had the same problem. Front triple converted to single on my daily commuter. As long as I used one of the three disks of the triple (initialy the middle and then the larger disk) the chain was slipping like hell.....
I re-installed the FD in order to keep the chain from slipping.
Some time later I bought a SINGLE SPEED DISK. The teeth are different, they are not shapped in such way to help the chain slip to a different disk when you want to change gears.... (as they are in a three disk crankset).
The chain DOES NOT slip anymore. I even removed the FD. No problem since.

Alright. Thanks for the help.
Looks like I'll just put the front derailleur back on until I can get my hands on a single speed chainring.