You Do Need power plug adapter while traveling from Bulgaria to Yemen.

Here is Why You Need An Adapter

The outlet and plug type used in Yemen are A, D and G and in Bulgaria are C and F. This is how they look like.

Bulgaria

In Bulgaria, outlets of type C and F are in use.

Type C

Type F

Yemen

In Yemen, outlets of type A, D and G are in use.

Type A

Type D

Type G