You can put nearly any standard 19" 90 degree CR23 tube in an Amplifone and it will work just fine. To get that real nice fine crisp picture that Amps are known for, however, you will need to use a medium resolution tube. Unfortunately, they are a bit hard to come by, being as though they were only used in medium resolution game monitors and the Amplifone. The standard tubes were used everything, including television sets.
-Ian