Over the years I've built a few speakers. I just recently 'decommitioned' my 32 year old Heathkit Amp when a great deal came up on a H-K receiver. I have JBL L-88's in the livingroom and J2050 in the den which also are used when we watch DVD's. I am thinking of giving the AMP to my college student daughter and building her some speakers. I am a H.S. shop teacher so I have good access to woodworking equipement.
Before, I just copied pervious designs or used an infinate baffle design large enough to work. That was pre-80's. Now with PC's, the net etc I have downloaded some design software (WinISD) and info on speakers is readily available.
Because the old Heathkit is 15w/ch in need a small but efficient enclosure. So a port is required. All the software comes up with round ports.
So, the question is, can I just use the Sq In.of the tube ID to make a rectangluar slot (of course keeping the depth the same are the tube length)?

To convert from a tube port to a slot port (round to rectangular) you need to 1) maintain the same area of the port and 2) same port volume. If you do these thing, the port will work fine (although slot ports tend to exhibit more port noise at times, since people tend to put them at the edges of the box).

One benefit of a slot port is that it can be "folded" (although you MUST be sure to remove the port volume from the volume of your enclosure, when calculating your enclosure size). A folded port can have a much greater area and volume than a tube port; and this will lead to much lower box losses (lower port Q) and tighter, cleaner bass output (particularly around the tuning frequency).

WJ