Hello folks,

I currently acquired a vintage Celestion G12-65 (15 ohm) guitar speaker (manufactured in January 25, 1982) in perfect condition. I'm trying to design a sealed back cabinet for it...

Here are the reissue version's specs from Celestion

Fs - 79 Hz (Their main page says 85 Hz? Does it make a big difference)
Re - 11.8 Ohm
Qms - 10.67
Qts - 0.61
Qes - 0.65
Cms - 0.13 mm/N
Vas - 51.4 lt
Le@1kHz - 0.78 mH
Pe: 65 Watts

Now, from what I read, before designing a cab I need to decide the Qtc of the cabinet. It's said that the best value for a guitar cabinet should be between 0.707 and 1.0. Let's say Qtc = 0.71
Also, again from what I read the F3 value should be below the lowest fundamental which is 73 Hz since I play in drop D tuning from time to time (But mostly I use standard E tuning, so should I chose 81 Hz?)

Another thing that I want is a good low frequency response so I choose to go with sealed back design without any ports or damping materials.

The problem occurs when I give all these information to HT Audio software to calculate the box dimensions...
Here are the steps that I took and the given error:

I appreciate all the help