Solar vacuum tubes with a diameter of 47 mm are now rare on the market, and are almost always replaced by positive vacuum tubes with a diameter of 58 mm.
The length of the vacuum tube of this specification is also shorter than that of the vacuum tube with a diameter of 58mm. Common tubes are 1.5 meters and 1.2 meters. (In addition to price reasons, it is not recommended to purchase vacuum tube solar power with this specification.)