These caves are not replaceable, because they really do not wear out. I used to have a similar soldering iron and was happy with it, unfortunately I left it somewhere and the next one I bought of this type was definitely worse. The problem with these soldering irons is the connection of the transformer winding with the sleeves that hold the tip - unfortunately, someone thought that this connection should be soldered and quite quickly (sometimes even several minutes of work) cold solder is formed, especially if it is unleaded solder, and today they are all.
In general, it is better to choose a Lutol type soldering iron, although the quality of these soldering irons has also deteriorated in recent years. I already have a third copy - the previous two have been replaced with new ones under warranty. They just smoked when soldering.
