The XH1.5 or XH1.5V miniature triode dates from 1935. The base is the Hivac small four pin, and the design use was for hearing aids but they were useful for miniature radios and radio controlled models.
Filament operation and anode supplies were both to be taken from dry calls.
The etched lettering reads HIVAC XH1.5.
The filament tension spring at the top and the single inverted V filament. The anode is spot-welded to the supports.
The anode side view and the HIVAC name.
The top of the filament spring and top of grid winding.
Here the filament can be seen entering the top of the grid.
The thin glass tube envelope is 15 mm in diameter and, excluding the base pins, is 54 mm tall.
Reference: 1020. Type XH1.5 was first introduced in 1935.