The T15 triode has B4 base and here the base cap is nickel with a paper band giving operating details.
The filament voltage would be 5.8 for a longer life. The T15 is rated at up to 6.0 Volts depending on use - The higher the volts the shorter the life. The anode is rated for up to 600 Volts.
The construction follows the earliest designs in that a horizontal bright cylinder anode has a stout wire helix grid running through it which is in turn pierced by a single strand of tungsten filament. The T15 has an anode dissipation of 15 Watts
The identification is etched on either side of the envelope.
The T15 with an M underneath The number below is 87.
A closer view showing the filament with corrugations to act as a tension spring.
The grid helix fixed to a backbone strut. The filament is held in folds of the supports.
Under the microscope the grid loops can be seen to be sitting in notches on the backbone.
With greater magnification the loops look to be spot welded to the support.
The balloon envelope is 56 mm in diameter and, excluding the B4 base pins, is 89 mm tall.
