The QY3-125 is an RF power tetrode with a maximum anode rating of 125 Watts dissipation. To keep within temperature limits forced air cooling is required above 50 MHz. Interestingly this valve is also rated for use as a class B modulator where a pair can generate 345 Watts of audio.
The graphite anode. At maximum dissipation this will glow red.
The anode is held to the top-cap by a four wire cage. Note the bend in the supports to hold the graphite in place.
The centre top of the anode is open.
This view shows the screen grid. The form is of a vertical wire cage the control grid is not visible but will take the same form with the wires aligned with those of the screeen grid.
The B5F base: the control grid connects to the lower pin with the screen grid connecting to both middle pins.
The wide glass tube envelope is 60 mm in diameter, and excluding the B5F base pins is 108 mm tall.
References: Datasheet & Adverts. Type QY3-125 was first introduced in 1953.