The output transformer shown in the AX84 schematic has a 16 ohm secondary with taps at 8 and 4 ohms. Assuming you connect your
speaker to the appropriate tap, the primary impedance will be 4000 Ohms. The primary impedance was not chosen by accident, the
value is taken from design examples in the EL84 datasheet that are similar to the AX84 output stage (250V plate current, 48mA plate
current, etc.). The general rule of thumb for a one-tube single ended output stage is to use a transformer with a primary impedance
approximately 1/10th of the specified plate resistance.