Braking down amp tubes schematics (EL86, ECC83, ECF80)
The squares seem to be bias currents, the circles bias voltages. In the first example, we see the top circle with 6.3 in it and the square with 0.7. They are connected to the heater element. If you go look up the datasheet, you will find that you are supposed to apply a voltage of 6.3 V RMS on the heater, and this should give an RMS current of maximum 0.7 A. 

The same can be found for the other tubes, where they will be the bias voltages and currents for a given operating point.