See if you can follow the wires from pins 4 and 5 on the valve base to where they come from at the power supply end.
The smaller transformer is almost certainly the one providing the heater supply, there should be a low voltage somewhere between 6 and 16V (depending if there is a regulator chip used) on one of the large horizontal capacitors between the two transformers.

If you are not confident about measuring voltages then take it to your local EE, there are high voltages present as well.