Well that's just a thought if you had some various values of resistors to try in your parts box. The resistor value would be on the order of one ohm at say 10 watts if you are drawing 4 or 5 amps. The resistor would be in series with the lamp load. Like transformer A terminal to the resistor, the other end of the resistor to the lamp, the other end (common terminal) of the lamp to the U terminal.
What accessory voltages are available and did you try them?