Lamp life is highly related to applied voltage (in the inverse sense), but light output in lumens is not nearly as strongly related to voltage. About a 10% drop in voltage gives about 4x the lifetime without a great drop in lumens. If you can, go for a bit less than 6V to the bulbs to keep them living longer. [ref.: http://www.zap-tek.com/webpage/Elect/lsn_4/014_lamp_res.html]

Does this thing have incandescent bulbs or LEDs?

 

OK, Ignore the suggestion about using the 4 diodes and continue guessing about other stuff that was not answering the question. The concept of diodes dropping was OK, but it needed to add them in the other direction as well to avoid cutting the power in half. Using the two sets of diodes in opposite directions will work out very well.