Tenths of volts on 12 volt output is be important. For
example, voltage at 12.2 may shorten life expectancy by 50%
compared to a voltage of 11.8. Little voltage increase can to
cause a significant reduction in bulb life expectancy.
Important here is the 120 VAC voltage. What is that - to
three digits?

Reducing 12.0 volts to about 11.0 volts will increase bulb
life expectancy by a factor of three times.

Voltage is also how one can change bulb life expectancy.
One important factor - how many amps are being drawn from the
transformer. A two digit number is good for this. A device
that drops 12 volts by say 0.3 volts would consume how much
power? A calculation necessary before lowering voltage on the
12 volt side. Even a couple of diodes placed reversed and in
parallel on transformer output would limit voltage from 12.0
volts to about 11.3 volts. But 0.7 volts times that ballpark
current is how many watts - to estimate how hot those diodes
may become.

The answer of 115 to 120 volts tells little. What is that
number accurate to three digits. And how does it vary? Does
line voltage increase to 126 volts?

Voltage spikes would have little to do with bulb failure.

Regular incandescent dimmer must be on the 120 VAC side of
transformer.

BTW transformer output voltage drops as load approaches what
the transformer is constructed for. IOW a transformer that is
too large will output much higher voltage than intended for
full load. How much current is being drawn and how large is
the transformer rated?

Ike wrote:
On 14 Dec 2005 10:11:37 -0800 "RickR" wrote :
...
Check a couple of things;
1. What is the rated life of your bulbs?

2000 hours and they are rated for 12 volts.

2. How long are you running them? ...

5 to 8 hours per night, depending upon the time of year. This means I
should be replacing them about once per year. The first bulb in the
set will burn out in as little as two weeks and will last as long as
two months.

3. As below, check your incoming voltage. Are you getting 115-120V?

Yes.

4. Check your out going voltage from the transformer. ...

I am getting 12 VAC on the output side, plus or minus a few
hundreds of a volt. It was my error to think it was 12 VDC.

5. Do you like having brighter lights? ...

Yes, but not at the expense of such short bulb life.

6!! Check for voltage spikes! This is probably your problem. ...

Unknown if I have spikes.

7. Check for overheating in the fixtures. Not usually a problem
outdoors but killer for lamps.

The transformers are well ventilated inside our garage.

Putting a dimmer in *will* get you longer life. ...

I tried putting a regular incandescent dimmer on the 12 VAC side but
the lights did not work at all. Oh well, it only cost $5 for the
dimmer to try it out.
...