The taps are to compensate for having different line voltages. The taps can be used to adjust the secondary voltage. If you built a project using a 12V transformer and now wish the secondary was 11V you can change the primary tap.

 

To get right to answering your question

I didn't look at all of their transformer specs but did look at a few. The only one that I saw was one for a Marshal amp, the had 120V and 100V, plus a few higher voltages on the primaries. But the other members have stated why.

 

Some of these replies seem to be thinking the OP is talking about a measured difference in voltage from the nominal. OP is talking about taps specifically labeled with these slightly different voltages. I've also seen them.

Most of us were likely thinking that he didn't need them if he didn't know what they could be used for.

I need to purchase a couple of small power transformers. I've always noticed that a lot of transformers have primary windings for 115V, 117V, 120V, and maybe even more.

 

The taps are to compensate for having different line voltages. The taps can be used to adjust the secondary voltage. If you built a project using a 12V transformer and now wish the secondary was 11V you can change the primary tap.

Primary taps are easier for the transformer company to wind. It also allows the transformer to be run at its best efficiency for lower voltages and avoid saturation at higher voltages

 

Just an opinion here - but a primary that can be adjusted by 10 volts (110 to 120) will make very very little difference. If that difference is unimportant then why the extra expense. Suppose a transformer is 10:1 (120 to 12); that would be 115 to 11.5V if you used the 115 tap; and if you use the 110 tap the output of that transformer would be 11 volts. Unless the output is critical, such as might be in an analog system, I just don't see the need for such tight control.

I believe I've seen such transformers, but that was on a very old tube type analog amplifier - a Magnavox with some HUGE transformers. The output was extremely beautiful and crisp. But with digital today - the main job of the transformer would be to reduce the voltage to within a decent range for operation. After that - regulation for the final control. But I'm no expert on analog or on transformers; so this is just an opinion. Yours may vary.

 

If the transformer is 120:12 with primary taps, then if one uses the 115V tap on a 120V supply, the output will be 12.52V.
The difference between 12.5V and 12V is very significant for a filament lamp. A projector lamp with a rated 50 hour life will be reduced to 30 hours if the supply increases from 12V to 12.5V.

 

As I said, I'm not the most experienced in these matters. But on the same circumstances, if the bulb is slightly under powered, it should last longer. And if long life is critical, rectifying to DC with filter caps and a regulator, one can dial in on the desired voltage without having to worry about line voltage fluctuations and brown-outs.

 

As I said, I'm not the most experienced in these matters. But on the same circumstances, if the bulb is slightly under powered, it should last longer. And if long life is critical, rectifying to DC with filter caps and a regulator, one can dial in on the desired voltage without having to worry about line voltage fluctuations and brown-outs.

You're spot on - sometimes a projector has a transformer fitted with a higher-voltage tap for the specific purpose of increasing the lamp-life. Because such lamps are expensive, it pays only to run it at full voltage when it is really necessary. But they've all been replaced by LEDs these days.