Filament transformer adjusts line voltage
The problem: I couldn't use my Heathkit oscilloscope in a house I lived in during the 1960s because my lab was too far from the power-line input to the house, and the line drop through the house was substantial. Depending on the time of day, the screen would shrink to perhaps half the normal display size. I checked the line voltage, and it was down to just approximately 100V. I lacked the funds to buy a high-wattage Variac to deal with the problem.
The solution: I had a couple of 12.6V filament transformers, rated at 3 or 4A. I simply connected one of these in my lab, with the primary winding across the ac line (Figure 1). Then, I connected the secondary winding such that one side connected to the ac line, and the other side provided the new, boosted ac line. Because the transformer had a center tap, I could adjust the line voltage in 6.3V steps. The beauty of this approach is that the transformer handles only the incremental power from the slight boost in voltage. And the technique uses less space and is less expensive than using a Variac.
Note that, by changing the polarity of the filament transformer's output, you can decrease rather than increase the ac output. This fact could come in handy in situations in which the line voltage is too high, causing incandescent-lamp burnout. Reduction in lamp life is a function of approximately the 13th power of the overvoltage (Reference 1). For the long, skinny, and expensive European incandescent lamps that some bathrooms use as a vertical light source, the lamp-life reduction can be significant. You can buck, or subtract, the line voltage to increase the lamps' life. Even at nominal line voltage, you can use the method to drop the voltage to an expensive or particularly inaccessible incandescent lamp.