A DBT is simply an **incandescent** bulb in series with the power line. An incandescent bulb has a fairly unique property that as more current passes through it, its resistance goes up. So if you have that bulb in series and the unit under test (UUT) draws more current than you expect, the resistance of the DBT will increase and limit current to the UUT to a greater extent than a simple resistor would. Couple of caveats - You *must* use an incandescent bulb. A CFL or an LED does not exhibit the same behavior and will be useless. The bulb must be sized considering the expected idle current the UUT would normally draw. If you expect that receiver to normally draw 20 watts idle, then you would want to use a bulb rated 3 - 4 times that wattage. In most cases, a 75 - 100 watt bulb is a good choice. With a low power unit you might start at a 40W bulb, and an ear blaster may need a 150W (or two 75's in parallel) bulb.
Some people like to use a variac to control the input voltage. While that can be helpful (particularly if you monitor the current as you increase the voltage), IMHO a DBT is a superior tool for protecting the sections of a receiver/amplifier that are not already fried.
Another thing to be aware of is that many new units are going to switch mode power supplies. Trying to 'start' a SMPS at lower than rated voltage can be problematic. This is because a SMPS looks like a negative resistance - it will draw more current at lower input voltages. If you have a SMPS that would normally draw 1 amp at its rated 120 VAC input, and you try to sneak up on the input voltage, you may find the power supply trying to start at 60 VAC by drawing 2 amps. Not a stable situation with either a variac or a DBT.
