Another low cost intro to high voltage, although not variable, is a classic dumb pass-through, one without a regulator or inline battery. A 5V 808 pass-through runs about $10 - $12. Add a 2A USB power supply (for example the Walmart IQ dual 2A USB supply, sold online for $10, free S&H if you have them deliver to your closest Walmart), under $25 total, lets you test 5V. I used one at work, and another in my car (they make 2A cig lighter USB supplies, also around $10) for many months. They would work fine with your 808 cartos -- 3 ohms is nice, and 2.5 ohms OK, though a bit too hot for my taste -- will definitely have more kick.