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Yes, voltages are measured with unit powered. Clip on the meter negative lead BEFORE you plug it in. Measure voltages one handed - a shock arm-to-arm (through the heart) can be deadly. Old-timers say to measure voltages with one hand in your back pocket (that's how they got to BE old-timers...).
A $25 digital voltmeter has a 10 megohm input impedance like a VTVM, so it will read the same. A $5 DVM may have a 1 megohm input, and MIGHT read lower in some spots, where there's a resistor in series with the voltage being measured. A analog V-O-M will work as well.
The filter caps are what stores energy and can have considerable voltage after it's turned off. The circuit will drain this energy quickly IF the tubes have had a chance to warm up. Turn it on and off immediately and the caps can store that energy for a LONG time. Clip your meter across the main filter cap and try it. Whenever you're about to reach inside a radio, it only takes a few seconds to check for voltage first... that's what I meant - if there's any doubt DON'T until you've checked for voltage.
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