02-09-2010, 06:31 PM
Think of it as a voltage divider. If your old fashioned voltmeter had a sensitivity of 1000 or 2000 ohms per volt, and you had a really large value resistor between the plate and the B+, then you would be almost shorting out the circuit when you measured the plate voltage with the meter. E=IR, and the current is so small, ere the measurement error of yesteryear.