Sanwa U-50D Operator's Manual page 17

DC, the internal resistance of the 250 volt range is
250X20,000 = 5,000,000 [Q) = 5,000(kl2), or 5 megohms.
When the plate or screen-grid voltage of a triode or
pentode of high impedance load and high amplification
factor along with small circuit current is checked by a
meter of small internal resistance, considerable current
flows from the meter into the series load resistance of the
tube. As a consequence, there is greater voltage drop,
and the meter reads lower than the true value: the con­
nection of the meter upsets the circuit to be checked.
Error can be minimized by the use of a meter of high
internal resistance, but unless it is infinite, some error
is inevitable even if a vacuum tube voltmeter be used
for checking such a circuit.
The internal resistance of a meter little affects the check­
ing of the DC voltage of B(-f-) power, or plate and
screen-grid of a power tube, because their impedances are
smaller than the meter impedance. It is the control grid
voltage that is disturbed by voltage measurement.
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