28-10-2015, 09:53 AM
(This post was last modified: 28-10-2015, 09:58 AM by Old Sparky.)
Possibly the confusion arises, as has been said, because we all treat the Negative line as common, usually chassis, which of course, it often is, whereas grids are biased Negative w.r.t (with respect to) chassis, which, to them is Positive, so, as Alan said, the meter is doing as it should. If you were to connect the Positive terminal of a DMM to chassis and measure, e'g. the HT line, the meter would show, say -200v when in fact the HT line is 200v. Positive WRT Chassis. IThis is because, unlike analogue meters, which must be connected the correct way round, most, if not all, DMMs aren't polarity sensitive, and indicate the polarity of the reading wrt whichever probe is connected to the (usually negative) chassis.
