ORIGINAL MESSAGE:
On Wed, 08 Apr 2009 18:12:18 +0200, Ulf Tjerneld <star@frizon.org>
wrote:
>
>I have noticed when I look on different HV supplys that sometimes the
>negative lead is connected to chassis ground and sometimes it is kept at
>a potential somewhat above ground by means of a resistor. Can anyone
>explain to me why, and if, this is important?
REPLY:
It is done for purposes of metering. In the olden days, the plate
meter was put in the B+ lead, a very unsafe practice since it puts the
entire B+ voltage on the meter itself. Later on, someone figured out
that one could put it in the B- lead, and in fact, the B- itself does
not have to be connected to chassis in a grounded grid circuit. By not
connecting it, this simplifies metering of the grid current as well.
In a grounded cathode circuit, common in tetrode amps, the meter is
still placed in the B- lead and then the + side of the meter is
grounded. One caution with this method, however: If the tetrode screen
current is derived from the B+, then the meter will show the
combination of plate and screen current, which is not good because it
makes tuning difficult. Tuning is accomplished by watching the screen
current and the screen current goes up while the plate goes down and
vice versa. Better to have a separate screen supply.
73, Bill W6WRT
73, Bill W6WRT
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