The diodes serve another purpose. That is, if the shunt should
open the current will flow thru the diode and not allow the
negative lead of a high voltage supply go to high voltage. The reason of
putting the plate current meter in the negative lead is so that there is no
high voltage on it if there is an open circuit. Same goes for the plate
voltage meter. It should be at the low voltage end of the resistor chain
and a set of diodes across it as well so if it opens up HV will not be on
the meter.
You really don't want the front panel meters hot. Several KV per
cm electric field can deflect the meter and cause some error. Another is if
you should bump into it and break the glass you are in for a real surprise
(ZAP).
Also you should make sure you use big slow power diodes. They
will not rectify RF and drive the meter crazy.
I knew a guy that had built a very nice homebrew amplifier. When he was
provided some drive to it his plate current would drop so low that it
seemed that the amplifier had an efficiency of greater than 100 percent. He
had used signal diodes across the meter. He replaced them with some
cheap power supply rectifiers and the meter readings became more realistic.
73
Bill wa4lav
At 05:36 PM 12/8/2002 -0800, Rich wrote:
> >I want to get away from the low value hand-made meter shunts. I plan on
> >using the meter (a 0-1 ma) as a voltmeter to measure the drop across
> >more standard values of resistance. Knowing that my Ohms Law is a little
> > rusty, I am looking for some re-assurance that I'm right. The
> >metering circuit is in the B- CT return and the B- is floating 10 ohms
> >from chassis ground. If I change the meter to a 1 volt voltmeter, do I
> >then need a 1 ohm resistor to measure 1 amp FS of Plate current and a
> >3.3 resistor to measure 300 ma FS of Grid Current ?. Of course there
> >will be a 1k resistor in series with the meter. Is this corrext ??.
> >
>To begin to calculate metering circuitry, one needs to measure the
>full-scale mV-drop across the meter movement as well as the full-scale
>current. For example, a 1mA meter indicates 57mV and 1.04mA at
>full-scale.
>- movement-R = 57mV/1,04mA = 54.8-ohms
>To make this meter read 1A full-scale:
>- With a 1-ohm shunt at 1A, there would be 1V drop -- but the meter
>wants only 57mV to indicate full-scale. Therefore, the series R between
>the shunt and the movement needs to drop 943mV at 1.04mA, since R=E/I, R
>= 943mV/1.04m-ohm = 906.7-ohms.
>
> >Next question is exactly where and what is needed for diodes to protect
> >the meter ? I have seen so many different commbinations and placements
> >that I'm totally confused. What is the current consensus ??.
> >
>2, 3A-rms/200a-peak diodes, cross-polarized across the meter movement
>will do.
>
> >Thanks...es 73
> >
> >Ed W3NR
>
>- Rich..., 805.386.3734, www.vcnet.com/measures.
>
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