Re: Panel meter insulation

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: Ray von Postel <vonpostel-at-comcast-dot-net> 

Brad:
Most panel meters are not rated for more than about 600 volts no matter 
what the scale on the face says.
If I wanted to measure plate voltages of up to around 3000 volts d.c. I 
would select a meter with a movement of something like 1 ma. but with a 
face that read 3000 volts.  I would mount it on an insulating panel such as 
bakelite. That panel would be mounted about an inch behind a glass window 
in the enclosure/cabinet.  The multiplier resistors for the meter can then 
be some distance from the meter itself.  The thing you have to guard 
against is an open connection in the meter movement.  That could place the 
meter at high voltage and you could accidentally touch it.  The meter 
movement adjusting screw on the front of many older meters is metal. 
Placing it behind the glass window protects against that and other 
hazards.  Always treat a meter reading plate voltage as if it is hot to ground.

If the problem is that you want to read the plate current without having to 
also read the grid current then I suggest you read the total plate and grid 
current in the cathode circuit and measure the grid current 
separately.  You can mentally subtract the grid current from the cathode 
current the difference being the plate current.  That is common practice.

Ray

Tesla list wrote:
>Original poster: "Brad Huff" <mailto:huffb-at-avalon-dot-net><huffb-at-avalon-dot-net>
>Does anyone know the insulation ratings of the quality panel meters that 
>were built in the 40's through the 60's(weston, simpson, westinghouse 
>etc.). These will be used for vttc service. Most people use one between 
>the cathode and ground which reads plate and grid current. I would like to 
>use one in the plate circuit if the meter is insulated to withstand plate 
>voltage for the 833-A. Any thoughts?
>
>