[BC] Transmitter metering

[Warren Shulz]

Added problems of telemetry were the Collins FM grounded screen grid
amplifier. Filament (cathode) was some -700 volts and plate supply was -
cathode to anode voltage.  Its not possible to run into a 0-5 volt
grounded telemetry input. Collins had some hall-effect "Secret Sauce"
modules to read current flow.

TFT and Mosley produced chopper amplifier to handle high common mode
floating device to jump the high common mode to a ground reference to a
0-5 volt DC signal.  Today the are classified as signal conditioners and
still exist by other.

Warren Shulz
WLS CGO

-----Original Message-----

 On Behalf Of
RichardBJohnson at comcast.net

This schematic clearly shows that a GG amplifier can read both grid
current and plate current independently, with the meters
at a low potential. If there is no "tab compression" in your reader, the
schematic should be readable. Otherwise --too bad!

        ---------------------
        |                   |
        |  HV power supply  |
        |___________________|
             |         |
            (-)       (+)
             |         |
      [Plate I Meter]  |
             |         |
             |         |________ To plate
             |__________________ To cathode circuit (often a bias
resistor)
             |
             |
       [Grid I Meter]
             |
            GND

However, remote-control is another problem! Remote control units of the
vacuum-tube days, needed samples referenced to ground (chassis)
potential. In the above circuit, there is no location where one would
not read both the grid current and the plate current when referenced to
ground. However, putting a sample resistor in series with the [Plate I
Meter] line would often be good enough. Sparta/Bauer marketed a MBB1
sample unit for just this problem. The MBB1 (Magic Black Box 1)
contained a single transistor oscillator, inductively coupled to a diode
detector. This could provide many kV of isolation. Once the sample
voltage was over about 0.7 volts (where the oscillator started), the
response was linear. 

Cheers,
Richard B. Johnson

----- Original Message -----
From: "Michael Patton" <tech at michaelpatton.com>

I once bought and installed a late-model (G-series) CCA 20 KW FM 
grounded-grid rig, and to my horror, I found that the "grid current" 
meter was simply set to measure and show a percentage of cathode 
current.  I tried to show the CCA folks the problem but they were never 
very interested in my rantings; good design was not a strong suit of the

revived Atlanta-based CCA.