[BC] Transmitter metering

[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.