[BC] WANTED - an FM tx.

[wpio fm 89.3]

On 11/10/2010 12:27 PM, RichardBJohnson at comcast.net wrote:
> Many early transmitters had their "plate current" meter on the HV feed going to the final amplifier tube. RCA used to have the meter set back
> behind the panel with an "empty" meter bezel in front of it to keep HV off the front of the transmitter. There were other schemes as well, including the Visual transmitters that just let the meter arc (oh well).
>
> The high voltage would attract dust. The meters were HV precipitators in effect. At one time, the FCC was strict. The "final amplifier plate current" needed to not only measure the anode current only, but it needed to be in the plate-voltage lead. When I worked for Wilkinson Electronics, I ended up being mentored by the chief of the FCC standards bureau in Laurel, MD. We "pushed through" the allowance to put plate-current metering in the negative lead of the HV power supply. It still measured only final amplifier plate current, even with an AM transmitter, if the negative returns from the modulator were properly connected, etc. Of course, the design needed to handle the possibility of any arc anywhere in the HV circuit. Eventually broadcast equipment manufactures ignored the FCC entirely and simply measured cathode current.
>

WOW, interesting that the FCC had a lot to do with plate current getting 
measured in the hot lead.  Seems it's not as accurate to sample from the 
low side.  It was always a mystery how, CCA for example, knew what to 
set the variable resistor at because the grounded grid is getting some 
of that current.

My guess was that they temporarily put a DC current meter in the plate 
lead, and used that figure for the low side xmtr plate current panel meter.

Randy