[BC] WANTED - an FM tx.

[RichardBJohnson at comcast.net]

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.

Methinks many broadcasters simply listen on their car radios nowadays, ignoring metering entirely.

Cheers,
Richard B. Johnson
Book: http://www.AbominableFirebug.com/

----- Original Message -----
From: "wpio fm 89.3" <wpio at gate.net>

(the Ebay CSI)

Now that was a nice tour.  Re: the plate current meter, I never thought 
that was a good idea to run the actual voltage through it.  In a Visual 
I used to work on, I recall it being problematic but I don't remember 
what the problems were.