Early RF Preamplifier



I found this preamplifier attached to the back of an RCA TRK-12 television I  restored. I believe it was either a homebrew by the set owner or a TV dealer trying to get a better picture to make a sale. It probably dates to 1939 or 1940.


The preamplifier uses a single 6AC7 for amplification. The input and output are tuned by way of the variable capacitors. B+ and filament voltages are taken from a modification to the TV chassis and made through a Cinch-Jones connector mounted on the shield.



The schematic of the RF preamplifier. I will try to fire it up on the bench and get a sweep response of the bandwidth, also insert an RF carrier and determine the gain and bandwidth on a spectrum analyzer.



Display from a Tek 7L12 spectrum analyzer. The preamp was tuned for best gain at 61MHz, the gain measured 10dBc. Overall the gain was between 11dBc at 55MHz and 9dBc at 83MHz. As you can see when it is peaked the bandwidth is pretty narrow, the audio carrier is down 19dBc. The carriers on this display are from a local cable system, channel 2 - 6 video signals were flat within 2dB and audio carriers were down about 15dB from video.

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