am/fm radio & Pilot 100 stereo demodulator

Posted on Feb 4, 2014

As the number of stations increased and the power levels also increased; receiver performance had to improve. The pentagrid converter, such as the 6BE6 tube (British EK90 valve), was introduced which was optimized for mixer performance. This included the isolation of local oscillator and input signal as well as allowing the first grid and cathode

am/fm radio & Pilot 100 stereo demodulator
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to work in an oscillator configuration. These tubes allowed a closer approximation to the multiplication of the two sine waves which produced fewer spurious mixing products. The British used a 7 grid tube with similar multiplication properties in FM broadcast receiver demodulators. They picked signals off the primary and secondary of the last IF double tuned transformer (which were 90 degrees out of phase) and multiplied them. The simplest modification seemed to be to use antiparallel diodes across the signal path to clip positive and negative peaks outside the +/- 0. 6 volt range (for two silicon diodes). I put the limiter at point A in the schematic. The limiter includes a 10k resistor between tank circuit A1 and the 470k resistor with the antiparallel diodes connected between ground and the connection point between the 10k and 470k resistors. I put a switch in the circuit to switch the diodes in and out of the circuit to compare the effect with and without the limiter. WMUC (88. 1 MHz) broadcasts with 10 watts of power and when the Pilot 100 is set to stereo, usually a grumbling splatter sound can be heard during silent passages which seems to be from interference from nearby stations, like NPR on 88. 5 MHz and probably other stations. Sometimes when WMUC is off the air I can hear a lot of noise at 88. 1 MHz which sounds like other stations bleeding over into that frequency, as well as another distant station on 88. 1...

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