Having designed and built several AM superheterodyne tube receivers and gained enough experience in tube circuitry, I decided to build what most would consider quite advanced for the homebrewer, an FM superhet receiver! This is the unit you see here. The receiver covers the FM broadcast band form 88 to 108 MHz, and has the following features: A double triode RF (radio frequency) amplifier for low noise reception, an independent VFO (variable frequency oscillator) and mixer for good stability, 2 IF (intermediate frequency) amplifiers and 1 IF limiter for improved sensitivity and quieting, a balanced ratio detector for easy alignment, AFC (automatic frequency control) for drift-free reception, AVC (automatic volume control) for constant volume on strong and weak stations, and an accurate dial for easy station location. The tube lineup is as follows: 6BS8 (RF amp.), 6AB4 (Mixer), 12AT7 (Oscillator & AFC), 6BJ6 (1st IF amp.), 6BH6 (2nd IF amp.), 6BH6 (IF Limiter), 6FM8 (Ratio detector & audio preamp.) and 6AQ5 (audio power amp.). Performance is excellent and the sound is smooth and clear. The receiver runs on 110 volts AC.
A solid-state STEREO decoder/amplifier
I built can be connected to this or any other tube or solid-state FM receiver with MPX output to listen to FM broadcasts in STEREO.
Controls from right: Tuning, AFC, volume and On-off switch. A 9:1 reduction pulley offers smooth and accurate tuning. AFC functions properly and provides hours of drift-free reception. Power-on light is right of speaker on top. Speaker grill was cut out from a pre-punched sheet and painted white.
The receiver can use a dipole antenna for weak stations, or a removable telescopic antenna for local stations. Components were placed in a "U" layout to save space and avoid feedback. Transformer on right isolates chassis from the mains. A 4 inch speaker provides strong and rich sound. Plate above speaker acts as a baffle.
Point to point wiring was used. Full wave rectifier is on left. Audio transformer is top left. A triangular hole was punched out to reach tuning capacitor terminals (top right). RF (radio frequency) amp. is bottom right. 3 stage IF (intermediate frequency) amplifier runs from right to left. Audio stages are next to audio transformer. Good component layout made wiring uncrowded and alignment easy.