Three Tube superregenerator with squelch
This is my latest airband RX. I was motivated to try this after stumbling on to a youtube channel by Mikrowave, and thought to give it a crack. The original had problems with insufficient squelch sensitivity , opening only on very strong signals, limiting the radios usefulness.
V1a is the RF buffer stage, V1b the superregenerative detector. The 330k gridleak and 56pf coupling capacitor set the quench frequency at around 80 kC. The detected output is amplified at V3a after the passive LPF removes any residual quench. To simplfy the filtering here, the quench frequency is much higher than is usual.
After passing through the squelch gating diode, the final audio stage V3b amplifies to speaker level. The high frequency audio components are amplified by V2b, audio modulation response is limited by the small 150pF coupling caps , detection and level shifting is accomplished in V2a by driving the grid negative by the diodes lifting the cathode by 1.4 volts. A negative going waveform at V2a anode ,with noise, keeps the .47 mfd capacitor negatively charged, reverse biasing the squelch gating diode, thereby cutting the audio. The screen of V2a is varied by the squelch threshold control to vary squelch gain and valve DC bias conditions, ie the anode standing voltage.
The SR sensitivity is -107 dBm, this is also near the minimum signal to open the squelch, so is adequately sensitive for weak VHF signals These sensitivity figures are standard with these common designs. With the values shown, rx coverage is from 110 to 144 mc. Audio stages use the ECF80/6BL8, the SR uses a 12AT7, ECC81. The squelch is effectively a noise squelch, full limiting occurs at around 2 microvolts RF input. Total DC current draw with the solid state "vibrator" HT supply is 1.2 amps at 12 volts, otherwise the radio can use a 12v AC supply, the backwards mains transformer then stepping it up to around 150 volts to provide mains isolation.Audio output is approx 400mW, adequate for a quietish room, Running the heaters on AC will probably introduce hum becuase of the very high audio gain, if so, rectify and filter this supply. Most of my valved designs use the ECF80, a triode/pentode TV valve, these are easily got and are cheap.