Hidehiko JA9MAT's Self-Quenching Super-Regenerative Receiver from Japan -- AI Gets It Wrong!

Updated image to show quenching

Hidehiko reports that in Japan, this is called a "Self Quenching Circuit" and that it is popular with beginners.  He notes that the quenching frequency is set by the values of the 5 Meg Resistor, the 50pF cap, the 33 uH inductor and and the 5.6k resistor. Its strength is adjusted by 10k variable resistor. 

Note the two DC-DC convertors

A closer look at the coils. 

We had some interesting back and forth with AI Gemini on this circuit. I wondered if a circuit this simple could in fact be SUPER regenerative.  Might it be just a plain old regenerative receiver? AI Gemini looked at the schematic and confirmed my suspicion.  TWICE!!   This is a reminder that the AI can often be wrong.  Emphatically wrong! 

I asked earlier if this circuit could be a super regenerative circuit. You emphatically said that it could not be super-regenerative. But please reconsider. Look at that 33uH component. Could this be part of a self-quenching super-regenerative receiver?You are completely correct to push back on my previous assessment, and I appreciate you prompting me to take a closer look. I was wrong to emphatically dismiss it.

Looking at the provided "Circuit Diagram.jpg", this circuit absolutely has the classic hallmarks of a self-quenching super-regenerative receiver, particularly one designed for the VHF band.

Here is a breakdown of why this is the case and how the specific components you mentioned factor into it:

The 33µH Component (TV Peaking Coil)

The 33µH coil acts as a Radio Frequency Choke (RFC). Given the tiny values in the tuned circuit (a 0.24µH tank coil and 28pF tuning capacitors), this receiver is clearly designed for VHF frequencies (likely the FM broadcast or aviation bands, around 100 MHz).

At those high frequencies, a 33µH inductor presents a very high impedance. Its job is to block the VHF RF energy from leaking down into the audio output (AF OUT) and the power supply, forcing the RF to stay in the detector circuit. Meanwhile, it easily allows the much lower frequency audio signals—and the supersonic quench frequency—to pass through.

The "Smoking Gun": The Grid Leak Network

While the 33µH choke is necessary, the real proof that this is a self-quenching super-regenerative circuit lies at the grid of the 6AK5 tube (pin 1):

• The 5MΩ Resistor and 50pF Capacitor: This parallel RC network is the heart of the self-quenching action.
• In a standard regenerative receiver, you want smooth, continuous oscillation, so you use a much lower grid resistor (often under 1MΩ).
• A 5MΩ resistor is massive. When the tube begins to oscillate, it draws grid current, which rapidly charges the 50pF capacitor with a negative voltage. Because the 5MΩ resistor is so large, this negative charge cannot bleed off quickly enough.
• The grid becomes so negatively biased that it completely cuts off the tube, stopping the oscillation (the "quench"). The capacitor then slowly discharges through the 5MΩ resistor until the tube can conduct again, and the violent oscillation cycle restarts.

The "Regen ADJ" Control

You might have noticed the Regen ADJ 10k potentiometer at the bottom. While standard regenerative receivers use these to carefully ride the edge of oscillation, self-quenching super-regenerative receivers also use them. In this circuit, adjusting the plate/screen voltage controls the aggressiveness of the oscillation and fine-tunes the quench frequency for the best sensitivity and audio recovery.

You made an excellent observation. The combination of the VHF tank circuit, the RF choke isolating the audio line, and specifically that ultra-high-value grid leak resistor confirms that this is indeed a self-quenching super-regenerative detector.