Simple 12AT7 VHF FM Tuner


This project came about simply as an improvement to my Improved One Tube Tuner.
While that circuit was about the simplest one could build for FM reception, it does suffer from less than optimimun aerial coupling.
So, this circuit was designed as a compromise between the 6C4 design and the exisiting 12AT7 circuit.
The audio quality and output level is the same as the Improved 6C4 design, but with the RF performance of the original 12AT7 circuit.
As it turned out, performance was very good. It seems the simplified quench filter of this circuit (the 150K and 470uuF) is quite adequate.
I won't launch into a discussion of circuit operation as that has been covered in the other articles.


I used modern power transformers for the power supply. The heaters are in series and powered from the 15V secondary of the first transformer, via a 10 ohm resistor.
This 15V AC supply is then stepped up by another transformer, which after rectification and filtering, provides the 140V supply.

Circuit of the tuner. Note the 15uH chokes used in the protoytpe are no longer available, and are now replaced by a superior home made version.

The tuned circuit is 4 turns of 18 gauge tinned copper wire, air cored with a diameter of 10mm. The RF chokes are made by winding 75cm of 25 gauge enamelled copper wire on a 6.3mm plastic former.
75cm represents a quarter wave at 100MHz, so the choke has a very high impedance in the FM band. Note that they do not measure 15uH as did the original commercially made chokes that were used, but the operating principles are not the same. Measured inductance is closer to 2uH. The particular 15uH chokes specified on the circuit diagram are 1) no longer available, 2) did not perform as well as the home made choke anyway.
Note that if oscillation seems poor, the connections to the choke may need to be reversed. This is because of magnetic coupling to the tuned circuit. If the coupling is in the wrong direction, it will cause negative feedback, and thus a reluctance to oscillate.

Like the later versions of the 6C4 and 12AT7 tuners, I've used variable cathode bias for the regeneration control. This eliminates the need for the 30V negative supply as used in the original battery operated receiver.
Sensitivity is good. For noise free reception -  30uV FM
                                                                               10uV AM

                                        Readable signal -  10uV FM
                                                                         3uV  AM

                                               Detectable -   1uV AM

Like the other super regenerative receivers I've described, reception is possible with no aerial in areas with good signal strength. Audio output is meant to be fed into a high impedance two valve amplifier of typically Rin >500K.

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