Two Tube Superheterodyne Receiver

 

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This was my first attempt to build a superhet receiver. I had searched the internet for some low tube count circuits, and discovered Jack Bryant’s KE4ID one tube superhet. I liked the simplicity, but his circuit lacked an IF gain stage. I decided that if I was going to build a superhet, it had to have an IF amplifier stage. So, I used Jack’s circuit as a starting point, and then added a second stage for IF amplification, and reflexed audio. Here is the schematic:

Larger Schematic Here


I simplified Jack’s front end to a single tuned stage mainly due to lack of parts. Some component values were tweaked a bit. AVC was also added. Lack of a double tuned front end has caused image problems with strong local transmitters, but the addition of a bank of wave-traps when using the long wire antenna has minimized the problem.


The second stage, V2, is a reflexed IF and AF amplifier using a 6HJ8. IF regeneration was added to this stage later, after it was up and running reliably. A few points of interest, regarding this stage, follow:

  1. The regeneration is largely capacitively coupled. I had noted that it would work with the tickler coil leads reversed. I later disconnected the ground end of the tickler, and moved the pot to the high end, and the regen still worked. However, it works best with the tickler connected as shown in the schematic, and there is a difference in operation (especially when the AVC is on) depending on which way the tickler leads are connected.

  2. Increasing the regeneration to the point where oscillation begins will cause the set to operate in synchrodyne mode. In this mode, selectivity of the IF becomes extremely sharp and it will separate out adjacent channels unless they are extremely powerful. I believe that the presence of AVC voltage helps to regulate the amount of oscillation, and keep the sensitivity up. I notice no discernible decrease in sensitivity in synchrodyne mode. One thing of note is that due to the mainly capacitive coupling of the regeneration, the IF frequency pulls about 20 kHz high when it goes into oscillation. Hence the IF shifts up to about 282 kHz. Oddly, this doesn't seem to hurt anything. Future plans are to move the tickler coil to the cathode side of V2 to see if the proportion of capacitive coupling can be reduced without harming performance.

  3. The audio filter provides a DC path back to the V2 grid providing approximately 50% AVC, while 100% AVC is fed back to V1. This arrangement has a side effect of increasing low frequency audio response at low volume settings, and better high frequency response at high volume settings.

  4. The V2 screen grid is connected to the high side of the second IF transformer primary, and the low side of the audio output transformer primary. In effect this gives high gain pentode operation for the IF signals, and lower gain lower impedance triode operation for the audio signals. Compared to having the screen connected to B+, this has less tendency to oscillate at audio frequencies, although the audio gain is somewhat reduced. A milliammeter connected in the screen grid circuit acts as a simple signal strength meter. It's relatively useless when the receiver is in synchrodyne mode.

  5. It occurred to me after the set was up and running, that there are three different modes of detection taking place in this circuit (sometimes all at the same time). There is of course diode detection from the diode section of V2. With strong local signals, there is also square law detection happening on the IF signal on its first pass through V2. Finally, there is synchrodyne detection when the regen is turned up to sustain oscillation. If I had considered this before I had built the set, I likely would have thought that it wouldn't work and may not have built it at all. However, these three detection modes seem to coexist quite happily.


I have used this receiver with both an outdoor 60 foot long, 12 foot high inverted 'L' long wire antenna as well as a 30" diameter tuned loop antenna. The loop antenna, consists of a ten turn tuned loop and a one turn pickup loop. Since the receiver's tuning capacitor has 3 gangs, I used the spare gang as the loop tuning capacitor. The loop inductance was designed to match the RF coil inductance so that they would track properly. However, I had overlooked the self-capacitance of the loop, and found it necessary to add a padder capacitor get it to track properly. Overall signal strength with the loop antenna was noticeably less than with the long wire antenna, but there was considerably less noise, and the receiver has more than enough gain to compensate. The loop directionality is a major advantage in separating multiple stations on the same frequency.


Headphones used with this set are Sony Sport Walkman MDR-W08—certainly not sensitive, but adequate for the audio signal that the receiver puts out.



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This page last updated: February 25, 2021

Copyright 2009, 2015, Robert Weaver