Receivers for Watching 7120 kHz

This page describes simple receivers intended for wide-bandwidth listening to amateur-radio radiotelegraph (CW) signals at one frequency. Although I built them for 7-MHz listening, they can be configured to work on other bands. Figure 1 and Figure 2 show two variations on one regenerative watch-receiver design.

Schematic diagram of 12AU7/ECC82-based regenerative receiver for watching 7120 kHz.
Figure 1—Regenerative watch receiver using 12AU7/ECC82 dual triodes. The detector is a synthetic tetrode; the calculated inductance of L1 is 1.42 μH; and the 105-V supply is VR-tube regulated. The resistive attenuation at the receiver input (approximately 18 dB, provided by a terminated 10-dB pi-attenuator pad in combination with mismatch loss) brings the noise floor ("band noise") of my antenna system down to a level just above the receiver noise floor at just above critical regeneration. (Other antenna systems may require more or less attenuation.) Although the regeneration control (REG) arrangement shown here uses a 10-kilohm control bracketed by fixed resistors to spread the circuit's critical-regeneration point across the control's rotation, you may want to begin with the regeneration-control arrangement of Figure 2 (50-kilohm control in series with 100-kilohm resistor) when setting up your tuned circuit and its regeneration tap.
Schematic diagram of 6AK6-based regenerative receiver for watching 7120 kHz.
Figure 2—The Figure 1 circuit modified for use with 6AK6 pentode tubes. (6AK6s are generally less expensive than 12AU7/ECC82s or 6C4s [the equivalent of one 12AU7/ECC82 section] on Ebay.) Although a triode-connected 6AK6 works in the triode AF amplifier portion of Figure 1, I wanted more gain than the amplification factor of a triode-connected 6AK6 can provide (μ of 9.3 as opposed to 20 for a 6C4 or 12AU7/ECC82 section) and so used the fourth 6AK6 as a pentode voltage amplifier as shown here. (A 6BH6 also works well in this stage with no other changes.) So smooth is the transition across critical regeneration with the 6AK6s that the simple regeneration control arrangement shown here is sufficient for operational use without further modification. This site's Homemade Equipment Pictures 3 page shows a practical implementation of this circuit.

Muting the Figure 2 circuit (for silence during transmitting periods) with relatively thump-free make and break required some experimentation. The gain of the 6AK6 pentode AF voltage amplifier is high enough that grounding and ungrounding its grid—a muting method I've successfully used for years with triodes in this first-AF stage—results in unacceptable bangs. Using a high-voltage power MOSFET to ground and unground the AF amplifier screen (with 1 kΩ of series resistance to slow discharge of the 1-μF screen-bypass capacitor) was pleasantly soft on make but too thumpy on break, as was grounding and ungrounding the pentode plate for dc. (Similar brute-force ground-the-B+ methods were sometimes used to obtain thump-free Morse-code-transmitter keying in the 1930s.) In the end, merely grounding and ungrounding the circuit's AF output terminal with a Radio Shack 12-V reed relay proved to be an acceptable solution.

This circuit can be easily scaled to other frequencies. So far, I've used it in spot-tuned form at intermediate frequencies (455, 1700, 1750, 1843 and 1850 kHz), and in tunable form at 80 meters (3480–3600 kHz) and 40 meters (7100–7125 kHz), with good results. In a future experiment, I'll drive this circuit with the 3180-kHz second-IF strip of a Yaesu FR-101S MF/HF receiver. I expect that the dynamic-range compression afforded by the FR-101's heavy automatic gain control will usefully augment the already good frequency-pulling dynamic range of this high-capacitance detector design.


Revised July 19, 2015 CE. Copyright © 2013–2015 by David Newkirk (DavidNewkirk@gmail.com). All rights reserved.
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