Some time ago came my son Tomas ok4bx with idea monitoring QRSS signals. I liked that idea and I decided made in my free time, I am in pension, that I will build something related with QRSS. At first I homebrewed DC RX on 30m – M0AYF design. Another device was MEPT with DDS module on 40 and 30m, when I made 250mW RF part and Tomas worked on logic unit and finally revive it. I am friend of OLD TIMER gears, then it was not too far to idea make QRSS DC RX with tubes. Today it is “anarchy” when there is 5W input power of heater and another half Watt in plate circuit. What a handicap, but I tried succeed – just challenge. And I did…

Common concept of QRSS RX is usually direct conversion circuit. It is important to have high frequency stability and good sensitivity and selectivity of receiver. It is not so hard achieve these parameters. Stability in my receiver is given by only one oscillator controlled by crystal. I was searching for crystal with large grinding in my drawboxes, and lucky helped me. I found one in thermostat which I bought from army stock sold out. Crystal is in glass package, but not in vacuum, perhaps filled with some kind of gas for minimal heat resistant – I must admit, I do not know it for sure. What is sure, that crystal in circuit with tube oscillate pretty stable. There is common Clapp osciallator in triode tube E1 connection with tuned circuit on ceramic coil 3uH in plate. I used this connection for good oscillator outpul level . Tuned circuit helps with own selectivity to increase spectral clarity of signal coming to mixer.

Input mixer (product detector) is realized as balanced with two tubes E2 and E3 in triode connection. It has advantage, that noise of this stage is lower and is not need to care of second grid cuircits with next components. In input is tuned band pass filter with inductive coupling. Here are also used army surplus coils 3uH with Q>350. Band pass filter here define selectivity and also partly receiver’s sensitivity. In tube cathodes is variable resistor for tunning tubes working points to symmetry. For tunning it, connect RF voltmeter into antenna connector, then set resistor to position of minimal voltage on voltmeter. DC voltage on both cathodes E2 and E3 should be same approx. 2,9V. Level of RF voltage from oscillator is 1.2V eff. Voltage can be set up with tap on oscillator’s anode coil. In my case it is on second turn from cold end. In anode circuit is used driving transformer from old transistor AF amplifier. Primary winding is tuned with condenser 15nF to 1kHz frequency range, secondary winding in grid of E4 drives AF amplfier in A class to maximal gain. In anode of E4, instead of output transformer, is used common transformer 220/10V-0.15A. Secondary winding is not grounded and is connected directly to jack and PC sound, so radio and computer are galvanically isolated.

I choose tubes 6Ž1B-V, they have small dimensions and highl transconductance. That tube type has wire leads and here is recommended keep them as long as possible to prevent heat dilatations and broke glass sealing. In my breadboard construction I soldered them 3rd pad far from hole leads came through.

Tube heating is serial-parallel and it can be supplied by stabilized 12.6V / 0.42 A. Anode volage is also stabilized at 55V and output current 12mA. In heater pathes are inserted 25uH chokes prevent RF coupling among each stage. It is important check heater voltage and if it is higher than 6,3V reduce it with approx 1.2kOhm resistor paralel to tube, just SMD package is enough.


All my effort I put it to this project had possitive result. Receiver quality is good for this purpose and it is given by used coils with high Q and used tubes type. I consider add band pass filter with one Xtal on input by M0AYF design, cause during contest there was seen interference from big gun sations.
This radio is unique, I do not expect next development nor production. I homebrewed it for fun. If there is anybody who try make it on its own and need help, do not hasitate to ask me.

Zdeněk Borovička OK2BX, 2/2013


THAT is a NICE design! 73

THAT is a NICE design!

73 /paul W3FIS

Good day OM! This is the

Good day OM! This is the possible EMP resistant receiver design I am looking for a long time. Please publish the data of L1,L2,L3 and T1 .I plan to use 6AK5 which is what I have, Congratulation for a neat design.Looking forward for your assistance. Thank you. Emi Constantino,Manila,Philippines