Magic Eye Tubes were used as RF-indicator tubes on radios from the 1930s until the end of the tube era and needle movement meters displaced them.
I love the glow of these tubes and want to use one as a rudimentary S-meter in my homebrew HF-transceiver. I purchased two Soviet 6e5c tubes from Ebay. I think both of them were used. I also bought two VU-meter PCBs from a Hong-Kong Ebay seller.
I could not find a schematic, but drew the circuit diagram based on the PCB on a piece of paper in order to understand the circuit. From my understanding it is a basic buffer audio amplifier followed by a DC coupled stage that rectifies the negative half cycle to provide the negative DC to drive the Magic Eye Tube.
Soldering the PCB was simple and I had all components in my junk box. The tube is mounted in a 8 pin PCB-mounted socket. There seems to be room for a LED beneath the tube. Nice if you want to pimp it up a notch or two. I do not want such modern nonsens in my old-school experiments, so I skipped that.
For the power supply I purchased a high-voltage supply from Ebay. Unfortunately, it does not seem to be able to provide enough current to drive the tube at any higher voltage than 180V, so it is a bit dim. I have to find an alternative solution. For the heater I used 6.3 V DC.
I used a signal generator to test the circuit. It was very satisfying to see the Magic Eye Tube perform its magic. The above video should convince you.
Future work include soldering up another tube and mounting them in a box together with my Nixie-tube display and some hefty dekatrons.
The WA2EBY amplifier is a famous linear amplifier project published in QST in March and April 1999 by Mike Kossor WA2EBY. As this amplifier can give up to 50W out with 1W drive, it is a perfect pair for my Softrock RXTX amplifier.
The articles can be found on the ARRL-site (search for WA2EBY) and are highly recommended reading.
I have had a suitable box laying around for quite some time that was perfect for the amplifier project. I decided to go for Manhattan style construction using mainly the parts I already had in my junk-box and not order the PCBs and toroid set which are available from different sources on Ebay. In other words: a low-cost project.
The first I did was to make room for the IRF510 mosfets and two heat sinks. I drilled the holes in the aluminum box with a hole saw.
Then I made the RF-detector part (used for RX/TX) switching. It is in the upper part of the picture between the two relays.
I followed the articles and the schematics as best I could. The only alteration I did was to use 12V relays (instead of 15V) and a 7812 regulator for the relays and a 7805 regulator for the MOSFET bias instead of zener diodes. The above picture shows the power supply part. It is a rather conventional design. I addeda heat sink on the 7812 to make it handle 28V without dying on me.
I used a 1dB pad on the input (can be seen to the right of the left relay). It is probably not necessary to use a pad but it might improve the stability and provide a solid 50 Ω load for the Softrock RXTX Ensemble.
Mike Kossor recommends using teflon wire in output transformer due to the possible high temperature. I did not have that and used standard stranded wire which will probably survive just fine. I used coax cable for interconnections to and from the IRF510s.
I used 5k potmeters for the bias adjustment. The value should not be critical since it only acts as a voltate divider. The article does not specify the bias current, and different sources on the Internet recommend between 10mA and 100mA. I decided to try 60mA and used the method described in W2AEWs excellent video.
I only made one of the many low pass filters in the schematic (for the 17m/20m bands) following the instructions from WA2EBY. There is no band-switching at the moment, but there should be plenty of room for that later.
The network analyzer plot shows that the filter is far from excellent. The -3dB point is as high as 20 MHz and the 2nd harmonic frequency for 20m transmissions is only about -23dB down. It does not mean that the filter is useless, but I will change it later to a steeper filter and lower the -3dB point when I`m at it. Nevertheless, the output waveform seems nice, and an FFT analysis show that the 2nd harmonic is within legal limits.
A picture of the final assembled amplifier. The switch on the front panel is on/off. One LED indicates ON, whereas the other indicates TX.
The rear of the amplifier. I used banana jacks since I like them, they are robust and cheap. A protective diode takes care of business should I ever connect red to black.
The verdict
Using a signal generator and a dummy load, the amplifier easily produces about 30 W on 1W input. The amplifier seems very stable, and I have had no problems what so ever. The power consumption and efficiency is about the same as WA2EBYs figures.
The above picture show the unit with an MFJ antenna tuner for size comparison. A nice pair. With my Softrock I usually drive the amplifier to about 10-15 Watts on 20V and this have given me many contacts on JT65/9. I live in an antenna restricted environment, and have struggled hard to get out with my mere 1W from the Softrock. I believe that this amplifier will give me lots of joy in the future.
First of all, this SWR meter in this article is not exactly homebrew, as I based the circuit on the schematic in the excellent book Arduino Projects of Amateur Radio. Although I bought the book and respect the copyright of the authors I will share my own schematic here. Why? First, the design in the book is not completely original and is based on similar designs from others. One example here. Second, I changed a few things, removed some stuff, and added some other things, to my own liking, so the design is not identical anymore.
The circuit is based on two AD8307 log amplifiers, which are connected to the forward and reflected ports on a directional coupler. The AD8307 amplifiers gives a DC voltage of about 25mV/dB of the input signal, which is amplified using an opamp (LM324). The opamp also provides the reference voltage to the Arduino (AREF) to ensure that the full ranges of the A/D converters are used.
I used the fourth opamp in the quad LM324 to provide input voltage reading. That is why the PCB in the picture has two power jacks. One input and one output. That way I know the drive voltage for the radio. I primary plan to use this device for my Softrock.
I created the schematic and the board in Eagle, and submitted the gerbers to Elecrow. I received 10 PCBs after about 25 days.
The board is not much larger than a standard 20x4 LCD.
I choose to use an Arduino nano rather than populating a AVR328 on the board and messing with USB converters. I am glad I did. I did one mistake on the board however. The Arduino was not powered from the 5V rail. The problem was that it was an error on the Eagle footprint for the Arduino Nano. I just downloaded the footprint file uncritically from the Internet without checking it. Other than that, It was fine.
Another problem was that I got oscillations on the LM324 opamps connected to the AD8307. The oscillations were around 40KHz and about 400mVpp on top of the DC signal. Hence it was impossible to calibrate the device. The solution was to desolder the output capacitors on the LM324. I do not think they are really needed on a DC design.
This was my first SMT design, and I am fairly happy with the result. The SWR and the power measurements seems accurate. I used the Arduino software from the book, and modified it to include a larger display and some other things. As you may notice, there are no buttons on the device. Originally I planned to use a rotary encoder and a bunch of menus, but as they say in the Soldersmoke podcast, menus are for restaurants. I totally agree. Besides, programming all kinds of features to the device is totally insane. This is a simple SWR meter and thats it.
The files are available here, should you be interested. Note that there is an error in the schematic. The Arduino is missing +5V, but it should be easy to fix.
I have always wanted to try out Nixie tubes in one of my projects, and here we go. Nixie tubes come in many different sizes and there are both top view and side view nixie tubes out there. I settled on some russian IN-12 top view nixie tubes as they are cheap, and should fit nicely on a front panel (for example on a radio).
The IN-12 was originally used in russian multimeters, radios, frequency counters. This R-155A Brusnika Radio Station serves as a good example on how nixie tubes were used in the cold war era.
Recently, this particular tube has been used for many hobby purposes such as clocks, geiger counters, etc. While I am mentioning it, the major page for steampunk and Nixie-fans is Bad dog designs having loads of excellent clocks using nixie tubes. Ok, enough drooling over other designs. Back to my modest attempts.
Testing a nixie
I purchased a 110-180V power supply kit from ebay. I highly recommend this kit from seller lumos-sk. It comes with an excellent build manual and was very easy to assemble.
The assembled power supply was set to 170V...
...and the IN-12 glows fine.
A PCB for four IN-12 tubes.
To control the nixies, I made a PCB in Eagle. The circuit uses two 74HC595 shift registers to drive four K155ID1 decimal decoders.
The schematic is shown above.
It is possible to cascade several of these boards to drive eight, twelve, sixteen, etc Nixies from just three pins on the microcontroller. I ordered the PCBs from OSH-park, and they were excellent.
However, I should have read Kevin Ryes blog before creating the PCB. I just took it for granted that the Eagle IN-12 part was correct, but as Kevin Rye found out, it is not. The pin numbering on the silk screen is wrong and the anode is connected to pin 5 instead of pin 1. However, if one ignores the numbering, my PCB works if the IN-12 is mounted on the back side of the board (i.e. mirrored).
Originally I planned to use IN-12 sockets (purchased from Ukraine) but since the IN-12 part was mirrored, the mounting holes were on the wrong locations and it made no sense to use the sockets. In addition, the parts I got were used, so I had to desolder chunks of 30 year old wiring. Totally boring work. So instead, I went back on Ebay and ordered some pins from the old Soviet Union (see picture above). They were excellent for this purpose.
To test the shift registers, I copied the Arduino code made by Imperkins over at Instructables. The only change was to add support for an additional shift register.
The above film shows the Nixie PCB in action.
As mentioned, it is simple to cascade several PCBs to control more nixie tubes. My secret plan for the future is to use eight nixie tubes as a frequency display for a HF radio, such as Farhans Minima.
I have been struggling to understand how my simple dipole antenna works (or doesn't work). The diycrap way to understand stuff is usually to read, build and measure, and then read some more. The key factor here is the measuring part, as I need to measure the standing wave ratio (SWR) on the feed line, as this is a key parameter. To measure the SWR, I need a directional coupler. And it is going to be homebrew.
Later, the coupler is going to be the basis of a SWR-meter, but for now, lets just look at the coupler.
The directional coupler design is classic and well known. Notice the input port and the output port on the upper line and the forward port and the reflected port on the bottom line. (My graphics software is Field Notes.)
Some theory
The principle of the coupler is based on two toroid transformers. The first is a current transformer and the second a voltage transformer. Each taking samples of the signal on the main line. The two transformers are equal, reducing the current and the voltage to the same level, meaning that the impedance is constant. The two transformers are connected in such a way that for a forward signal, the signal cancels out on the reflected sample port, but adds up on the forward sample port. And vice versa, a reflected signal adds up on the reflected sample port but cancels on the forwards sample port. Since we now have a sample of both the forward signal and the reflected signal, it is straightforward to calculate the SWR.
It is simple to construct the directional coupler. The transformers are FT50-43 toroid cores with 32 turns of 24 AWG enamel wire. The primary winding is simply a piece of RG58 through the torioid (i.e., one turn). Different designs use different toroids and number of turns. I settled down on a design found in Arduino projects for amateur radio.
I used a aluminum box and BNC connectors. I used copper clad boards as shielding here and there. I did not have any 50 ohm resistors in my junk box so I used two 100 ohm resistors in parallel. They are all 2W resistors, which is totally unnecessary and overkill.
Rudimentary testing
Testing the forward port. The output port is connected to my 50 Ohm dummy load. As signal source I used my GW Instek GFG-8255 signal generator, which unfortunately maxes at 5.5 MHz.
8.2 Vpp on the input port resulted in about 244 mVpp on the forward port. Hence, the coupling factor is about -30dB. The signals are not in phase, but that does not matter for voltage measurements in a SWR-meter.
Testing the reflected port
8.2 Vpp on the input port results in 1.60 mVpp on the reflected port. This translates to a reflected signal of -74dB. The directivity is the reflected signal (-74dB) minus the coupling factor (-30dB) which equals -44dB.
Testing over the HF band
Later I borrowed a TTi TG2511 function generator which goes all the way up to 25 MHz. I tested with 10 Vpp on the input port and got these results:
frequency
coupling factor
return loss
1.8 MHz
-30 dB
-84 dB
3.5 MHz
-30 dB
-80 dB
7 MHz
-30 dB
-75 dB
10 MHz
-30 dB
-72 dB
14 MHz
-30 dB
-69 dB
18 MHz
-30 dB
-66 dB
21 MHz
-30 dB
-65 dB
25 MHz
-30 dB
-62 dB
The directivity is between 54 dB and 32 dB. The numbers seem reasonable, but indicates that the coupler should not be used for VHF/UHF.
Future work
The plan is to build a power meter and SWR meter using AD8307 logarithmic amplifiers and an Arduino. I will probably base the device on the design from the book Arduino projects for amateur radio.
I have created a simple 50 Ω dummy load to test transmitters. I also added a simple RF diode detector so I can measure the peak voltage, and calculate the power.
The dummy load consists of eight 100 Ω resistors rated at 2 W so the load should handle 16 W, at least for short periods. I constructed the dummy load using a combination of ugly construction and Manhattan style, by gluing pieces of PCB (as isolation pads) on top of a ground plane PCB. Then I soldered the components directly on the copper without drilling holes.
RF Probe
The RF probe part consist of a simple 1N4148 diode and a 0.01 uF ceramic capacitor. I only had a 50 V capacitor in my junk box, but it should be sufficient given that this is a 16 W dummy load and 16 W translates to 40 V peak. I added female banana connectors, which connects to a multimeter. The power can be calculated by Ohms law by subtracting the forward voltage on the diode from the measured voltage, then multiply by 0.707 (to get RMS), then square the result and finally divide by 50 Ω. Some homebrewers add a voltage divider to their RF probes using a 4M7 resistor to get RMS voltage directly. I did not bother since I am sure the input impedances on my multimeters varies.
The calculations might seem a bit cumbersome, but I might print out a small lookup table and glue it to the box to have some ballpark figures. However, this is not a precision instrument. The forward voltage on the diode varies with load and can be somewhere between 0.4 and 0.7 V. I simply use 0.5 V in my measurements.
L200 and L201 is wound to about 0.9uH, whereas C200 and C202 is 100pF and C201 is 330pF. Everything is soldered to a PCB (ugly construction) and I added two switches and a coax to enable on and off switching of the filter.
Ugly construction...
I tested the filter using a 25 MHz function generator and my Rigol 1052 scope. The filter was connected to a dummy load. The first two pictures below show 1 Vptp at 10 MHz with the filter off and on respectively. The two pictures thereafter show 1 Vptp at 20 MHz (on and off).
10MHz lowpass filter switched off
10MHz lowpass filter switched on
20MHz lowpass filter switched off
20MHz lowpass filter switched on
As a three pole filter it should have a roll off at 18db/octave. This seems about right when I compared 12 MHz and 24 MHz (9.3V and 0.9V ptp respectively). I measured the -3dB point to be around 14.7 MHz. The purpose of the filter is to reduce the the 2nd harmonic when transmitting in the 30m band (10.1 MHz). The second harmonic is down about 13dB.
Function generator on burst mode, Oscilloscope on FFT, center frequency is 10.5MHz and filter is off
Same as above, but lowpass filter is switched on
I also looked at the FFT on the Rigol scope while using burst mode on the function generator. In the pictures above, the center frequency is 10.5MHz and the grid is 12.5MHz in the X axis and 10dB in the Y axis. Here it seems like the filter contributes to 20dB attenuation at 23MHz which is about right. I have not measured the second harmonic while using the Softrock, since I have no real spectrum analyzer. However, as the filter is working, I guess I can legally transmit on 30m. 73!
