7MHz Crystal Oscillator Design

After constructing a 40m wire dipole that works with my SDR setup, I needed to start working on a transmission system. At the heart of virtually any RF system lies a stable oscillator, and crystal oscillators are ubiquitous in many low-power (QRP) rigs simply because they are so stable. After some rough math and a lot of simulations in LTSpice, I came up with this design to give me somewhere around 7 dBm of power.

The first stage is simple Colpitts oscillator topology with a “bent” 7.03 MHz crystal resonator. The 30 pF variable capacitor (C1) provides around 2 kHz of tuning. The output stage is a common-collector (Q4) and emitter-follower (Q3) with a negative feedback loop. I forget exactly where I saw this configuration, but I thought I would try it out and see if it worked. As you might have guessed, the output is loaded with higher order harmonics resulting in a waveform that doesn’t resemble a sine wave at all. I made sure to include a simple second-order low-pass Butterworth filter on the output to filter the output.

Pictured above is the ugly constructed version of the oscillator in all it’s dead-bug style. I built it on a scrap piece of double-sided FR4 and overall it’s performance came out fairly close to what LTSpice had predicted. I got around 6 dBm of output and the second harmonic is around 29 dB down (around -23 dBm). That’s not the cleanest of signals, but it’s about right for the filter. Below is the output shown on my HP 8595E spectrum analyzer.

For the next phase, I’ll likely be adding control of the oscillator via an ATTiny chip. This will give me the ability to automate on-off keying of the device turning it into a simple CW beacon. One thing that could be improved is the current draw (~40 mA) from the emitter-follower at Q3. Basically, it’s a class A amplifier so it’s not the most efficient design in the world, but it provides enough power for driving an ADE-1 and could run off a small solar panel if I wanted to use it in the field.

Testing a 40m Wire Dipole on Mt. Hood

Here’s a quick video showing how I deployed a DIY 40m dipole antenna on Mt. Hood a while back. Though not the most interesting video, I think it shows the results pretty well. I used an AirSpy HF+ supplied by KK7B for a project I’m working on. A Panasonic Toughbook CF-30 helped stave off the rain.

The dipole itself is two sections of 65 ft speaker wire connected to an SO-239 connector. I took a couple of sections of PVC and drilled holes in the end so I could attach some paracord and hoist it into the air. I tuned the antenna using a fancy MFJ-259C antenna tuner. Bethany and I both had very cold fingers after pulling the antenna down.

I’ll be posting more on this as it develops… for real though.

2m AM Exciter

Here’s a project that I’ve been working on for a class recently. This is a very basic 2m AM Transmitter that operates from 144.25 to 144.31 MHz. It does put out a low powered RF signal (around 5 mW PEP) that has been copied over 1 mile from the transmitter. This is an ongoing project that will likely evolve into something else, so I should get time to do some more in-depth posts as time goes on.

Basic Voltage-Controlled Amplifier

I’ve been looking at the theoretical principles that govern the voltage-controlled amplifier (VCA) recently and came across some simple VCA designs that caught my attention. A common technique involves a JFET and an opamp to achieve VCA type operation by using the JFET as a voltage-controlled resistor somewhere in the input path. I’ve seen a good deal of these circuits out there, but I had trouble getting good simulation results from what I had seen. I cam across an article by Rod Elliot of Elliot Sound Products which covered some nice history and interesting discussion of the VCA in general. After going over the information there, I came up with this design based on some of the more basic designs presented.

Basic VCA LTSpice Schematic

One of the things I’ve been most interested in is trying to accomplish this with a single supply which is common in most stompbox setups. In the schematic, there’s an emitter-follower stage just to act as a buffer for the input signal followed by an inverting opamp stage. To make this work, both C1 and C2 are required to effectively AC couple both input signals (Vin and Vctl) to the opamp stage. There’s an RC network that is supposed to tame the distortion in the output by taking a portion of the output and connecting it to the input of the J201. The article explains this in decent detail. In simulation, the it seems to smooth out the non-linearity of the JFET as Vctl changes. Using the J201, LTSpice gives a decent linear-like response over a range of around 500 mV (0 to -500 mV at the input) and operates decently with a 500 mVp signal.

VCA Linearity Test

For use with an LFO, I found that the best results happen with a slight negative voltage offset and a signal who’s amplitude peaks at 0V (i.e. 250 mV sinusoidal signal with a -250 mV offset). Of course, this is all highly dependent on the threshold voltage of the J201 which can range from -0.3 V to -1.5 V according to the Fairchild datasheet. It will be interesting to see the results of this circuit on a breadboard.

LFO modulated VCA Simulation

[1] Gray, P. (2009). Analysis and design of analog integrated circuits. New York: Wiley.

[2] Sound.whsites.net. (2017). VCAs. [online] Available at: http://sound.whsites.net/articles/vca-techniques.html [Accessed 1 Sep. 2017].

The Cicada: a push-pull vacuum tube amplifier build

Well, 2017 is more than half over now. This means I should probably post something to get at least one post in for this year. I’ve been meaning to do a write up about this project for a while, but time isn’t as freely available as I would like it to be. Go figure. I had been wanting to try a push-pull tube amp build for years and scrapped together a design from parts I had lying around. I play a Gretsch G5265 baritone guitar with bass VI strings (this was its original configuration when I got it back in ’08), and I’ve had a hard time finding an amp that really suits the sound of the mini-humbuckers and it’s very deep tone. I decided to etch together a blackface Bassman-like amp with a tone stack capable of giving it the upper-mid range that it needs to bring out the high notes of the instrument. Unfortunately, I still haven’t gone through a rigorous analysis of the design other than some tone stack simulations in LTspice, but I’ll give out the schematic and details of the build.

6L6GC amp schematic

First off, the secondary voltage of the transformer should read around 300 V peak or around 600V peak-to-peak. Unlike a typical Bassman, it lacks a choke and the negative feedback resistor value is pretty large by comparison. I just swapped out values until I liked the sound and the oscillations stopped happening. I might swap this out with a 22k resistor again, but I can’t complain so far about the tone. There is one side of a 12AU7 (V2b) which is currently unused. I might make a simple phase shift oscillator of that later to make a tremolo circuit. If someone wanted to swap out 12AX7s in this design, the thing would be hella dirty and loud for a 40-ish watt amplifier (I still have to measure the actual wattage delivered to the load).

6L6GC amp tone simulation
6L6GC amp tone simulation

Here is a plot of the magnitude of the tone stack simulation with the treble at noon, the bass at 9 o’clock, and the mid all the way up. The behavior of the stack is interesting as changing any one of the pots can have a sometimes dramatic effect on the position of the mid center frequency. This is one of the made drawbacks of passive tone stacks since the change in resistance of any potentiometer changes the transfer function for the network. However, the tone stack does lend itself to also listening to recorded music when setup properly. We’ve actually used this as a PA head in conjunction with a subwoofer which worked out really well. It’s bright nature actually seems to work well for vocals and low-mid while the sub picks up anything under 150 Hz or so.

6L6GC amp guts

I have a number of pictures that show how the layout progressed from right to left inside the chassis. Basically, I just started on the right side closest to the power transformer and worked towards the input. The biasing circuitry is towards the front of the amp by the large filter caps in the picture.

6L6GC tube glow

I was reading through Theory and Application of Electron Tubes [1] and discovered that the blue plasma color is actually caused by a spray of electrons from the cathode that actually overshoot the anode and slam into the glass. In the picture, you can see how the blue light only cover the area surrounding the big gray metal anode inside the glass. My new JJs that I put in there don’t have such an extreme blue color to them during operation.

Lastly, pictured above is the final build with the steel cage situated on top. The chassis is aluminum while the cage is steel. Both are manufactured by the fine folks at Hammond. I picked these up off CE Distribution’s website for a decent price. There’s definitely some room for improvement in the overall, but I really didn’t think it would work at all. I do think that it’s one of the nicest sounding amps I’ve ever owned regardless of the fact that I built it. I’ll post some sound samples eventually with some different instruments so anyone who stumbles upon this post can hear what the amp sounds like at lower volumes at least.

[1] Reich, H. (1944). Theory and Application of Electron Tubes. New York [usw.]: Mc Graw-Hill.

Milling PCBs with Linux…

This is a quick outline of my LinuxCNC setup. Basically, I have an old HP Compaq Pentium IV that’s running Xubuntu 14.04. I had to install the Xenomai real-time kernel in order to make it work with LinuxCNC 2.5.2 which controls my Chinese CNC3020T.

I’ve been using 0.1mm 30° engraving bits for etching which have worked nicely. For drilling, I’ve got some carbide 0.7mm drills. I haven’t broken any of them thus far. I’ve setup a Dremel 3000 with a 220-01 rotary tool work station for additional drilling by hand.


For sanding, I’ve been starting off a freshly etched board with a 180 grit sanding block and then moving down to a 320 grit for smoothing. So far, I’ve been impressed with the results. The following picture shows a single pass isolation trace after sanding at ~75x magnification.




I believe it’s time to put an end to this kind of insanity. There can be no economic growth if human beings continue to be reckless with the environment that supports them. Clean air, water, and soil are necessary for all life to flourish. That ought to be a priority.

Differential Amplifier w/ Cathode Follower

Differential Amplifier with Cathode Follower

This is a circuit designed for a classroom project whereby the instructions were to “improve” a differential amplifier circuit. The differential amplifier design is essentially an exercise in understanding the inner workings of an opamp, and it effectively works in the same way. The figure below is a schematic for the differential amplifier without the cathode follower output stage. In the LTspice simulation, the input signal is connected to the V+ terminal and the V- terminal is connected to ground. There is no feedback loop between the output and either input terminal making this a high-gain open-loop configuration. However, adding a resistor from V- to ground and one from the output terminal to V- would accomplish the same results as a non-inverting opamp.


I wouldn’t say that adding a cathode follower “improves” the output stage of the amp. It was more an experiment in comparing different solid-state output stages with a vacuum tube stage operating at very low voltages. However, I will say that this thing sounded amazing with the couple of guitars I tested through it. Putting a potentiometer in the feedback network allowed me to play with different gain settings. It’s a very bright sound overall giving lots of high-end sparkle, but the breakup was quite remarkable. I suspect this might be a very usable configuration for a tube mic preamp or a number of audio applications. Hopefully, I will get an opportunity to revisit this before too long.

For those of you interested in the ins and outs of this experiment, you can download our full report here.

Already Dead Tapes releases Document II

“As we march along on our mission to create a diverse and expansive creative community, we reach a new milestone: 200 releases. To this end, we bring you ‘Document II’, a retrospective cassette that comes with a Golden Ticket good for downloads of our last 100 releases. This is a one-time limited edition of 200 golden cassettes with hand numbered gold fold j-cards and golden tickets.

The mix itself is a collection of just a few of our favorite tracks from the last one hundred releases, a diverse collection of tracks that span genres, from artists all over the world. As always, thank you for listening.”
– Joshua Tabbia, Already Dead Tapes, 2016