We recently purchased two kilns for firing ceramics. These kilns have the older-type manual controls that require you to adjust the settings during the firing to achieve the correct temperature profile. Of course we can improve on that!
Check out this new project on the Projects tab under Kiln Control, and look for periodic updates there as the project progresses.
I ended up making a couple of modifications to the KG-650 to improve the output. The first was to add a 2 Megohm multi-turn trim potentiometer at the output of the oscillator to reduce the signal level feeding the mixer stage. I adjusted this potentiometer until the output waveform looked much better as you can see below.
If you refer back to the circuit diagram from an earlier post, I clipped the connection between C-8 and pin 6 of V-2. The loose end of C-8 was then connected to the center pin of the potentiometer, one end of the pot was connected to ground and the other end to pin 6 of V-2.
Next I made one more modification to improve the modulation level, The original modulation looked like this.
As you can see it is way over-modulated. By changing R-5 from 33K to 1 Megohm the resulting modulation looks like this.
Not exactly ideal, but certainly less modulated than before. This worked very well when I attached it to the radio.
So that’s it for the KG-650, should be a useful addition to the lab.
The new 12AT7 tubes arrived (I ordered two just to have a backup) and with great anticipation I plugged one in… and found that the output waveform hadn’t really changed that much. Hmmm….
That had me staring at the circuit diagram and doing some more careful analysis, which basically showed that the device is working as designed. Then I entered the circuit into LTspice and ran a simulation – and sure enough the output waveform looked very close to what I’ve been seeing. Turns out that folks on the internet also mention the non-sinusoidal waveform that this thing puts out. So it has actually been working as designed all along. The design is basically a compromise as to what can be achieved with just two common tubes.
Once again here is the output waveform with nothing connected to the output, and the level turned all the way up. The frequency is set to approximately 600KHz in the AM band.
So what happens to this output if you actually connect it to a radio? I connected the output to the antenna input of my old Hallicrafters S-40 with about a 3-ft length of 300 ohm twin lead. Here’s what the waveform looks like at the output of the KG-650.
Sorry for the blurry image, but you can get the idea – there is definitely some 600KHz in there but an awful lot of harmonics as well. At the radio antenna inputs it looks like this.
Once again apologies for the poor photo. This is basically a slightly attenuated signal from the one at the output of the KG-650, which is to be expected.
The radio is actually able to tune this signal at 600KHz on the dial, and if I turn on the modulation in the KG-650 you do get a tone in the radio. Not a very pure tone, but a tone nonetheless. So it could be used as a rough alignment tool, which is why I bought it in the first place.
Now I need to decide if I’m going to leave it the way it is and just use it that way, or if I’m going to try to improve it. Decisions, decisions.
It looks like a new 12AT7 tube may be in order for this unit. Here’s what I found as I was tracing through it. You can refer to the schematic, there’s a link to that in the last post.
First I removed the 6C4 audio oscillator tube so I knew the audio modulation circuit was not going to affect the measurements.
Then I started looking at the RF modulator and what I found was that it was oscillating quite nicely, but the amplitude of the oscillation was so high that it was overdriving the second triode stage, thus causing the distortion. Here are the signals on the gate and plate of the right-hand triode in the circuit diagram (pins 7 and 6 respectively).
Both signals are at 50V/division and ground level is one line down from the center of the display.The gate signal is on the bottom and the voltage on the plate is the top waveform. The frequency is set to about 200 KHz.
Looking at the data sheet for the 12AT7, it’s clear that by the time the gate voltage gets to about -4 volts, the tube should be shut off completely and the plate voltage should be at maximum. But what we see here is that the gate voltage is reaching about -70 volts before the plate voltage reaches maximum. So it appears that the gain of this tube is way down from the specification. That’s not too surprising since the tube appears to be original – it has a “made for Knight” stamp on it.
I did then replace the 6C4 audio oscillator tube and checked that circuit, which seems to be working fine and provides a nice sine wave at around 600Hz to modulate the RF signal.
So now it’s time to order a 12AT7 tube and see if this diagnosis is correct.
Sorry it’s been such a long time since the last post. I wanted to show you the latest piece of equipment that I picked up today at a hamfest – a Knight KG-650 RF generator. This should be really handy for aligning and testing radio gear, assuming I can get it to work properly. Here’s what the front panel looks like:
Here’s what the insides look like – appears to be in decent shape:
So I powered it up slowly with a variac and watched the power supply voltage carefully – the supply looks very clean with very little ripple. So I went straight to measuring the RF output. Unfortunately the output is not a clean sine wave, instead the lower part of the waveform is distorted as you can see below. This happens across the entire frequency range. The good news is that other than the distortion, the unit seems to work. Next step is to trace the signals through. Luckily it is a fairly simple circuit – I’ll attach the circuit diagram as well.
It’s been a long time since the last post and a lot has happened over the summer. The North Lab has moved into a new and better location with real windows! NSE labs also obtained our first custom printed circuit board – the power supply board for the B-17B bass preamplifier. Below you can see a picture of the board and an action photo of the South Lab CEO assembling it (he visited the North Lab for the construction phase, so you can also see the new workbench in the photo). Best thing was – it worked!
And here is the completed power supply board:
Obviously this is not an extremely complex board. The objective was to prove out the process of designing and manufacturing a custom printed circuit board, and that objective was accomplished.
The next step in this project will be to design a printed circuit board for the rest of the B-17B preamp. And then a power amplifier. And to that end the South Lab CEO came through again with a very inexpensive Class-D amplifier kit. So far we have built two of these and they work very well – up to 30W bridged output without the need for a heatsink. Nice for testing all kinds of things.
Then things got a little sidetracked. After all these years I finally decided to take the amateur radio technician class license exam and obtained my ticket in July. Then the FCC was kind enough to grant my request for a special call sign – K9NSE. So NSE labs is on the air! More details on that adventure can be found at this link.
We’ve also started into another project, this one again coming from the South Lab. It is a mid-side processing circuit. More details on that in a future post – and hopefully it won’t take so long to get that next post out.
The B-17B preamp prototype is finally complete! Here is a picture of the unit working with both Gain and Master controls set close to 0dB output.
The output buffer and VU meter driver circuit was added on a daughter board as you can see in the side view picture below.
So now everything is ready for some musician testing in the South Lab. Unfortunately this will probably wait until Memorial Day weekend when the prototype can be hand-delivered. After that testing the next step will be to order printed circuit boards to ensure the final model is rugged enough for musician use.
With some work I was able to fit the components onto a board layout within the size limits of the Cadsoft Eagle freeware version. Here is a picture of the layout.
The tube socket for the 12AX7 will be mounted on the backside of the board to allow the tube to be visible outside the case of the unit. The components external to the board will be the AC inlet fuse, AC filter cap, power switch, pilot lamp, the power transformer, the input and output jacks,the two VU meters, and the five potentiometers.
Meanwhile it is time to take a pause in development while the North Lab is being relocated. The new lab will be a significant upgrade over the bare concrete floors and dark spaces of the current lab. Definitely not-so-evil!
You may be wondering what has been happening with the B-17B preamp. Quite a bit actually, but mostly I have been getting educated on using Cadsoft Eagle to do schematics and circuit board layouts. My first attempt was just the VU and buffer section – basically the low voltage stuff. Here is the circuit diagram and board layout from that work.
I was considering if I should send this board out to a board fabricator and get it built, but it occurred to me that if I am going to order a PC board perhaps I should try to get the entire preamp circuit on there. I knew this would be a challenge because the freeware version of Eagle is restricted to 80×100 mm board size (3.15 x 3.94 inches). I like using the Orange Drop capacitors but they are quite large and take up a lot of board space. But hey – it couldn’t hurt to try, and I was bound to learn a lot along the way.
The first challenge was learning how to create new parts in the Eagle library, since I couldn’t find existing libraries for many of the parts used in this design. So there is now an NSE Labs Eagle library where I’ve created the parts that I couldn’t find elsewhere.
Below you can see the full B-17B circuit so far. The board layout is not done, all I have done so far is place the components and they are still just connected by airwires.
This circuit does not contain the VU drivers yet. I’m debating if I should switch to a dual package for the opamp and just build the VU drivers external to this board. That would allow changing to other analog or digital VU meters in the future, for instance. The high-voltage supply is also not on this board. It wouldn’t be a big deal to keep that supply external to the PC board either. But as you can see most of the components should actually fit on the board.
That’s it for now. This is still a work in progress and I am learning a ton as I go. Stay tuned!
The prototype B-17B preamp circuit has been built. This includes the power supply and preamp tube circuit, but not the VU and buffer circuit. Here it is:
If you look closely at the picture above you can see the tube glowing.
It actually worked the first time it was turned on. There were a couple of things found in initial testing, which is not surprising.
The first issue is that the power supply is actually at about 350V, rather than the 290V predicted by simulation. One contributor to this is that the transformer is actually putting out 134V RMS instead of the 120V RMS rating on the secondary winding. The primary side of the transformer is at 119V RMS which is within spec. The maximum anode voltage for the 12AX7 is 330V so further analysis is required.
The second issue is that the first stage amplifier is adding compression earlier than expected. Below is the output of the first stage with 1V RMS 200Hz sine wave input.
As you can see the bottom part of the waveform shows some rounding, If the input voltage is reduced to 0.65V RMS then the rounding goes away as shown below.
The first stage was predicted to have more headroom than this.
The second stage appears to function more or less as predicted. Here is the output of that stage with the Gain control set about halfway up, and you can see plenty of compression on the bottom part of the waveform.
When the gain control is reduced the compression goes away. It’s hard to tell if this happens at about 1/3 of the rotation of the gain control as predicted, it seems to happen a bit before that.
So there’s still work to do to figure out how to adjust for these discrepancies. But good progress so far – it works!