NXTracer
Martin Manning
Pedro Cimini
Thoughts? --
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Martin Manning
New features / Modifications:
Use modern single supply 5V OpAmps
There are these days many advanced OpAmp that deliver just the right performance in terms of low-offset and slew rate, but (unfortunately?) only come in an SMD package and with a low (single) supply voltage of 5 V or even less. Switching to a single OpAmp supply is not without issues, but the advantages of using modern OpAmps have become so obvious that the time has come to see if these issues can be overcome.
Delete the +15 V and -15 V power supplies
Moving to single supply 5V rail-to-rail output OpAmps automatically eliminates the need for the +15 V and -15 V power supplies, resulting in a considerable reduction of complexity, component count and board space. It also eliminates the need for the over-voltage input protections that are now used to protect the input of the two Programmable Gain Amplifiers (PGAs).
· Seems like a solid choice as long as there aren’t any compromises that come along with it.
Extend the control grid bias range 0 to -100 V
An extended control grid bias range is high on the wish list of many people working on power audio amplifiers. The goal for the NXTracer is to extend this range to at least 0 to -100 V and preferably even higher (more negative).
Redesign of the control grid bias circuit
With the increase of the control grid bias range, it is a good moment to revise the complete grid bias circuit. The present circuit, which uses one of the PWM outputs of the PIC, is a bit of an odd concept which worked well, but with many affordable and readily available DACs on the market these days, a more straightforward approach is perhaps more logical.
· I would like to have some positive grid voltage available, perhaps to +20V. Something that functions like the add-on “Grid Loupe” accessory would be nice, which then begs for the capability to measure grid current in triodes and pentodes. In uT3 with Grid Loupe, grid current was only measurable for triodes as the screen channel is used for the measurement. The Grid Loupe is also able to set exactly zero volts Vg1, which the uT3 g1 supply is not able to do.
Reference cathode to ground instead of to the supply voltage
In the uTracer3 the cathode of the tube is referenced to the plus of the supply voltage rather than to ground. This choice was originally motivated by the fact that normally the output voltage of a boost converter can never be lower than the supply voltage, and to extend the range of the control grid bias circuit to -50 V. However, it appears that with a simple trick the output voltage of a boost converter can be lowered to 0 V, and with the grid bias circuit being redesigned it is a good moment to change to a more conventional configuration whereby the cathode is referenced to ground.
· This quirk of the uT3 occasionally caused some confusion, but it was not that much of a problem.
Use uTracer6 NMOS high voltage switch circuit instead of pnp bipolar circuit
Because very high voltage pnp transistors are hardly available, I switched to an NMOS based HV-switch for the uTracer6. High voltage NMOS transistors, certainly for voltages at around 600 V are readily available at low cost. Moreover, the uTracer6 high voltage switch has proven to be very reliable and robust. So I plan to use the same circuit for the NXTracer.
Increase maximum anode voltage to (at least) 450 V
With the tube cathode referenced to ground, and a high voltage switch that can easily handle voltages of more than 500 V, it becomes feasible to use the entire 450 V voltage range of the 100 uF reservoir capacitors. It could even be extended to 500 V if the same capacitors are used as in the uTracer6. Unfortunately, these capacitors have a long lead time, and are sometimes difficult to get. So it would be great if the circuit could be made suitable for both types.
· Increased robustness is welcome. For my purposes, 400V is adequate. An increase to 450V seems to be of little value.
Easy modification for higher anode/screen currents
As demonstrated in the uTracer6, the NMOS high voltage switch will allow for higher currents, possibly up to 1A. It would be great if the current range could be increased with minimal changes to the circuit.
Easy modification for low anode and screen voltages
Battery tubes usually operate at anode and screen voltages well below 100 V. In the uTracer3+ these tubes could only be accurately traced with some major circuit modification, including a PIC with modified firmware. It would be great if for the NXTracer there was a simpler way to test and trace low-voltage battery tubes.
· Low and high current modes for increased accuracy would be an attractive feature. Ranges of 100 mA and 500mA would be nice, with perhaps a third lower range of 10 mA or a higher range of 1A. My uT3+ is set up as a 500 mA machine, and it functions adequately for low currents (below 1 mA), but I’m sure it would be better if a low range were available.
AC heater synchronization
In the extension board for the uTracer6, an option was introduced to synchronize the pulsed current measurements of the uTracer with the AC main frequency. This makes it possible to use an AC transformer to power the heater of directly heated tubes, making it possible to trace heavy duty directly heated transmitter tubes. Both the hardware and the software for this option are very simple, and it would be great if it could be added to the NXTracer.
· This is a desirable feature for rectifier tubes with 2-3A directly heated cathodes as well.
Allow for easy experimentation / modification
Make it simple to modify the circuit for high voltages, currents etc. e.g. by adding extra resistor positions, jumpers etc.
· Always nice to leave some room for future modifications or updates.
What remains the same:
PCB size and (approximate) position of the terminals
To make the NXTracer a drop-in replacement for the uTracer3 with minimal changes.
· This is a noble goal, but I would give it up if it means leaving desirable features out.
RS232 communication, but with connector for serial connection and FDTI cable (uTracer6 style)
The RS232 communication interface of the uTracers is a point of endless discussions. I still plan to use it for the NXTracer because it is a standard, reliable and to keep the NXTracer compatible with its predecessor. But, just like for the uTracer6, I intend to make it easier to use with TTL level serial adaptors and FDTI cables.
· A modification for TTL has worked well for me.
PWM controlled heater supply, but at lower frequency
Again, here I refer to the webpage on the uTracer6.
· An on-board supply that can provide accurate heater voltage at current of up to 2A at 6.3 VDC would be nice. Monitoring heater current would also be useful.
The choice of PIC processor, the 16F884
This is also a point of much debate. Yes, I know, there are more modern processors, but the 16F884 is doing a great job. In the past the migration from the 16F874 to the 16F884 processor has given me a lot of headaches, because although MicroChip claims they are fully compatible, there were many small changes that had to be taken care of. For now, the 16F884 is still listed as “recommended for automotive,” so my guess is, it will be around for some time!
· I’m not knowledgeable enough to have an opinion on this
· I can say that I have been a satisfied user of Ihor Smal’s uTracerJS browser-driven UI, and it seems to me that it is a better solution than the Windows only UI provided by Dekker. When I was running a Windows installation the FTDI USB-TTL cable had to be replaced for compatibility, where the old cable still works fine for uTracerJS in MacOS.
I would like to see the continuous mode remain available, along with the shorts and leakage test and on-board calibration resistors I have added to my uT3. These are off-board, and so probably not an issue.
Nick de Smith
Nick de Smith
I also don't see why the project needs to be completely open - it's Ronald's project to with exactly as he pleases. If folk don't like that, they are completely free to go away and do their own thing.
The fact that there unscrupulous, ungrateful and sometime rude folk out there who may or may not plagiarise his work is not his fault!
Martin Manning
theold...@gmail.com
Agreed!
First of all, Ronald Dekker does not read the forums. I fully understand why; I once had a project published (via a magazine) and it is unbelievable how rude and inconsiderate some of the comments were. If you don’t like the project, either provide thoughtful and constructive feedback, or move on. As you say, it is his project.
Having said that, I too have made suggestions to Ronald directly. I was one of his V6 beta testers. I have always found Ronald considered of my suggestions, but in the end, it is his decision. He is very correct in that small changes in a production series is a huge headache to support. Myself, I would like the whole project to go to SMD parts, simply because I am set up for that, and comfortable with it. But that’s me. Unless there was an assembled option, it would be difficult to produce as a kit. Through hole parts are going the way of the Dodo bird.
Am I going to upgrade to the new version? Not likely. I do not see enough change to warrant that for my use.
I sometimes wonder what some of the uTracer users use their tester for. Some of the (occasionally intense) discussions are about minute changes that have no real impact on a tube in a real application. Tubes were / are used in circuits with components of 10, sometimes 20% tolerance. So what if the anode current is off by 0.5 mA??
Sorry, I just realized I’m on my soapbox.
Anyway, if someone has a suggestion, contact him directly. I have found him to be very responsive, and really appreciates feedback given in a constructive and supportive voice.
Bill v.
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