yet another ultra simple vacuum tube (valve) curve tracer build - Page 1
Forum section: electronics / projects

author: crazyhog
author_rank: Regular Contributor
timestamp: October 31, 2017, 11:30:51 am
content: Hi all,After seeing pics of old Tek vacuum tube scopes, I was instantly smitten. I've since acquired a few of these sexy beasts in various state of disrepair (535, 502, 585, 545B, 547, and 564B). Yeah, I've gone overboard! To repair them I needed a way to check the tubes and hence I wanted a curve tracer. Yes, I've heard of tube testers and I am not interestedI've read various approaches other folks have employed for vacuum tube curve tracer builds. The two that stand out are:valve wizardhttp://www.valvewizard.co.uk/curvetracer.htmluTracerhttp://www.dos4ever.com/uTracer3/uTracer3_pag0.htmlI like both solutions a lot and am grateful for these generous folks for sharing their ideas. I'd highly recommend anyone interested to read the excellent and detailed documentations at the above websites.I've since come up with a very simple approach which I will document here. Certain parts of my solution were inspired by both the valve wizard and the uTracer. I will high light my simplification to the problem, and my proof of concept in this post.One of the challenge with vacuum tube curve tracer is the need to sweep the anode (i.e. plate) of the vacuum tube with high voltage (400v and more).The Valvewizard simply rectify the AC voltage from a transformer and apply it directly to the valve. For Europe this is good enough. However, the US AC voltage is only 120v and definitely no good. Also some tubes require much higher voltage than 400v which the valve wizard cannot handle.The uTracer made a brilliant observation that high voltage is only needed during the actual testing; a very short span of time. Therefore a tiny boost converter to step up the voltage and a capacitor to store the energy for the momentary testing need will suffice. However, the uTracer has to coordinated everything using a 20 mhz micro-controller (i.e. charge the capacitor, initiate test, data capture, communicate with a PC, etc.). Keep in mind the test duration is ultimately limited by the speed of the 20 mhz micro-controller.There are other complications (e.g. short circuit in the tube, test voltage above 400v, etc.) that arised from the boost converter + capacitor solution which is detailed in the uTracer documentation.My solution is similar to uTracer but much simpler. My observation is we can take uTracer's brilliant observation to the next logical level and thereby simplify everything. I have a Rigol ds1054z DSO! It can capture 1 billion samples per second!Using the Rigol DSO, the curve trace test duration can be done much shorter (i.e. energy requirement much less) compared to uTracer's 20mhz micro controller. Therefore, there is no need for a micro-controller to coordinated a boost converter and a storage capacitor to hold enough energy for the slow controller to finish the test. Instead, we can store all the energy needed for one test sweep in a tiny 15mH inductor!Other issues that complicated the uTracer (e.g. handling short circuit within the vacuum tube, test voltage much higher than 400v and etc.) gets eliminated or simplified significantly. Anyway, I will detail these as I work through the the project build.Just to be clear, I know I have a different set of requirements from uTracer. For example, uTracer can be use by folks without access to a scope. Both uTracer and valvewizard are excellent solutions for what they aim to achieve. I am just sharing how I arrive at my solution by learning from others.Proof of concept:Note the actual value of the components is only approximately right; this emulation is only intended to illustrate the idea and as a quick sanity check.The 20k ohm resistor represents the tube's anode to cathod resistance (got it from a tube datasheet)The circuit works as follow. First establish a current (~20ma) through the inductor and have it store the energy as magentic flux. Most of this current flows around the tube (i.e. most of the current are through the switch). When we press the push button and hold it down, the inductor magnetic field collapses and generate 386v (from a 1 volt source!) to force the 20ma through the 20k ohm resistor. As the inductor loses energy, voltage sweeps from 386 volt down to 0. This whole event (0 -> 386v -> 0) is done in 3 microseconds. The Rigol DSO, at 1G samples/second, will capture 3000 data points. It is impossible for uTracer's controller to do the test so quickly which is why it needed more circuitry, code and energy storage. The utracer needs 1ms.Proof of concept breadboard:The dip package is a 6 pack schmitt trigger for button debouncing and to drive the fet hard. The fet is used as a switch. The tiny 156c can is the 15mH inductor. The circuit is powered with 5 volt.Note that I can easily get 800v test voltage just by changing the resistor that establish the initial current through the inductor to a smaller value (i.e. more current).Proof of concept scope trace:The yellow trace shows 421v at the top of the 20k ohm resistor (tube anode) when the blue button is clicked. The blue trace is voltage at the gate of the fet switch. The whole event is over in 3 uSeconds and plenty of data is captured for curve tracing!I feel quite good about completing my curve tracer project. I will liberally borrow other ideas from valvewizard and utracer to complete this project. Ebay will provide the a supply board for the control grid and filament for the price a hamburger! BTW, while I need a negative voltage for the grid, I don't think I need a separate plus supply and negative supply because of my simplification to the plate voltage sweep above.There is really not much more now that I have the anode voltage sweep and data capature and presentation sorted. Heh, I will enhance this project so that I can even use my old Tek tube scopes to do tube curve traces! Storage scope is not necessary!

author: ivaylo
author_rank: Frequent Contributor
timestamp: November 01, 2017, 06:53:35 am
content: Yes, please finish your project and publish the results

author: flowib
author_rank: Contributor
timestamp: November 01, 2017, 09:06:11 pm
content: One of the main advantages to having a tester that can cook a tube at DC voltages and currents near its maximum published ratings is that this weeds out any tubes that might have other issues. Furthermore you can burn in the tubes to stabilize cathode activity.Grid current and cathode to heater insulation are two large factors that contribute to vacuum tube failure in circuit, most of the time a tubes goes gassy or has some insulation failure before it reaches the end of its useful life.

author: Cyberdragon
author_rank: Super Contributor
timestamp: November 02, 2017, 04:41:31 am
content: A curve tracer like this will not function as an effective general purpose tube tester. Yes, it will give you a transconductance curve, but for the few microseconds the tube starts to conduct. As stated above you need to do initial basic functional testing (leaks, shorts, exc) first before you attempt any transconductance test. Also, doing the transconductance test as fast as possible isn't always the best idea. Some tubes will conduct fine for a few seconds then just start fading away, which this machine will think are fine.QuoteWhat we're trying to say is...you still need a tube tester, then you can put the tubes on this machine. By all means make your own tester too, but don't let partialy working tubes fool you with just this device.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: November 02, 2017, 05:06:49 am
content: Pulsed tests are valuable in many applications, but one must be aware of their limitations.Four important considerations come to mind:1. Interelectrode capacitances. Can you hold the other electrode voltages accurately constant during this transient? Are you sure they won't ring or oscillate, or pick up an offset from the ramp?2. System bandwidth. Are you certain your waveforms are not being rounded off, peaked, or distorted through the signal path? You don't have to worry much about this at DC, or near-DC (say, mains frequency), but a 1x probe won't correctly read the rising edge of that waveform there -- compare 1x and 10x readings!This isn't a problem for that scope, in and of itself, but you need to be careful that the entire signal path in front of the scope -- from the pulse generator to the tube socket and back -- is clean.3. Tube physics itself. There are two approximate time constants in tube behavior.a. The first, and fastest, is the intuitive case: it takes time for electrons to travel from the grid to the anode. This is the transit time, or more commonly referred to as transit angle, as it's usually measured at a single frequency. Typically, for receiver tubes, this is only a few nanoseconds, so it should not affect measurements here. (This sets the practical upper frequency limit of the tube. Comparable to BJT recombination, or MOSFET spreading resistance and transit time.)b. The second is cathode effect. This is poorly understood (I think?), but probably arises in part from history-dependent cathode emission. Thermal time constants of the electrodes may also be relevant (it doesn't take long for a thin grid wire to change temperature -- though except for high (pulsed) power levels, that's probably more down in the ms range).Cathode emission can be likened to a wet sponge, where the fluid is hot electrons. The cathode coating is porous and only modestly conductive (it's a semiconductor when hot). In quiescent operation, the cathode remains well-stocked with electrons, but as current demand changes, the electron density (sponge "wetness")changes. It takes time for this change to absorb into the cathode material, hence having a subtle effect (<1% change in gain, say) on the tube's operating curves, with a us to ms time constant. (The visible effect of this would be the curves tracing out a loop of sorts, depending on how fast and how often the curves are plotted.)In tubes intended for pulsed operation, this limits the duration of a pulse, or more specifically, the amount of charge available in a single shot. Typically, radar modulator tubes were capable of a dozen amperes, from a cathode not much larger than a regular 4CX1000, but with oxide cathode, but this is only available for a few microseconds at a time. Pulses too long and too high cause the cathode to become depleted, pushing into saturation (current saturation, where cathode current ceases to increase from a further increase in plate/screen voltage) and causing damage (ion bombardment, cathode stripping).For curve tracing purposes, neither of these effects will be very important if you're just after quick, crude (say 10% accurate) curves. If you are seeking utmost accuracy, perform the test at different speeds and see if it's repeatable. And cathode depletion can be avoided by not going to excessively high currents, of course.4. You need a different inductor for each curve (or rather, the area under it) you take. You can limit peak voltage by controlling the switch, but the circuit can quickly become much more complicated than you had expected. (Indeed, dynamic systems are always more complicated than novices expect, and even rookies fall into traps with them!) At that point, it might be better to use the old standby: an isolation transformer and variac.Tim

author: ivaylo
author_rank: Frequent Contributor
timestamp: November 02, 2017, 05:32:16 am
content: Sorry, may be a dumb question, but can contemporary SMUs be used for testing tubes? Some reach 200V+, have multiple channels, etc...

author: T3sl4co1l
author_rank: Super Contributor
timestamp: November 02, 2017, 04:53:48 pm
content: Sure. Just mind accuracy limits at low currents, since tubes are relatively low current devices.Tim

author: vk6zgo
author_rank: Super Contributor
timestamp: November 02, 2017, 11:01:46 pm
content: Quote from: flowib on November 01, 2017, 09:06:11 pmExactly!Back in the day, I used to test a lot of receiving tubes.The most common reason for rejection was heater/cathode leakage.On several occasions we tested the Characteristics (you can do this with the top level Avo testers) & apart from the H/C leakage, the tube would have been fine.Another thing which often occurred was the needle coming up nicely, then after several minutes dropping off to a "fail" reading.

author: flowib
author_rank: Contributor
timestamp: November 04, 2017, 04:14:55 pm
content: I am currently thinking of building another parametric tube tester, I have a L1-3 (russian and darned good) however its card system isnt capable of testing every tube.most if not all recieving tubes have their characteristics specified at anode voltage =250V the most rudimentary tester you could make involves just regulating +250V for the anode, and a variable G2 at 30-50mA (doesnt have to go to 0)i started building a tester fully tube regulated, however this got heavy and cumbersome pretty fast.Building one with linear regulators is possible, however dissipation will be 20-40W in most worst case scenarios per supply.. Flyback or boost converters could be a solution to keeping dissipation within limits. If you want to go oldskool you can use a SCR preregulated supply, like the HP6209BThe DC supply for the heaters is quite simple, any supply that has CV/CC is good enough, or AC heating in the latter case you have to make sure you can adjust for mains variations...