Modular vacuum tube analyzer - Page 1
Forum section: electronics / projects

author: flowib
author_rank: Contributor
timestamp: October 29, 2017, 09:33:29 pm
content: Good evening,It seems fitting to introduce myself: I'm 22 from the Netherlands. My main interests hobby related are analog circuitry. Furthermore, i have a fascination with vacuum tubes, and how to measure their characteristics. I look at them from a purely technical point of view, so no audio nonsense here.I am currently thinking of building a completely analog measuring circuit that will measure the most important parameters for vacuum tubes, not because I lack the functionality (I have measuring equipment that will measure grid current and anode/screen current at almost any operating point). Furthermore i am already in the possession of a curve tracer.I don't plan on ever going commercial with the project, as the market is limited, and there are already quite good solutions available (which i dont like), i have no problem posting schematics or gerbers.Some explanation: most vacuum tubes have some important parameters such as: anode current at certain bias voltages, and a corresponding mutual conductance (expressed in mA/V) at the same point) Hence one would like to have a variable negative voltage as a bias supply, and some axillary adjustable high voltage supplies to provide plate and screen grid voltages. Further functionality would be a way of measuring the mutual conductance dynamically.GoalsThe aim of this project would be to design a completely analog and modular circuit, whilst still providing the option for voltage control so any of the supplies could be controlled by a DAC and ADC plug-in board.My current thinking is to divide the system up in multiple boards, that can plug in to a rack. The rack is to be powered by a single 24V bus from which all the other voltages are derived. This is due to the fact that i don't want to build a system like others that need a custom multi tap high voltage transformer. Furthermore keeping the system modular allows for swapping out boards to provide different functionallity.The de facto standard in Europe has been 10x16cm with a DIN edge connector for a long time. DIN41617 is one of the older standards that is less used nowadays but still available, however it has better contact properties then the DIN41612 connectors. I was thinking on standardizing on the 31 pole DIN41617 connector for the project. These are available for about €2 each. 10X16cm PCB's are about €5 each shipped from China.RequirementsAnode supply: 10-350V @ 150mAScreen grid supply 10-350V @ 50mAGrid supply,Heater supply: 0-15V @ 5AMutual conductance measurement (S in Europe) 2/20/200mA/V measurement range, this is to be done by superimposing a sinewave on the grid signal and measuring back the AC component over a resistor in the anode supply, more on this later.Auxiliary supplies: +-15V @ 1A for operational amplifiers. +-10V 50mA buffered reference voltage and +5V 2@ for any digital add on circuitry.Most of the low voltage supplies can be generated by buck converters, the high voltage supplies require flyback converters, this has the added benefit of being able to measure the current in the negative lead of the flyback secondary. There are already examples of suitable flybacks from LThttp://cds.linear.com/docs/en/datasheet/3751fc.pdf.I'm thinking of using these as pre-regulators for a linear regulator in the negative lead on the secondary side, in that way current can be read back referenced to analog ground.ChallengesThe major challenge in this project will be keeping the switchers quiet, and managing the grounding.Mutual conductance or S parameter measurementThere are a couple of ways to measure the steepness of a valve, one of which is to shift the grid voltage this is done on AC testers such as those made by AVO, and can also be done by parametric instruments such as the U61 by metrix, of the 370/375 by Neuberger.The downside to this approach is that curves are inherently unlinear, hence there will always be a measurement error.Applying a sinewave of know amplitude to the grid of a vacuum tube will give a proportional AC current in the anode lead this can be measured back by a resistor in series with the anode. However this shunt resistor itself will mean a DC error in the anode voltage.The solution is quite simple, and was used in the Russian built analyzers L1-3 and L3-3 the latter being an upgraded version. Here a sinewave is superimposed on the bias voltage, and the anode AC component is measured over a resistor that is inside the feedback loop of the VA regulator. This voltage is then passed through a bandpass filter and amplified to drive a moving coil meter. This has the benefit of filtering out any un-linearities and hum produced by the D.U.T itselfhttp://www.jacmusic.com/Tube-testers/L3/Schematics-Versions/MM-Circuit-Diagram-L3-3.jpg

author: oldway
author_rank: Super Contributor
timestamp: October 30, 2017, 07:20:29 am
content: You probably knows the vacuum tube curve tracer Tektronix 570.You can download the service manual here...http://bama.edebris.com/manuals/tek/570/

author: flowib
author_rank: Contributor
timestamp: October 30, 2017, 12:41:08 pm
content: Yeah, the 570 is nice, however a 576 is nicer, much more accurate... and no room heater..Anyway, I was thinking about the control grid drive circuitry, A buffer from AN18 would suffice to give +-100V swing on the grid, however one requirement is that grid current also needs to be measured. I was thinking about ways to achieve this, and thought of a floating supply on the output so that an opamp can sense the voltage over a resistor in series with the output.However vacuum tubes are known to have grid current when they are driven slightly positive. For normal recieving tubes 200µA would be a sensible range for grid current, however when driving the output positive with respect to ground, the tube will pull significant grid current (In the range of milliamperes, hence i thought about bridging the shunt resistor with a reed relay to give 200µA and 200mA measurement range.There are two 3V3 zener diodes in series with the emitters of the output transistors, if these transistors are biased in class A for most of their operating voltages, the current sense opamp that needs about 1.25mA max can be fed from there. the opamp introduces an error, that is compared by a differential amplifier on the ground side of things.Attached you will find the Original schematic from AN18 and my version that i cobbled together.

author: albert22
author_rank: Regular Contributor
timestamp: October 30, 2017, 02:01:36 pm
content: Years ago there were several vacuum tube curve tracers on the web. Most of them were discussed at audio forums.Now I can only find this one:http://www.dos4ever.com/uTracer3/uTracer3_pag0.html

author: Cyberdragon
author_rank: Super Contributor
timestamp: October 30, 2017, 03:48:12 pm
content: There's more...http://www.valvewizard.co.uk/curvetracer.htmlhttp://el34world.com/Forum/index.php?topic=7684.0

author: nickds1
author_rank: Contributor
timestamp: October 31, 2017, 10:58:54 am
content: Quote from: albert22 on October 30, 2017, 02:01:36 pmThe uTracer is great - I've had one for several years - and the designer, Ronald Dekker, is a serious guy - a professor at iTU and at Philips research labs in Holland. He also provides top level support - the kits is good and easy (the SMDs are all pre-mounted).

author: flowib
author_rank: Contributor
timestamp: October 31, 2017, 11:30:44 am
content: My aim with this project was to provide similar functionallity to the Roetest,http://www.roehrentest.de/With a hardware defined mutual conductance measurement, as found in de L1-3Furthermore the Roetest uses linear power supplies and a custom transformer. with the use of the LT flybacks i wanted to avoid having to purchase an expensive custom transformer and it simplifies current measurement on the HV supplies. Everything should run from a cheap 24V 10A SMPS.This adds complexity in the sense that a lot of switchmode power supplies have to be developed, and everything needs to be rather well filtered/ground layout needs to be good.I have a pack of LT1076 and most of the parts for this project.I was thinking of building 5 modules:By keeping all the power supplies on one board, the connection for the 0V from the +24V bus to AGND is only on one board. Which avoids ground loops. The high voltage will be flyback based.Power supply board: +-15V 1A buck converters, 5v 2A buck converter for digital circuitry and an auxilary 5V 1A for panel meters in the modules +-110v 50mA for the grid driver.Grid driver + Wienbridge oscillator + dual bandpass filter on board #2Screen grid supply + linear post regulator on its own dedicated board.Anode supply +Linear post regulator+ BNC connector on front panel to connect AC measurement to the wien bridge.*I dont think i am going to develop a heater supply, i have 5 delta eurocard rackmount switchmodes that have CV/CC functionality.