The “MCUTracer”
Part I
A Microprocessor
Controlled
Vacuum Tube (and
other) Curve Tracer
Part II
A Microprocessor Controlled
Power Supply for the MCUTracer
By
Jack Walton
And
Martin Hebel
Ver: Feb 6, 2004
There have been a number of articles in Audio Amateur, Glass Audio, Electronics (nee Wireless) World etc. that describe tube and transistor curve tracers that would satisfy one or more of my interests in examining the characteristic curves of vacuum tubes and semiconductor devices. Nevertheless, none of the designs are flexible enough the accommodate the potential uses which a microcontroller, analog-to-digital converter and PC can be used for. This article describes a modular, microprocessor-based curve tracer, using the Parallax Inc. Basic Stamp BS2. The design is sufficiently flexible to allow the unit to plot and display several variables in real time.
Parallax Inc. has a macro for Microsoft Excel[1], StampDAQ available on their website which takes the data from the microprocessor and sends it directly to the PC for use in the popular spreadsheet program. With the MCUTracer and Basic Stamp you can plot data in real time, save the results or perform statistical analysis. While other microprocessors could be used for this article, we decided to stick with the Basic Stamp as Parallax did fund the development of the software used here.
In Part II of the article (A Microprocessor Controlled Power Supply for the MCUTracer) we show how an existing high voltage power supply, the Heathkit IP-17, can be POOGED for control by the MCUTracer, supplying and measuring B+ and C- voltages as well as Plate Current. In Part II we use a more robust analog acquisition and plotting tool, StampPlotTM, written by co-author Martin Hebel.
The MCUTracer basic setup can be adapted to a variety of purposes, such as semiconductor curve tracer, spectrum analyzer, logic analyzer etc. As it is configured for Part I of this article it can test two devices at the same time. Thus the two triode sections of 12AU7’s, or a pair of 6L6’s (or with a little ingenuity a quad) can be quickly compared, the results stored or printed.
Tube and Transistor Tracers:
Most tube and transistor curve tracers use a ramp generator to provide the voltages for plate and grid, gate and drain etc. As the drive voltage is swept, an oscilloscope display is triggered (or the X-Axis is driven) and displays the results. Borberly described a tracer in Audio Amateur in 1990[2] and provided a list of references to such designs. Petrowsky offered a clever vacuum curve tracer using the X – Y axes inputs of an oscilloscope[3]. While Petrowsky’s Tube Tracer was useful in helping determine the merits of a particular bottle, it would not allow the user to store and record data, or to compare data among lots of tubes for matching. Although Joe Carr did not offer a specific tracer design in Ham Radio[4] he did lay out the fundamentals for power tube testing and provided the schematic for a gm tester for high power transmitting tubes. More recently, George Steber had an article in “Circuit Cellar”[5] which used a PC sound card to trace current and voltage. In addition, Maxim has an application note, of course using Maxim IC’s which show how a PC Printer Port can be used as an IV Curve Tracer[6]. While all of these articles provide solutions, using StampDAQ simplifies design implementation, and you don’t need to be a Visual Basic or C++ Guru to get the device to work. Of course, the problem with a ramp generator is that the measurements lag the movement of the ramp due to the time necessary for analog to digital conversion.
The MCUTracer, as shown here, departs from the strictly analog approach by using an inexpensive and easily programmed chip, the Basic Stamp II from Parallax[7] to monitor the output from a Linear Technology 10-bit ADC. While Part II describes a digitally modified power supply for the MCUTracer, the old “Armstrong Method”, i.e., twiddling the knobs on an adjustable high voltage supply to adjust grid, screen or plate voltages by hand. The data collected by the MCUTracer is channeled to your PC using the recently published, Microsoft ExcelÔ Macro, StampDAQ from Parallax Inc. and available for free on the Parallax website (www.parallax.com )
Circuit Description:
The circuit consists of a 6 non-inverting op-amp front ends. Plate and Grid voltages are stepped down and calibrated with multi-turn adjustable potentiometers. No voltage divider is necessary for the Plate Current measuring channels since the level is within the acceptable range for the ADC.
The 6-channel, 10 bit ADC is decoupled from the op-amps with 51 ohm ¼ watt resistors and 100nF capacitors. In application, I found that extraneous noise could be reduced with this method which, incidentally, is also suggested in the Product Design File from Linear Tech.
The input resistors for the high
voltage measurement circuits should be made up from three ½ watt 270K resistors
to minimize the non-linearities of carbon resistors at high voltage. Do not use ¼ watt resistors in this
application.
Power for operational amplifiers
and ADC is derived from the 12.6 Volt Tube Filament Transformer output of
the Heath IP-17 Power Supply as shown
above. The voltage is rectified, filtered and regulated to +/- 5.9 Volts
with a pair of LM317L/LM337L regulators.
Construction:
The MCUTracer is built on a 3 X 6 inch printed circuit board which houses the opamps, ADC, Basic Stamp and male 0.100” Molex headers for use in the microprocessor controlled power supply.
The Printed Circuit Board PCB X-Ray (with top layer traces as dotted lines) and the Reflected Image of the bottom (trace layer) are shown below:
Reflected PCB Pattern 1
TopSide PCB Pattern 1
Stuffing Diagram 1
Instead of agonizing over a complex switching arrangement for the MCUTracer, I decided to place pairs of 7, 9-pin and octal sockets. Each pin of the tube socket pair is connected to a flexible probe fitted with a miniature banana plug, that is, all pin 1’s are connected to a probe with, in this case, a black lead. There are 9 mini-banana jacks which provide plate, screen, control grid, cathode and heater voltages. For those enamored of the 300E, 6CW4, or Compactrons, knock yourself out and use the appropriate socket!
Typical Setup
Plate Current is measured by reading the voltage drop from
cathode to ground across a resistor.
A typical setup, showing an unmodified Heath IP-17 Power Supply
is shown below:
In the lowest current mode (10mA) the voltage drop across 250 ohms, 2.5 volts, is measured. A 100 ohm resistor is used for 25mA and 25 ohms for 100milliamps.
Testing Two Tubes
The MCUTracer is designed so that more than one tube may be tested. A general outline of how this can be done is shown below:
Testing Tubes with Screen Grids:
The screen grid can be connected to the plate with a jumper cable. (From the picture of the MCUTracer at the beginning of the article you can see that multiple B+ outlet jacks are available.) If you want to add resistance to the path between the B+ supply and the screen grid just fashion a jumper cable with the appropriate resistance in the path. Of course, safety considerations are paramount and any such resistance should be contained in a small box or wrapped in a couple layers of heat shrink tubing to avoid the risk of injury.
The Program:
The code below will
operate the MCUTracer with any high voltage power supply. The program uses the full 10-bit range of
the LTC1093 ADC to measure positive voltages using the “UNIPOLAR” multiplexer address
bit. Negative voltages are measured to
9-bits using the “BIPOLAR” ADC setting.
This lower precision is necessary for there to be a “SIGN” bit
indicating whether the Grid Voltage is positive or negative. Choosing
the mode is done by deselecting the UNIPOLAR bit when the MUX address is
selected. The negative voltage is “Two’s
Complemented” sent to the spreadsheet.
In Part II of this
article we list the code which is used to control the High Voltage Power Supply
and the StampPlot macro’s:
'{$STAMP BS2}
'********** Tube Curve Tracer using StampDAQ **********
'Using the StampDAQ Excel Macro, the data
'is Dumped into an Excel spreadsheet where it can be
stored, charted and analyzed.
'Data Is sent out to the PC via Pin16 - the Programming
Port of the Basic Stamp
'The Baud rate and transmission mode IS 9600,n,8,1 -- using
the constant"84"
'The Bipolar Mode is used to read the negative grid
voltages
'In the Bipolar Mode the resulting data must be "two-s
complemented"
'**********
Variable Assignments
***************
DIRL = %11011111
Set Pins I/O
ADCV VAR Word 'Voltage read by ADC
I VAR Byte
'Counter
MUX VAR Byte
ADCS VAR Word(6) 'ARRAY to hold
the 6 measured values
SPIN CON 16 'SERIAL PIN
- P16, Programming port
Baud CON 84 'Baud mode
for a rate of 9600, 8-N-1
CLK CON 0 'Clock
CS CON 1 'Chip Select
DIN CON 2 'Data Out from ADC
DOUT CON 3 'Data In MUX Address and Mode
'********** Initial
Variable Values ****************
I = 0
HIGH CS
PAUSE 1000 'Allow
data communications to stabilize
SEROUT SPIN,Baud,[CR]
'Send
Carriage Return to purge StampDAQ buffer
CONFIGURE:
'****************LABEL COLUMNS A to D with TRACER Outputs
SEROUT
SPIN,Baud,[CR, "LABEL,TIME,PT_1V, PT_2V,PT_1I,PT_I2,GR_1V,GR_2V",CR]
SEROUT
SPIN,Baud,["CLEARDATA",CR]
PAUSE 1000
MAIN:
FOR I = 0 TO 5
LOOKUP
I,[99,119,115,103,105,123],MUX 'This
selects the address of the LT1093
LOW CS:TOGGLE
CLK 'Select
the LT1093 and toggle the clock line once
SHIFTOUT DOUT,CLK,1,[MUX\7]:PAUSE 1 'Shift
the Address into the MUX of the LT1093
SHIFTIN
DIN,CLK,2,[ADCV\10] 'Pause 1 millisecond to
access the data
TOGGLE CLK 'Toggle the clock
line
PAUSE 1 'Pause to settle
the LT1093
HIGH CS:PAUSE
50 'Unselect the LT1093
ADCS(I)=ADCV
NEXT
ADCS(4)=~ADCS(4)+1
& 1023 'Two-s complement the negative
grid voltage
ADCS(5)=~ADCS(5)+1
& 1023
SEROUT
SPin,Baud,[CR,"DATA,TIME,",DEC ADCS(0),",",DEC
ADCS(1),",",DEC ADCS(2),",",DEC ADCS(3),",", DEC
ADCS(4),",",DEC ADCS(5),",",CR]
PAUSE 50
GOTO MAIN
Setting Up the MCUTracer:
The setup procedure described here makes use of an unmodified high voltage power supply. If you modify the high voltage supply as described in Part II, you will use the Static, rather than Sweep Function.
You should exercise great care in setting up the
MCUTracer since lethal voltages are present on the circuit board. Death or serious injury can occur if you are
not very careful in the setup and usage of the MCUTracer. If you have used a 3-resistor high voltage
divider make sure that any exposed nodes are insulated to avoid personal
injury. Use an insulated screwdriver to
adjust the trimming potentiometers.
Each bit of the ADC will measure Vref/1023 millivolts, or 4.00mV with the LM4040-4.1 volt reference. Adjust the B+ output of the supply to 100 volts on the appropriate HV header. Adjust the trimmer pot so that 1.000 volt is present on the appropriate output pin of the opamp or input pin of the ADC). With this setting, a 100 volt Plate reading will be represented by 250 bits from the LT1093 ADC to the Basic Stamp.
Similarly with the C- supply, apply –12.50 Volts to the Grid Voltage input header and adjust its trimming potentiometer until the voltage at the corresponding pin of the ADC measures -1.000 volts. The full scale reading of the Grid Measuring circuit is a little over –50 volts, and each bit will represent 50 millivolts. If you can only “get close” with the trimming potentiometer you can always make a correction in the Excel spreadsheet for the actual value.
The connections from the Curve Tracer to the Power Supply
are made with a cable with banana plugs, the plugs attached to the MCUTracer
(and not the other way around for safety’s sake!). Make sure that the B+, C-, heater and ground connections are
correct. Make the connections from the
tube pins to the appropriate input of the MCUTracer. Importantly, test your
tubes for shorts before using the MCUTracer! There’s no point blowing up your power supply if this simple test
isn’t made first. Do not remove any
of the plugs from their jacks while power is supplied to the MCUTracer!
Running the MCUTracer
After you have checked that the setup is correct apply power to the MCUTracer through the High Voltage Power Supply. Allow the “device under test” (DUT) a minute or so to warm up. While the warm-up is taking place, start Excel and open the StampDAQ macro within the program. The StampDAQ icon will appear on the spreadsheet prompting you to “Connect”. Adjust the B+ and C- voltages of the power supply and current measuring range switch to levels which are appropriate for the DUT.
When you press the Connect button data will be transmitted from the MCUTracer to your PC. Therefore, after connecting, you can sweep the B+ voltage by hand and you will see the B+, C- and Plate Current will print in separate rows of the spreadsheet. To go to a different C- voltage, just pause the MCUTracer, adjust the C- voltage and reconnect.
Graphing the Data in Excel
To see the results in real time, set up a generic “Scatter Plot” graph with Column B, Plate Voltage, as the X-Axis or independent variable. Column C, Plate Current, is set up as the Y-Axis. An example of the finished product, a characteristic curve for an ECC86 is shown in the following graph:
If you set up the graph prior to Connecting the
results will be displayed in real time just as if you had a Tektronix 576 or
577 Curve Tracer!
The chart below shows the differences between the two halves of an old 6SN7GT which I found with a box of tubes:
Another example, a not so pretty junk box 12AT7 with Grid Voltages ranging from –2.0 to –10.0V:
Conclusion:
The Microprocessor Controlled Curve Tracer is not a daunting
a project. For the investment of a few
hours time you can have an instrument that can chart voltage and current in
real time. While the first application
described in this series of articles is a vacuum tube characteristic tracer, it
is apparent that the modularity of the device lends itself to a wide variety of
applications in which one or more variables are plotted against each
other. In part II of this article we
will describe a Digital to Analog Converter/Voltage Amplifier controlled by the
Basic Stamp, using a StampPlot Graphical User Interface to automatically provide the voltages
necessary to test tube characteristics.
About the authors:
Jack Walton is an investment consultant and entrepreneur in Short Hills, NJ. His educational background is in chemistry and physics, and finance. He has had a lifelong interest in electronics, ham radio, photography and the arts.
Martin Hebel is an Assistant Professor in Electronic Systems Technologies at Southern Illinois University where he teaches process control and microcontroller programming. He is also a partner in SelmaWare Solutions and the developer of StampPlot and StampDAQ software and co-author of BASIC Stamp texts. During the summer he hosts robotics camps.