DIY Test Equipment -- MCU Tracer (Vacuum Tube Curve Tracer)
   

Ver:1/16/03

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, based upon the Parallax Inc. Basic Stamp BS2 with a design that is sufficiently flexible to allow the unit to plot and display several variables in real time.  In addition, the problem of keeping high voltages off a computer motherboard is addressed with the use of readily available opto-isolators. The Basic Stamp II was chosen since there is a large community of users for the device, a number of good books written on the devices, and the ease with which it is programmed.  The prototype MCTracer is shown in the photograph below:

 
   

The MCTracer is designed so that  two devices can be measured and matched.  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.

 
   
   

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[1] 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[2].   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[3] he did lay out the fundamentals for power tube testing and provided the schematic for a gm tester for high power transmitting tubes. 

 The MCTracer, as shown here, departs from the strictly analog approach by using an inexpensive and easily programmed chip, the Basic Stamp II from Parallax[4] to monitor the output from an Analog to Digital Converter and feed the readings to the serial port of a personal computer.  A six input, 10 bit analog to digital converter  (Linear Tech LTC1093) and my trusty (but modified) thirty-year old Heathkit IP-17 adjustable power supply (or other similar adjustable supply such as the Eico 1030) are used to read and provide the plate, grid and filament voltages to the device under test.   Instead of a ramp generator, the “Armstrong Method” (i.e. using your arm) is employed.  The user sets a grid voltage (C-) and simply turns the knob controlling B+ to generate a plot of plate current (Y-axis) as a function of plate voltage (X-axis) while the microprocessor collects data and transmits it to the PC.  Multiple plots can be obtained by pausing the microprocessor, setting a new grid voltage, and again sweeping the plate voltage by hand. (In Part II we describe a Digital to Analog Converter and Voltage Amplifier that automates the entire process.)  Timing circuits and a ramp generator are unnecessary as the free-running analog to digital converter repeatedly transmits data to the PC at 15 millisecond intervals (66 samples per second) using a recently published, Microsoft ExcelÔ Macro, StampDAQ for Parallax Inc. and available for free on the Parallax website (www.parallaxinc.com )

 
   
Circuit Description:  
The circuit consists of an op-amp front end with adjustable voltage dividers and a Linear Technology LTC1093 10 bit, 6 input successive approximation analog to digital converter. The LTC1093 is capable of bipolar measurements by setting LOW the Unipolar bit of its instruction set. This eases the design and provides flexibility in measurements. The LTC1093 requires careful attention to detail in placing the ground planes since the device uses a 10-bit capacitative DAC. As pictured above, I used copper foil to place a ground plane on the top of the PCBoard as well as the bottom. The inputs to the ADC are filtered with a small RC network to reduce noise.  
   
Linear Tech LT1013 and 1014 opamps are specified in this design because of their low leakage current, high accuracy, and a modest degree of protection provided by their input circuitry. Other high quality opamps could be used in their place. (Note, Texas Instruments now provides a direct substitute for these op-amps.) The high voltage resistive voltage dividers employ 3 ½ watt resistors stacked together to avoid the problems of non-linearity at high voltages. Importantly, two power supplies are used so that the analog opto-isolators keep high voltages from straying into the circuitry of the MCTracer, and potentially onto the computer motherboard (with disastrous results!). High voltages can creep across a circuit board due to grease, dust, fingerprints, poor soldering techniques and a variety of other reasons. Thus, I used an epoxy coating for the area of the circuit board where high voltages might be present, particularly around and underneath the input headers.
 
 
The first power supply for the opamps, ADC and voltage reference derives its current from the 12.6VAC filament windings of the high voltage supply, is pre-regulated with a 7815 voltage regulator, and split into positive and negative 5.6 volt rails with an artificial ground using LM317LZ and LM337LZ low power voltage regulators. Power for microcomputer and the isolated side of the opto-isolators is obtained from a wall-wart transformer and regulated with an
 
 
   
The schematic diagrams for the various components of the MCTracer are illustrated below:  
   
The basic amplifier unit consists of a non-inverting amplifier with a gain of 2.0 and a resistive voltage divider. The divider is adjusted with a 15 turn potentiometer so that the maximum non-inverting voltage is equal to 2.5 Volts. R(a) for the high voltage measurement circuits should be made up from three 270K resistors to minimize the non-linearities of carbon resistors at high voltage. Dual supplies were chosen to make the unit “modular” in that positive and negative voltages could be measured and digitized. The voltage range and values for R(a) and R(b) for each of the 6 amplifier stages are given in the table below.
 
 
   
   
   
'{$STAMP BS2}
'********** Tube Curve Tracer using StampDAQ **********
'The tube curve tracer uses a Heath, Eico, Lambda or other high voltage
' power supply to generate characteristic curves of vacuum tubes. A 4 input,
' 10-bit analog to digital converter (LTC1093) is used to read two Plate Voltage,
'Current and Grid Voltage. 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 StampThe Baud Rate and transmission mode is 9600,n,8,1 --
'using the constant"84"
'********** Variable Assignments ***************
I VAR WORD 'Counter
J VAR BIT(2) ‘Sign
CLK CON 0 'ADC CLOCK
CS CON 1 'ADC CHIP SELECT (Active LOW)
DOUT CON 2 'MUX Address Data
MUX VAR WORD 'Channel Address (for voltages and currents)
D_IN CON 3 'Data In PIN
ADC VAR WORD 'Data from ADC
A VAR WORD(6) 'ARRAY to hold the 4 measured values
S_Pin CON 16 'Serial Pin - P16, Programming port
Baud CON 84 'Baud mode for a rate of 9600, 8-N-1

'********** Pin Assignments ************************
DIRL = %11111111 'Initial LowByte set all outputs
'********** Initial Variable Values ****************
I = 0:J=0
High CS
Pause 1000 'Allow data communications to stabilize
SEROUT S_Pin,Baud,[CR] 'Send Carriage Return to purge StampDAQ buffer


CONFIGURE:
'Label COLUMNS A to F with TRACER Outputs
SEROUT S_Pin,Baud,[CR, "LABEL,TIME, Plate_1,Plate_2,I_1,I_2,Grid_1,Sign1,Grid_2,Sign2,",CR]
'Clear all data columns (A-J) in Excel
SEROUT S_Pin,Baud,["CLEARDATA",CR]

MAIN:DEBUG "**MAIN**",CR
'Read 10-bit positive voltages and currents:
FOR I = 0 to 5
Lookup I,[99,115,103,119,105,121],MUX
Low CS
Shiftout D_IN,CLK,1,[MUX\8]
Shiftin DOUT,CLK,2,[ADC\10]
PAUSE 30
HIGH CS
A(I) = ADC 'Place data in Array
Pause 1
NEXT

FOR I=4 to 5
ADC=A(I)
J(I)=ADC.bit9
ADC=ADC^%0000000000
A(I)=ADC.bit8<<8 + ADC.LOWBYTE
NEXT

SEROUT S_Pin,Baud,[cr,"DATA,TIME,",DEC A(0),",",DEC A(1),",",DEC A(2),",",DEC A(3),",",DEC A(4),",",DEC J(4),",",DEC A(5),",",DEC J(5),CR]
GOTO MAIN
 
   

1] Borberly, Erno “A Simple Curve Tracer”, Audio Amateur, Volume 2, Issue 4 (1990) page 8ff.

[2] Petrowsky, William J. “From Oscilloscope to Curve Tracer”. Glass Audio, Volume 2, Issue 2 (1990)  page 10ff

[3] Carr, Joe,  “Testing Power Tubes”, Ham Radio, April 1978, page 60ff.

[4] Parallax, Inc. www.parallaxinc.com The Basic Stamp II is available from many vendors including Jameco