VACUUM TUBE TESTER KIT LIST:
1x main board
1x LCD display
1x 0-115-230v 10VA transformer
1x AC-DC 9V 1A power supply
1 set wires (with potentiometer and switch)
1x 4-6.3V/4A power (input 90-260VAC)
3 x potentiometer knobs
We also have a front panel for the tester, here is the link of it:
https://www.ebay.com/itm/153160710601
This tester is ready to use and can work with most audio tubes,
rom small triodes: 12at7, 12au7, 12ax7,6sl7,6sn7 to power triodes / pentodes like: 2a3, 300b,
el84, 6v6, 6l6, el84, el34, kt66, kt88, etc...
Transconductance ( Gm ), Internal resistance ( Rp ) and Gain ( µ ) can easily be calculated using
2 simple measurements.
Rectifiers and diodes can also be tested.
User guide of the tester:
1 GETTING STARTED AND OPERATING THE SYSTEM
Getting started is very simple, because most of the elements are already connected. You only need
o plug the different connectors , followed by the T1 and T2 to the mains using a switch and a fuse
1A.
Connect the four wires (cathode, G2, plate and G1) to the appropriate tube socket pins. Do not forge
o slide the ferrite beads on to the wires (except for the cathode).
Connect a transformer or an appropriate power supply to the tube’s filament socket.
Make sure that the module is on an insulated support and that there is no short circuit that can
possibly occur. Switch on power.
Pre-adjust the V plate voltage (Plate), Vg2 (grid 2)*, and Vg1 (grid 1) according to the characteristics
of the tube to be tested.
Example of measurement :
EL84 tube, the datasheet indicates :
Pin 2 : G1 (grid 1)
Pin 3 : K-G3 (cathode and grid 3)
Pin 4 : F (filament)
Pin 5 : F'(filament)
Pin 7 : A (anode and plate)
Pin 9 : G2 (screen or grid 2)
Test conditions : In the absence of indications just refer to the curves. According to these curves, it
would be possible, for instance, to choose a plate voltage of 300V, a G2 voltage of 250V and a G1
voltage of -4.0V. Under these conditions, the nominal debit would be 90 mA.
1 Set the display voltage selector as to show the plate voltage.
2 Turn the Vplate potentiometer to get a 300V reading on the screen.
3 Set the display voltage selector as to show the G2 voltage.
4 Turn the Vg2 potentiometer to get a 250V reading on the screen.
5 Turn the Vg1 potentiometer to get a -4.0V reading on the screen.
6 Plug the tube and leave it to heat up for about one (1) minute.
Choose the corresponding displays by using the voltage switch.
Check that the neon indicator is turned off or is blinking. For certain tubes, this light could be turned
on continuously without blinking even if there is no short circuit. In general , this kind of phenomenon
s created by tubes which have high transconductance or power or by valves and cathodic indicators.
t is not dangerous to carry out tests even if the light is turned on.
Push on the "TEST" button and read the value of the plate current in mA.
Release the button. The test is completed.
More measurements : transconductance , internal resistance and gain are possible and simple.
Tubes matching is done by a simple method. Test several tubes of the same type without changing
he adjustments in order to pair up (or in quartets) tubes with plate currents whose measurements are
closest.
For double tubes like : ecc81, ecc82, ecc83, ecc88 – 6sl7, 6sn7, etc…The measurement of any of the
nternal triodes can be undertaken by swaping the connection to either the plateof triode 1 or triode 2
all other same electrodes connected together ). This can be easily undertaken with a simple
nverter.
The testing of valves, diodes , cathodic indicators, gas regulators is possible and described on §6.
2 OPERATING PRINCIPLES
Using an impulse mode, the tests are carried out every 0.5 seconds. The time lag for obtaining the
esults is very short : 800 uS (0.0008 seconds).
Between the test windows, grid 1 is at a negative potential and the tube is in a cut-off state. Its
dissipation is almost non-existent.
This mode allows for the use of very little energy on the high voltage supply (plate and G2), and
prevents the use of heavy and costly transformers, while at the same time allows for tests using high
voltage and plate current (450V / 340mA).
A microcomputer PIC clocked at 12MHz carries out the acquisiton , measurement and management
of defects and of the commands for the LCD display.
The plate supply is organised around an self-oscillation converter (Royer) controlled by a comparator.
A high-value condenser (47µF) serves as a reservoir and supplies the voltage during the800µS test.
This condenser discharges slowly. When the plate voltage has to be lowered, through the Vplate
potentiometer, it would be useful to briefly push the DISCHARGE button, which would put a discharge
esistance in parallel with the supply output and accelerate the process. SURCHARGE warning (see
below) could then be displayed, and will disapear when the button is released.
The G2 supply, which has a lower debit, is organized around a conventional circuit : transformer,
ectifier, filter, stabilization using a group of zener diodes. A power mosfet controlled by the Vg2
potentiometer delivers the G2 voltage.
These two supplies, plate and G2, are protected by current limiters. The peripheral circuits also have
his type of protection.
The display of the plate and G2 voltage is carried out in an alternating manner through selection,
using the voltage switch (connector J1 - pins 4, 5 and 6). The G1 supply circuitry is like G2 but
because of the low current, it is not buffered.
A short circuit tester for the plate and the internal electrodes of the tube is effected through a neon
ght (connector J2, pins 8 and 10). This neon lights up continuously in the event of a strong current.
Otherwise, it will blink or will remain extinguished. This simple circuit is just an aide for the tests and
has its limits. It could stay lighted up continously when used with high power tubes or tubes of a
specific type (diodes, magic eyes…) are present.
Some tubes are very unstable and the connection wires can be seen as resonant circuits. Likewise,
he plate, G2 and G1 wires have to be equipped with a ferrite bead filter to block any possible auto-
oscillation. These beads have to be positioned beside the tube socket closest to the connections.
The microcomputer monitors the plate voltage current. All excess will automatically limit the output
voltage, SURCHARGE will appear on the screen, and all tests will be suspended until the problem
disappears or until the TEST button is released.
3 ADVANCED TESTS
The cathodic current of a tube is an important element to determine its state and to carry out
matching.
Nonetheless, this parameter gives only a preview of the tube overall performance. With the DuoKit
you will be able to carry out three other very complementary tests, which are :
The slope or transconductance : Gm : expressed in mA/V, uS or umhos
The internal resistance Rp : expressed in Ohm
The gain µ which is the result of the two preceding values
Testing the slope :
Carry out a first test, and note the corresponding cathode current Ik1.
Without making other adjustments, increase or decrease the voltage on grid 1 (Vg1) by one volt, and
note the new cathode current Ik2.
The Gm ( transconductance ) is the difference between the lk1 and lk2 currents, expressed in mA / V,
or in uS, or in umhos, by multiplying the value by 1000 : 1mA/V = 1000 uS = 1000 umhos.
This feature, measured on several points, enables one to know the linearity of a tube.
Testing internal resistance :
Carry out an initial test, note the Vp1 plate voltage and the corresponding lk1 current.
Without making other adjustments, increase or decrease the plate voltage (Vplate) until a significant
change in the current is created. Note the new value of the Vp2 plate voltage and the new lk2
current.
The internal resistance Rp is the difference of the plate voltage divided by the difference of the
corresponding currents : Vp1 - Vp2 divided by Ik1 – Ik2
Calculating the gain :
As indicated above, the µ gain is equal to Gm (in mA/V) x Rp
All tube features are then known, and a very precise matching can thus be made.
4 VALVES, REGULATORS AND CATHODIC INDICATORS
Testing particular tubes like diodes, valves, rectifiers, gas-regulated tubes and cathodic indicators can
be done with the DuoKit.
Diodes :
With the internal module supply, low-power tubes can be tested to a maximum current of
approximately 15 mA,. For currents above this, follow the procedure for valves testing.
Before plugging the tube, turn the Vplate potentiometer fully anti-clockwise and, if needed, push the
DISCHARGE button several times to bring the plate voltage to a value < 5 volts.
The connection is done similarly as for other tubes : wire K to the cathode and the Plate wire towards
he plate (s).
When the tube is hot, slowly turn the Vplate potentiometer and observe the increase of the current,
comparing it to the curve presented in the tube specifications.
Valves :
Valves or rectifiers tubes can be partially tested with an internal plate supply of the module. However
he absence of the command grid prevents the operation of the impulse mode, and the permanent
debit may exceed the capacity of this supply.
n this case, use an external variable d.c. supply. This d.c. supply will be adjustable from 0V to the
desired voltage (depending on the curve of the tube).
Minus of the supply will be connected to GND (J2 pin1 or 2), and the positive pole to the terminal
corresponding to the plate (s). To the tube socket but not to the module plate wire.
The connection and the sequence of tests are identical to the diodes.
Gas-regulated :
The test of these tubes is carried out with a resistor in order to limit the current. The xxx voltage will
be seen directly on the display. Connections between terminal K and plate.
Cathodic indicators :
The test for luminosity of these tubes can be done simply by connecting its electrodes to the
appropriate terminal and by adding the resistance to the plates, according to the schematics of the
manual.
5 Technical features
Main Board's Dimensions : 146 x 133 x 28 mm ( L x W x H. max )
Power Supply : 220V – 230V or 115V / 50-60 Hz or 12V DC via DC to AC converter
Power request : 25 to 35 VA max.
Approx. 2 amps under 12VDC, via DC to AC converter
Protection fuse : 1 A fast fuse (5 x 20 mm)
Measurement mode : pulsed, 8 bits / 12 MHz micro-computer
Adc sampling : 10 bits monotonous ( 1024 )
Measurement window : 800 µS
nterval between measurements : 0.5 second
Measurement accuracy : better than 5% ± 1 digit
Grid 1 voltage: 0 to -100 volts
Grid 2 voltage :0 to 350 volts
Plate voltage :0 to 450 volts
Maximal mesurable plate current : 340 mA
Grid 2 supply max. current: 60 mA
Grid 1 display resolution: 0.1 V ± 1 digit
Grid 2 display resolution: 1 V ± 1 digit
Plate display resolution: 1 V ± 1 digit
Plate current display resolution: 0.1 mA ± 1 digit until 34 mA
1 mA ± 1 digit for I > 34 mA
nternal short-circuit protection.
Optional AC-DC regulated supply for tubes heating :
Heating voltages : 4 V – 5 V – 6.3 V
Output current: 3.5 A maximum
Output voltage tolerance : better than 5 % ( regulated )
