A Short Introduction to the VssBurst DC Static Parametric Tube Tester:
The Vssburst Tube Tester is a three power supply direct current static tester. It is not an in circuit tester. It was designed to supply a single tube in a single tube fixture the exact voltages necessary to learn the emission profile of a thermionic valve / electron tube. Every parameter is adjustable so any tube of any kind from any manufacturer can be tested precisely, consistently and in the manner the user wants. This makes the test result of universal application, instead of being specific to the circuit in which the tube is being tested. Any number of different testing methodologies can be employed.
We have provided a separate paper, “Understanding Vacuum Tubes and How To Test Them” that will explain in depth the history of thermionic tubes, all the principles that govern tube testing, what tube data sheets mean and how to use them. We also provided an in depth look at the standard test methodologies that the VssBurst tester can be used for. If the user has never tested tubes before, we vividly suggest he become acquainted with the concepts and terminology first.
VssBurst Tube Tester
Brief list of features:
All power supplies are DC regulated constant current or constant voltage.
Supply A, Heater Supply: 0-26V DC 3A, one decimal voltage and current meters
Supply B, Cathode Supply: 0-300V DC 500mA, two decimal high precision voltage and current meters, with by pass.
Supply C, Grid supply: 0-26V DC 3A, one decimal voltage and current meters
B Supply voltage meter is upstream of cathode, mA current meter is downstream of anode.
Insert tube under test in appropriate tube socket assembly and connect direct current leads from supply(s) within the test fixture to reference pin, and alter voltage to get current measurements.
A simple schematic diagram:
Reference Circuit Design: Vh fixed 0-26 DC, Vk + Vg DC variable = Ia.
The V symbol within a circle indicates it’s a voltage meter (Vh, Vk, Vg).
The I symbol within a circle indicates it’s a current ampere meter (Ia).
The A and B Supply have current and voltage meters that are tied to the supply, not to the test circuit, thus are not indicated in the schematic.
The resistor symbol within a circle with a V above it indicates a Variable Voltage Power Source, and therefore the current indication is voltage dependent (Vk, Vh, Vg1).
The resistor symbol within a circle with a T above it, indicates that the current measurement is temperature dependent.
The resistance symbol R indicates a resistance value is needed between the anode and grid 2.
TUBE UNDER TEST
Using the VssBurst Static Parametric Tube Tester
The VssBurst Parametric Static Tube Tester was designed to give you the ability to provide the TUT a variable set of measured parameters. Those parameters will allow you to consistently judge output parameters.
The actionable variables are the (V) voltage values.
The observable values are the (I) current values.
The Voltage and Current Meters:
The meters on the A and C supply are a high quality precision 1% tolerance parts, with single decimal point meter.
The two meters on the B Supply are high precision electronic devices.
The mA meter is a high precision 0.5% tolerance part with a 0-200mA range. The ammeter can be bypassed or replaced with different meters.
The Voltage meter is of similar quality with a 0.5% tolerance 300V DC meter.
The VssBurst Power Supplies, A,B and C.
All supplies are Constant Current / Constant Voltage Supplies.
In Constant Voltage Mode you set the voltage at a predetermined level, and the circuitry will keep the voltage stable while the current stabilizes as the tube warms up.
The supplies can also be run as a Constant Current source by limiting the amount of current the supply delivers via the current dial.
Any unstable voltage reading on any of the supplies is a short.
Power Supply A
The A Supply is a sophisticated direct current regulated power supply capable of a maximum output of 26V at 3 Amps. There is a little head room built in but you should NOT attempt to run it at full power continuously. The LED multimeter display will indicate the amperage on the left and the voltage on the right. It’s a constant current, constant voltage design (CC/CV). Which means that if you set the voltage at 12.6V it will keep the voltage stable and allow the current to fluctuate with demand. You could potentially run it in a constant current mode, which would mean you could determine the maximum amount of current you want the supply to provide, with the down side that if the tube under test requires more current than the pre set limit it would not reach the voltage you set.
The unit also has a standby switch, which must be pressed for the unit to output.
Short protection comes from the constant current design and from a short circuit protection circuit. The supply will click once every 7.5V (aprox- depending on load) as it works through the voltage range. If you hear constant clicking the supply is failing to control the short circuit: you should immediately place the supply in standby or reduce the voltage to 0V. Remove the offending short as soon as possible.
Power Supply B
The B Supply is the high-tension supply. It’s a 300V DC regulated power supply, capable of putting out 500mA at full power.
The B Supply is also a CC/CV regulated power supply.
Power Supply C
The C supply is identical to the A supply.
The current draw on the Grid is very low and only measurable if there is a problem with the TUT.
Since the current draw on the grid is so low we could have used a low power supply. The problem with that is that if a short develops, the supply would get damaged.
By supplying a 3A design we limit the damage potential of a mechanical failure within the tube.
If a short develops stop all testing and dispose of the tube.
Test Fixture is the ABC Circuit.
THE A, B, C Circuit:
The schematic above shows how simple the circuit is.
Using the provided test leads connect the appropriate potentials to the tube pin jacks.
The A Supply feeds Heater supplies
The B Supply negative feeds the cathode
The B Supply positive feeds the anode
The C supply negative feeds the grid
The Tube cathode is shorted to the C supply positive which floats to ground.
The Tube anode is shorted via a resistor to grid 2.
We have designed test fixtures for every kind of tube socket imaginable.
We will provide with the unit the standard 8 and 9 pin socket fixtures.
We can also provide any fixture the user might need, or provide him with a generic blank board for him to add his own socket.
We can also assemble the fixture with any socket provided by the user should he not be familiar with soldering.
Again, before turning on the unit, please make sure you have a test fixture inserted and that all six hold down screw nuts are securely in place.
Insert the appropriate tube into the socket, making sure that the orientation is correct.
Make sure that Supply A and C are in standby mode and that the B Supply is set to its lowest setting.
Find the Tube Pin Out
Find the relevant Specification Sheet for the Tube Under Test.
Find Pin Out Diagram.
We will use the example of an EL34B, which would give you:
Pin 1 = No Connection (NC) or G3 in some applications
Pin 2 = Heater 1 (H)
Pin 3 = Anode (A) (PLATE)
Pin 4 = Control Grid 2 (G2)
Pin 5 = Control Grid 1 (G1)
Pin 6 = NC
Pin 7 = Heater 2 (H)
Pin 8 = Cathode (K)
Connect the test leads
The tube under test socket assembly has 6 jacks.
The two on the left are the A Supply, and therefore you would connect either lead to Pin 2 and the other to Pin 7
The two at the bottom are the B Supply, and therefore:
The Negative “-“ lead would go to the cathode, which is pin 8
The Positive “+” lead would go to the anode, which is pin 3
The two jacks on the right are the C Supply.
The negative “-“ lead would go to grid 1, pin 5
Completing the ABC circuit
Grid 2 is shorted to the ANODE via 1k ohm resistor.
Of importance here is the fact that grid 2 usually has to be “at a lower potential than the anode you are connecting to”.
If you use a resistor you will lower the voltage sufficiently to affect that result.
You can play around with that value as you see fit depending on what you want to achieve.
First connect the cathode to the negative terminal of the B Supply.
Then use a second lead and run it from the top of the Cathode lead and connect it directly to the Positive terminal on the C Supply.
If you are going to be routinely testing tubes in the 300V DC range- use a resistor.
If you are going to use the tester in the normal under 200V DC range there is no need to “short”, just make the connection.
Find the tube’s Limiting Values
The first thing to do is find the “limiting value” for the tube under test (max Vk, Vg1, Ia)
The three limiting values:
Vk, or the maximum voltage that the cathode can handle
Ia, or the highest amount of current that the anode can handle
Vg, or the highest voltage that the grid can handle.
As shown in the schematic: Vh fixed 0-26 DC, Vk + Vg DC variable = Ia.
Or in plain English:
Set the heater voltage on supply A,
Set the cathode voltage on Supply B
Set the grid voltage on Supply C
Complete circuit and observe emission value on B Supply.
With the VssBurst tube tester the user selects the voltages to be applied.
The user could choose a high cathode voltage with a high grid voltage, or a lower cathode voltage with a lower grid voltage.
The best part about the VssBurst tube tester is that if you exceed limiting values, you will be in fact measuring them.
If the heater voltage is too high it will register as heater to cathode leakage.
If the grid is to high, likewise, it will register as grid to anode leakage.
If the cathode voltage is too high it will register as current and voltage on the A and C supply meters.
Leakage is decreased resistance between tube elements. How you qualify that short is up to the user.
Apply Voltage Observe Current
With the ABC circuit completed:
Apply the correct specified voltage to the heaters via the A supply, and observe the current draw: Fixed Vh, observable Ih.
Apply the appropriate range of voltages to the cathode via the B Supply, and observe the current draw: Variable Vk, observable Ia.
Apply the appropriate range of voltages to the controlling filament, the grid, and observe the change in Ia on the B Supply meter.
At fixed Vh, with observable Ih, the higher the Vk and the lower the Vg, the higher the observable emission Ia.
The User is not limited to any specific test and can use whatever test methodology suits his in circuit application.
Nontheless we suggest at the very minimum the tests indicated here:
Heater to Cathode Emission
Vh = Ia
With the heater brought up to temperature, observe B Supply mA meter. It should indicate no more than 0.01mA (0.0001A) worth of emission. Generally a tube with a “heater short” will indicate 15-30mA worth of emission at the anode. Sometimes the cathode meter will indicate voltage not supplied by the B Supply. It is often unclear if the heater is directly making the cathode emit, or the emission from a damaged heater is being picked up by the anode.
It is a sign of a bad tube. Be very careful not to throw the tube into runaway emission when applying voltage to the cathode. We would suggest disposing of this tube without testing it further.
Grid to Cathode Emission
Vh + Vg1 = Ia
This is an observation. Under normal tube conditions the grid emission should not be noticeable at the B Supply mA meter. As with heater to cathode emission, it goes to indicate serious problems with the tube. Its often the result of heaters set to improper voltage, or tubes that over heated due to run away emission.
Cathode to Elements Emission
Vk = Unstable Vh-Ih, Vg1-Ig1
If changing the voltage the at the cathode changes the current and voltage meter readings on the A or C Supply, the tube is damaged and should be discarded. This might even be a physical short but is usually due to a cathode where the coating has come loose. This might appear at a B Supply voltage as low as 30V, but will become more pronounced as higher voltages are applied. This is related to the two aforementioned tests but far more pronounced. Do not proceed any further, the tube is non functional in any application.
Ia = Vk – Vg1
This is the emission test.
You start with a range of voltages to apply to the cathode/anode.
We have selected “standard” voltages that “High Tension” batteries used to carry: 45, 67.5V, 90V and 135V.
Optimally a single test would be sufficient: the cathode voltage (Vk) at which your circuit operates best, with the grid as defined (Vg1).
The grid is first set to 0 V on the C Supply (Vg1 = 0), and on the B Supply to 67.5V (Vk = 67.5), then mark the result.
Repeat with B Supply at 67.5V with the C Supply at -2V, -4V, -8V and -14V
Repeat with B Supply at 90V with the C Supply -2V, -4V, -8V and -14V
Repeat with B Supply at 135V with the C Supply at -2V, -4V, -8V and -14V
Two simple correlations: the higher the cathode voltage (Vk) the higher the emission in mA (Ia), the higher the voltage at the grid (Vg) the lower the emission in mA (Ia) on the B Supply.
Zero Emission Test
Ia = 0 @ Vg1
Here we are trying to find the voltage value at grid 1 (Vg1) that is required to shut down all emission at a pre-set voltage at the cathode (Vk).
Simple correlation: the higher the cathode voltage (Vk) the higher the grid Voltage (Vg1) required.
Fixed Grid Test
Fixed Max Vg1 with variable Vk
From the specification page of the TUT, find the grid’s specified cut off value, which is the maximum (control or limiting) grid 1 voltage (Vg1).
Then with the B Supply at 0 V increase it until you get emission. We suggest 0.01mA (Va) as the goal.
That is the cathode voltage (Vk) at which the grid can no longer shut down all emission.
Observed value Ih
This is an observed value.
Nothing to do- just write down the heater current draw (Ih).
Note that every tube will start high and then drop as it reaches temperature. Don’t write it down until after all the above tests have been done, but keep an eye on it, it should be close to what the manufacturer suggest. If the indicated heater current is 1.5A and its 1.51 its fine- if its 1.60 then its probably shorting internally to the cathode.
Heater Voltage Drop Test
Ih – 5%
Drop the voltage at the heater (Vh) by 5%. Which in the case of a 6.3V filament that would be 6V, then wait a minute and see where it stabilizes if at all. This test can also be done at +5% if your vintage amp was designed for 115V and your current mains runs at 122.4VAC. If you get a 10% mA drop in emission (Ik) the tube is “old” and at the end of its life. If its within the bogey value the tube is still strong. This is similar to the Lifetime Test or Gas Test on older tube testers.
Stressed Stable Emission Test
Max Vk with variable Vg at Ia limiting value
This test really stresses the tube. We suggest a baseline cathode setting (Vk), say 90V, and double it.
At a cathode voltage (Vk) of 180 the tube will not be stable- be careful it does not exceed the meter’s 200mA limit, or go thereafter into runaway emission.
You need to know what the maximum emission (Ia) is from the datasheet. That is the limiting value, please don’t exceed it or the tube will be damaged.
Its usually best to set the grid voltage (Vg1) -4V to start with, and then keep increasing the grid voltage (Vg) until such time that emission stabilizes- that is, stops increasing.
At that cathode to anode voltage (Vk), that grid voltage (Vg) is the minimum bias required to have a stable emission.
You can skip this test in most cases. It is not useful with modern equipment. Most circuits will not try to bring the Vk that high anyways, but some older circuits did indeed specify the highest Vk with the highest possible Vg1.
After you complete this test, you can bring the Vk back to its baseline, and with Vg1 = 0 you will be able to observe a higher Ia than previously recorded. This is because of all the heat the stress test generated. Once the temperature falls back to normal levels the emission should go back to its previous observed level.
Non Circuit Conventions used:
Ih = Current at the heater expressed in amperes
Vh = Voltage at the heater expressed in volts
Ik = Current at the cathode expressed in amperes
Vk = Voltage at the cathode expressed in volts
Vg1 = Voltage at the grid expressed in volts
Ig1 = Current at the grid expressed in amperes
TUT = Tube Under Test in a parametric voltage driven system.
The Cathode (Vk) is always Negative
The Anode (Va) is always Positive
The Grid1 (Vg1) is always Negative
The Heaters (Vh) are polarity neutral.
G2 is tied to the Anode
G3 is often tied to the Cathode, sometimes internally some times externally.
A Voltage indication is an actionable command, which means the indicated Voltage requires you to set it to such value.
A Current indication is a verifiable observation, which means that the current indicated on the meter is a result of Voltages applied.
A Current limit is available and will inhibit the ability of the emitting tube element to reach set voltage value.
A voltage setting on a particular supply (A,B or C) requires a polarity indicator, positive or negative (+ or -).
A voltage setting on a particular tube (Vh, Vk, Vg) does NOT require a polarity indicator since it is assumed that the user understands that current moves in one direction only, namely from the cathode to the anode and from the cathode and grid to ground.
The heaters are filaments. What you supply them is what you supply them. In our Constant Voltage model, you set the voltages and whatever current it draws it draws. Thus both measurements are upstream of the tube under test. Measurements are internal to the circuit producing the direct current.
You take the Voltage measurement before it enters “as supplied” to the cathode and you take the current measurement after it exits the anode. Thus the voltage measurement is upstream of the cathode, and the current measurement is downstream of the anode. Both measurements are in the test circuit.
In our Constant Voltage model, you set the voltages and whatever current it draws it draws. Thus both measurements are upstream of the tube under test. Measurements are internal to the circuit producing the direct current.
And there you have it!
The VssBurst Static Parametric Tube Tester is not limited to the above tests.
Any test can be specified and used on the tester.
A much longer treatment of all the concepts above employed can be found on our more exhaustive treatment, “Understanding Vacuum Tubes and How To Test Them”.
This is available below in sections or as a pdf download that can be printed and read.
.Here is a sample test result for an EL34B: