Chapter 4
TUT: TUBE UNDER TEST
Using the VssBurst Static Parametric Tube Tester
- 1: SAFETY:
This is a very high quality precision instrument.
It requires the user to know the basic principles of electricity.
In the Military an ORDER is given in CAPITAL LETTERS.
We are not talking about safety, we are PRACTICING SAFETY PRECAUTIONS.
We are not asking you kindly pretty please don’t shock your self, WE ARE TELLING YOU BE CAREFULL.
THIS UNIT CAN GIVE YOU A SERIOUS ELECTRICAL SHOCK.
DO NOT ATTEMPT TO USE THIS UNIT IF YOU ARE UNFAMILIAR WITH BASIC ELECTRICAL SAFETY PRECAUTIONS .
DO NOT ATTEMPT TO USE THIS UNIT IF YOU ARE UNFAMILIAR WITH THE UNIT’S BASIC OPERATING INSTRUCTIONS.
DO NOT ATTEMPT TO OPERATE THIS UNIT WITHOUT THE FULL SAFETY MEASURES IN PLACE.
The unit will not power up unless a test fixture is inserted in the panel. There is a cut off switch.
DO NOT DISABLE THE CUT OFF SAFETY SWITCH.
DO NOT OPERATE THE UNIT UNLESS ALL SIX SCREW CAPS ARE HOLDING DOWN THE TEST FIXTURE AT THE JACKS.
THERE ARE NO POINTS OF ELECTRICAL CONTACT BETWEEN THE EQUIPMENT AND THE USER IF THE TEST SOCKET IS INSERTED AND THE SIX HOLDOWN SCREWS FOR SAID TEST SOCKET ARE FIRMLY IN PLACE.
OKAY, WE WILL ASK NICELY… On the bottom of the test fixture, there are the two output sockets for the B Supply- its 300V DC supply, take that very seriously and make sure everything is secured properly before operating. Thank you.
There will be a brief introduction to the tube tester itself and what it was designed to achieve. Then we will proceed with simple testing methodology that should be able to give you a reasonable understanding of the tube under test.
- 2 The VssBurst 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.
- 3 The Voltage and Current Meters:
The meters on the A and C supply are a high quality precision 1% tolerance parts, nonetheless they are a single decimal point meter. It will give you a correct measurement of say 12.6 volts but it will not tell you if that’s 12.61 or 12.69 volts. The two meters in the B Supply cost more than to build the A supply itself. You might want greater precision but frankly a tenth of a volt precision on the heater is not necessary. If you are that obsessive about it, use an external multimeter with the precision that makes you happy.
The two meters on the B Supply are high precision electronic devices. They are very sensitive devices. They do not like to be abused and they don’t like to go out of range. The specification calls for a maximum of 200mA- please do not exceed that or you will damage the unit.
The reason for a 200mA meter is that if you use a 500mA meter it will simply be incorrect at 5mA. The point is to be able to test the full range between a 2mA 12AU7 and a 100mA 6550. We can provide a very accurate 20mA meter, but as soon as you insert a pentode you would destroy the meter. This 200mA meter is precise within the full range.
On the left of the main test fixture you will see two banana jacks. That is the connection between the B Supply Anode output, the current meter, and the test fixture. If you break the connection the meter will no longer indicate current. It was designed in this fashion so you could run an external multimeter in case you want to run more than 200mA or take a more precise measurement. If you are really interested in knowing that your dual triode is 2.5001mA and 2.5009mA use a high quality benchtop external multimeter. The existing meter gives you two digits of a hundredth of a mA precision. You don’t need more than that. If you feel you do, spend accordingly.
If you want greater precision plug in the positive lead of the multimeter into the top jack and run the negative lead directly to the anode jack. The shorter your multimeter leads the more precise the measurement.
4.4: The VssBurst Power Supplies, A,B and C.
All supplies are Constant Current / Constant Voltage Supplies.
In Constant Voltage Mode (with current set to maximum value clockwise on the current dial) 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.
As a practical example if you were to set maximum current at 1Amp on a EL34 tube, it would never reach the required 6.3V even though you set the voltage to that figure, since the EL34 requires 1.5A. It is useful on the C supply since the grid uses very little current, and any measurement above 50mA is probably a grid short.
Any unstable voltage reading on any of the supplies is a short. Reduce voltages to 0V immediately, wait for the tube to cool, and discard it.
Any unstable Voltage or Current reading on the A supply is a short.
Any unstable Voltage or Current reading on the C supply is a short.
Any current reading on the C supply is a short.
At higher B Supply voltages that could be indicative of electromagnetic interference from the cathode to the grid or heater.
In practice that means the user has exceeded the performance envelope of the thermionic valve, and damage might have occurred.
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.
There is a Stand By switch the purpose of which is to allow the user to set the voltage before applying power. The voltage would subsequently stay constant and the current would at first be far greater than the final value reached when the TUT has reached its operating temperature.
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. There are very few tubes that will achieve that level of power output. As a matter of fact I know of only two consumer tubes that achieve those power levels. The point is that should the tube go into runaway emission, you will need that head room so as not to damage the test equipment. But neither the tube nor the power supply was designed to go that high. The B Supply is also a CC/CV regulated power supply. For our purposes you should not allow more than 200mA to reach the mA meter. If you exceed that current measurement you will damage the meter and very possibly seriously damage the tube.
Please see note on the Meters (4.3).
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 short. If a short develops stop all testing and dispose of the tube.
Please run the Current knobs at the 12 o’clock position. This way if a short exists, current will run out before damage to the equipment occurs.
If you hear a single click it is simply working its way through its power range. If the clicking is constant (more than one click) its means it cannot control the current and the tube has a short.
The unit also has a standby switch, which must be pressed for the unit to output.
4.5: Test Fixture is the ABC Circuit.
THE A, B, C Circuit:
The picture below 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
The Tube anode is shorted via a resistor to grid 2.
The V symbol within a circle indicates it’s a voltage meter (Vk).
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 (TUT) temperature dependent.
The resistance symbol R indicates a resistance value is needed between the anode and grid 2.
- 6: Test Fixture
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.
Most 8Pin tubes have a notch that has to be aligned.
Most 9Pin tubes have a space between Pin 1 and Pin 9 that must match the tube socket.
Make sure that Supply A and C are in standby mode and that the B Supply is set to its lowest setting.
- 7: Find the Tube Pin Out
Find the relevant Specification Sheet for the Tube Under Test.
Find Pin Out Diagram.
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)
- 8: 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
- 9: Completing the ABC circuit
Grid 2 is SHORTED to the ANODE.
Shorting & Connecting:
Shorting just refers to running a connection between two pins.
Use whatever word works for you. We use the term because at the very least it suggests you use a resistor in line to do so.
When you connect two branches of a circuit you can do so without using a resistor. Either way you can use the words interchangeably.
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.
Lets take the specifications from a Tung-Sol 6550:
PENTODE CONNECTION
PLATE VOLTAGE, 250 VOLTS
GRID 2 VOLTAGE, 250 VOLTS
GRID 1 VOLTAGE, -14 VOLTS
PLATE CURRENT, 140 MA
What the above would indicate is that G2 can be tied directly to A without the use of a resistor since the Anode (plate) and G2 are at the same voltage.
But those are limiting maximum values.
But the circuit regardless will not work well with a 1 ohm resistor if you apply anything more than 45V DC at the cathode (Vk = 45).
We suggest a 1k (1000) ohm resistor. You can play around with that value as you see fit depending on what you want to achieve, but sticking to 1k ohms makes it at the very least consistent.
Cathode Short
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.
- 10: Find the tube’s Limiting Values
The first thing to do is find the “limiting value” for the TUT.
That is the maximum value in current that the cathode can emit and the anode receive. In the above example the limiting value is “PLATE CURRENT, 140 MA” (Ia = 140). If you exceed that value you will damage the tube. If you run the tube at 250V with the grid at -14V you might or might not exceed the Ia value of 140mA. The important thing to know is not to exceed that emission maximum value.
Similarly you need to find the limiting value for the Grid. Its usually indicated in volts, as in “-14V”.
You can usually run the grid voltage at a higher value, but above that value, the grid starts emitting in a noticeable manner, it might even be picked up as emission in the B Supply mA meter.
- 11 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.
- 12 Sample Test Methodology
First, a sample spreadsheet with some boxes to be filled in with the correct observable values.
This is just a sample, you could come up with a different test circuit.
The tests we will illustrate the functioning with have worked well, but they are not exclusive.
We will use an EL34 pentode to demonstrate.
Lets explain:
- 13 Heater to Cathode Emission
Vh = Ia
This is an observation. 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.
- 14 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.
- 15 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.
- 16 Emission Test
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.
We are ignoring the 45V test since it does not stress the tube in any way; if you have a real expensive tube it might not be a bad idea to start there though.
Having said the above, 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
From the chart below you will see 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.
- 17: Zero Emission Test
Ia = 0 at Fixed Vk with variable 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).
From the chart below you will see a simple correlation: the higher the cathode voltage (Vk) the higher the grid Voltage (Vg1) required.
- 18: 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.
- 19: Heater Current
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.
- 20: 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.
- 21: 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.
- 22: Sample Results EL34B
Please see at page:
Some Observations:
TUT#1 has some grid 1 leakage into the anode. TUT#2 does not. TUT#1 has some heater leakage into the anode. TUT 2 does not. Its not quite a “short” but close to it.
TUT#2, based on its fixed grid (Vg1 = 14) with cut off voltage at the cathode of 97.5 is very good, and pretty much on bogey based on experience with this tube.
From the emission test you can see that Tube #1 is prone to running high and had difficulty settling down on a Ia value. Tube #2 instead was steady and tests on bogey.
These were two out of a lot of four tubes. Three were all on bogey, TUT#1 was not.
- 2: Testing a Double Triode
A double triode is no different from a simple triode, only that there is only one heater (or two internally connected heaters), and two triodes which are controlled separately.
The procedure is no different from the EL34 example we used above only that you will have to repeat the test on each triode within the tube.
The cathode is connected to the C supply by connecting on top of the cathode lead to the C Supply positive terminal.
There are no secondary grids to deal with.
I will use the example from two standard tubes still in production.
The first tube is the 12AX7 which has many versions and manufacturers.
Essentially it likes somewhere between Vk 100 to 200- with 200V being a maximum value.
Its grid should be set optimally at Vg 2.00
As you can see from the test results, if you set the grid at 2V (Vg1), emission starts at 125V (Vk)
The maximum value at the cathode is somewhere above 200V but I usually do not go that high.
Double Triodes are delicate little things, which means somewhere on the specification sheet there will be either a “Limiting Value” or a “Max Value” expressed in terms of cathodic emission. On the Philips spec sheet for the ECC83 (12AX7) that is given as Ia = 1,2mA.
That means they do not want you to exceed that number- either by running too high a Vk or too low a Vg.
On the 12AU7 for example Electro Harmonix clearly states “Plate Current, not more than 22mA”
In any event pay close attention to the mA meter on the B Supply.
What that means is that while the Vk or Vg have a wide tolerance, total emission should not. Emission is the limiting factor.
One last point- these tubes have very small heaters, they take longer to warm up.
They will emit after 20 seconds but a stable result takes more time.
- 21 Sample Results 12AX7
Please see at page:
This is a nearly perfect 12AX7A. Regardless of voltage at the cathode the emission is similar on both.
Triode 2 (6,7,8) will always test slightly higher because by the time you get to it the temperature will have been affected by the stress test on Triode 1.
Notice how the stress test is not stressful at all- Even with the voltage at the cathode at 200, you can keep grid 1 at 0V and not exceed the current limiting value.
These could have easily been tested at 100V, 150V, 200V with stress test at 250V.
- 22: Sample Results 12AU7
Please see at page:
This is a good example of a well balanced double triode. There is very little leakage from the heater or the grid. The emission is also reasonably well balanced. From the spec sheet I could gather a limiting value of Vk=250, with a LV on Ia of 22. Thus the Vk values of 80-120-160 are perfectly appropriate. On the stress test, once runaway emission had started it took Vg1=4 to shut it down, but then I could dial that down to Vg1=3 and maintain stable emission.
Afterworlds:
That should do it!
And remember there is no right or wrong way to test tubes, just remember not to exceed the limiting values and everything should go well.
Enjoy, be pragmatic and don’t fry the tubes!!
LASTLY:
Please do send us feedback.
If there is something you do not understand its because we have failed to explain it properly.
If there is something that we missed, please let us know, so we can update this paper.
Where we have made a mistake, please let us know so we can correct it.
And finally any comments, positive or negative are always welcome. One learns more from one’s mistakes because one usually commits the most mistakes while trying something new….
Please send all comments to: hmf@vssburst.com
- 23 Further Reading:
http://frank.pocnet.net
Very Exhaustive source for data sheets on pdf.
Department of Defense, Test Method Standard. MIL-STD-1311C, 31 January 2001. FSC 5960
If you want to see how to test tubes in circuit for a specific application, ie- tubes designed specifically for a single circuit, this is the one to read. You can download it from the internet its “Approved for public release; distribution is unlimited.” Save as pdf and bring it to your commercial printer for double sided printing.
They for example prefer to run the Vh- 5% test at -10%. I assume because the equipment must operate off of diesel generators and mains fluctuations will be larger.
Theory And Applications of Electron Tubes. Herbert J. Reich, Ph.D, McGraw-Hill Book Company, 1944
If you like mathematics to describe empirically verifiable results of theoretical investigations, this is it. Exceedingly well written and never surpassed. Back from the day when people wrote in the King’s English and in the Queen’s hand.
We quote: Section 15-27 Determination Of Static Tube Characteristics: “It would seem at first thought that the determination of static tube characteristics by means of direct voltages and d-c meters is such a simple procedure that it requires no discussion…” He validly brings up the point that all static tests are temperature related, namely that a highly stressed tube will reach temperatures which invalidate the empirical results. If you, for example run our SSE V test before the Vh-5% test with Vg = 0 and Vk = 120 the increase in heat because of the stressed test will give you a higher baseline result (Vh to spec) than that written down for the original Vg=0, Vk =120 test.
Getting The Most Out Of Vacuum Tubes, Robert B Tomer, Howard E. Sams & Co, 1960.
1950’s common sense. Covers all testing ideas of the era. Well written, emphasis on being pragmatic and empirical, and ignoring all the transconductance bunk.
Available for download on the internet.
RCA Receiving Tuba Manual, RC-30, as reprinted by Antique Electronic Supply.
Principles of Power, A Practical Guide to Tube Power Amplifier Design, by Kevin O’Connor. Power Press Publishing.
Very good book if you want to jump from DC to AC and see how tube parameters apply to circuit design. Simple and systematic exposition. Does not require a Degree in Electrical Engineering to understand the principles.
Ready Set Go! An electronics reference for the everyman, Kevin O’Connor, Power Press Publishing, 1999.
If you are looking for an introduction to electrical terms and concepts, this book is useful.
Tube Testers and Classic Electronic Test Gear, Alan Douglas, Sonoran Publishing LLC, 2009.
A walk through memory lane of all historically significant tube testers.
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