Tube testers TV-7/U and variants; made by various U.S. manufacturers in contracts for the Signal Corps.

• Principle: Tube tester
• Operation modes: Tube tester with „Mutual conductance“ test

• Mains operation: 115 V nominal, 103.5 - 126.5 V; 45 VA

• 610 x 400 x 330 mm, 35 kg

• Spare parts: Tubes 5Y3WGTA, 83, neon incicator NE-45 and bulbs NO. 47 and 81

The tube tester TV-7/U of which exist different variants is a compact mains-powered tube tester which was used in the U.S. Signal Corps, the tube table therefore mainly includes the U.S. tubes. Numerous tube sockets are built in and a tube table is located in the lid, so that the set can be put into operation without any further accessories.

It should be noted here that the TV-7 was built to be powered with the American mains voltage of 115 V AC and the set must not be connected directly to the European 220 / 230 V mains.

The different variants differ only slightly, only the latest variant TV-7D/U has an additional measuring range F of the measuring switch.

The main instrument is provided with a 0 - 120 dial, which does not display absolute values, but the value for a usable tube with sufficient emission must be taken from the tube table, i.e. corresponding tube data must be available for all tubes to be tested.

• POWER turns the set on, it must be supplied with 110 V AC voltage and must not be connected directly to the 230 V mains supply commonly used in Europe. When the set is switched on, the red PILOT lamp lights up.
• LINE ADJUST is used to adjust to the mains voltage, which may be between 103.5 - 126.5.
• FILAMENT VOLTAGE: tube heating voltages of 0.6 - 117 volts can be selected, the BLST position is used to test the continuity of ballast tubes and the heating voltage is switched off in the OFF position.

Rotary switches connect the electrodes of the tube sockets to the corresponding voltages

• The two FILAMENT switches apply the heating voltage to the corresponding electrodes of the tube socket.
• GRID: connects the (control) grid of the tube to the corresponding grid voltage
• PLATE: connects the anode of the tube to the corresponding anode (plate) voltage
• SCREEN: connects the screen grid of the tube to the corresponding voltage
• CATHODE: connects the cathode of the tube for corresponding measurements
• SUPPRESSOR: connects the suppressor grid of the tube for corresponding measurements
• The cables can be connected to the G (control grid) and P (plate, anode) sockets to apply the respective voltage to the tube cap connector, depending on whether the tube cap connector requires grid or anode voltage.

• FUNCTION: the left-hand positions SHORTS are used for short-circuit tests, the right-hand 5 or 6 positions RANGES for adjusting the display range of the measuring instrument, the only the TV-7D variant has an additional range F.
• BIAS: the controller adjusts the grid bias voltage
• SHUNT: the control adjusts the sensitivity of the measuring instrument.

Push buttons are used for the various tests:

• LINE ADJ: the measuring instrument and the LINE ADJUST switch can be used to adjust the set to the grid voltage, which is not always completely accurate.
• DIODE: diodes or rectifier tube systems are supplied with AC voltage
• MUT.COND: The TV-7 is a „mutual conductance“ test device: with this most important switching position, anode and screen grid voltages are applied to the tube undergoing the test. It is marked in red.
• GAS 1 / GAS 2: is used to check amplifier tubes for vacuum loss
• OZ4: applies an ignition voltage to the tube OZ4
• RECT: applies an alternating voltage to a rectifier tube under test
• METER.REV: changes the polarity of the measuring instrument

Available indicators:

• the METER / measuring instrument with a 0 - 120 relative scale
• SHORTS neon indicator to indicate short circuits inside the tube
• The PILOT indicator light indicates that the power supply is switched on.
• The FUSE lamp lights up to indicate overloads or burns out in the event of an overload.
• NOISE: for testing the tube for microphonic effects

The tube tester provides the most common sockets in the USA, 4, 5, 6, 7 pin tubes, OCTAL and LOCTAL, NOVAL, miniature 7 pin, ACORN and sockets for round and linear subminiature tube connections.

The tube data table booklet in the lid contains the settings for the various tube types, sorted in ascending numerical order (the first digit of the post-war U.S. tubes often stands for the heater voltage) and alphabetical order, as well as a comparison table for VT and CV tubes.

In the tube data table, each line shows the heater voltage to be set (FIL), the positions of the switches to supply the heater voltage to the tube (letters) and the bottom five switches to connect the other electrodes, which are set from left to right using the switches on the set. The BIAS column shows the position of the BIAS control for the grid bias voltage and the SHUNT column shows the setting of the SHUNT control for the instrument sensitivity, which can only be set in measuring switch range A. The PRESS column indicates which test button is to be pressed, and the MIN VALUE column indicates the minimum acceptable value for tubes with usable emission. For tubes with several systems, the electrode switches must be altered.

Accessories include additional adapters (E104 for testing subminiature tubes with long wires with the octal socket; E105 for testing certain transmitter tubes in the octal socket) and, for the TV-7D, socket saver adapters, which can be plugged onto the corresponding socket for frequent testing of miniature and octal tubes. These adapters can be replaced when they are worn out, so that the socket built into the set is spared.

To test the tubes, the set is connected to a 115 volt power supply and the PILOT light illuminates red when switched on. Now press the LINE ADJ button and adjust the mains voltage with the LINE ADJUST control until the pointer of the instrument is exactly above the LINE TEST centre line. The set is then switched off again.

The manual describes the settings to test the neon indicator for short-circuit testing.

As a first step, the settings in the tube data table are looked up; replacement tables are available to test European tubes and American military tubes with VT and British tubes with CV designations.

The tube must not be plugged in yet! First select the appropriate heating voltage with the FILAMENT VOLTAGE switch. Then all switches (the FILAMENT switches with the letter designations for the heater voltages) and the lower five switches for control grid, anode, screen grid voltage, etc. are set to the correct position.

Now set the BIAS regulator and, if necessary, the SHUNT regulator according to the tube data table. The measuring switch is set to position 1.

Now the tube is plugged into the corresponding socket, some tubes with a top cap connector require the cable with the clip on the cap connector, depending on the data table, the cable is plugged in at G (grid, if the cap connection leads to the control grid) or P (if the cap connection requires anode voltage).

Only now is the TV-7 switched on, the indicator lamp lights up red and the tube heats up. After a few seconds, press the LINE TEST button again to check that the mains voltage has not dropped under the load.

For the short-circuit test, positions 1 - 5 of the FUNCTION measuring switch are now switched through; if the SHORTS glow lamp lights up, there is a short circuit between two electrodes; you can also try to sort these out by tapping the tube. When switching through, one element of the glow lamp often lights up for an instant, which is normal. Some tubes with internal connections can cause the glow lamp to light up during the short-circuit test, which is noted in the tube test table. If both glow lamp elements light up, there is a short circuit in the tube, if one element lights up when tapped, the tube is suspected of having something loose inside. A tube with a short circuit should be discarded and no further tests should be carried out.

Now set the FUNCTION measuring switch to the position indicated so that the measuring range of the instrument is set correctly:

Tube test of amplifier tubes: only to test amplifier tubes, the MUT COND button is now pressed briefly and the value read off. For a usable tube, the value must be higher than the minimum value stated in the tube data table. The manual contains a comparison table for all measuring ranges showing which display value stands for which value in Micromhos. Excessively long measuring time can lead to overloading of the tube and should be avoided.

Vacuum testing of amplifier tubes: For tubes with multiple systems, the gas test is carried out with the settings for the amplifier elements (triode or pentode system). The tube is heated with the corresponding settings. Now press button 4 / GAS 1 and set the BIAS control to a reading of 10 (if the BIAS control cannot be turned down this far, set it to 100). Now, while holding down the GAS 1 button, also press the GAS 2 button. If the display increases by more than one division mark, a grid current is flowing and this will indicate the tube vacuum is insufficient.

Emission test of rectifier tubes: An emission test can be carried out for diodes, diode systems in multiple tubes and rectifiers. An alternating voltage is applied between the cathode and anode of a rectifier tube; the rectified electron flow results in a pulsating DC voltage that increases with the electron density or emission.

According to the specifications in the tube table, buttons 2, 7 or 6 are pressed for the test, never press button 3 for rectifiers. To test diodes and diode systems, an alternating voltage of 20 V is applied between the cathode and anode when button 2 is pressed, and an alternating voltage of 35 V is applied when button 7 is pressed to test rectifier tubes. To test cold cathode rectifiers such as the 0Z4, button 6 labelled 0Z4 is pressed and a voltage of 330 V is applied.

Testing stabiliser tubes: Voltage stabilisers should actually function with a variable voltage, the TV-7 has only one setting in the tube table (FIL to BLST), but this only works with a fixed voltage (GAS 1, i.e. vacuum test, 150 V), the set is not suitable for testing stabilisers such as 0A2 and 0B2.

The TV-7 can be used to test tubes for microphonic effects caused by vibrations; this and also the testing of voltage regulation tubes and thyratrons as well as magic eyes are described in the manual and in the remarks in the tube table. The manual must also be consulted for checking subminiature tubes with different electrode wiring.

Unusual but original is the idea of testing dial lamps, for which the centre contact in the 7-pin socket and the heater voltage setting is used.

Tube tester with measurement of the mutual conductance, the tube steepness is measured using alternating current.

The tube tester consists of various assemblies.

Firstly, it contains a sophisticated power supply. All voltages of the set can be set to the current mains voltage on the primary side using a control resistor, thus calibrating the set.

The centrepiece is a large mains transformer. The various heating voltages from 0.6 to 117 are provided with 18 taps, which are selected with the FILAMENT VOLTAGE selector switch and fed to the connections for the tube heating with the FILAMENT switch. Windings 5 and 6 provide the heating voltages for the two tubes 83 and 5Y3GT.

Windings 1 and 2 provide the anode voltage for the rectifier tube 83, which is used to generate the anode voltage for the tube under test.

Windings 4 provide the anode voltage for the rectifier tube 5Y3GT, which is used to generate the screen grid voltage for the tube under test.

The anode voltage of the device under test is rectified by the two-way rectifier 83, depending on whether a positive or negative half-wave flows with the applied AC voltage, the current flows via one or the other rectifier plate and accordingly via the series resistor R113 to cause a positive or via R115 to cause a negative deflection of the measuring instrument, due to the inertia the pointer remains in the centre position.

A combined grid voltage consisting of a 5 V AC voltage and a DC voltage set with the BIAS controller is applied to the grid of the test object; the total grid voltage now fluctuates with the amplitude of the AC voltage. If this is positive, the entire grid voltage becomes more positive, the current to the anode increases, flows via R113 and the instrument shows a more positive deflection; during the negative half-wave of the grid AC voltage, the grid voltage becomes more negative, the anode current decreases and a lower current reaches the instrument via R115.

The anode current now depends on the total grid voltage (DC + fluctuating AC voltage); the measuring instrument displays the difference between the maximum and minimum anode current, i.e. the anode current depends on the fluctuation of the grid voltage. This is referred to as transconductance or slope and is an important characteristic of amplifier tubes. The data on the relative 0 - 120 dial of the instrument can be converted into micromhos using a conversion table, taking into account the measuring range. 1000 micromhos correspond to 1 mA/V, a more common unit of measurement for the slope.

As a standard, a tube is rated as usable if the emission reaches 60 % of a new tube according to factory specifications, i.e. a NOS tube should reach the minimum value / 0.6.

5Y3WGTA as rectifier for grid voltages, 83 as rectifier for anode voltage.

The TV-7(*)/U series of tube testers was developed for the U.S. Signal Corps. The sets were built by various manufacturers in the form of contracts.

• TV-7/U: C.Lorenz AG and others
• TV-7A/U
• TV-7B/U: Hickock and others
• TV-7C/U: Canadian version from Stark Electronic Instruments Inc. with an adapter for European tubes
• TV-7D/U: last variant with additional measuring range F and socket saver adapters that can be replaced when worn; e.g. Molded Insulation Co.

• Manual and tube data tables can be found in various places on the Internet

• TV-7B on the website of www.radiomuseum.org
• TV-7 Tube Tester Page from EB5AGV
• TV-7 Tube Tester information on the Old Timers Tube website
• TV-7 Information, Jogis Röhrenbude