The tube rectifies the AC voltage into DC; a load resistance comprising either one resistor, or a series of resistors is placed in series with the cathode to develop a pulsating DC voltage. The Load switch selects a predetermined voltage-load combination one of 9 different selections that is appropriate for the tube, and the resulting cathode current is measured and output to the panel meter, where the user can easily read the test result as Bad, Questionable, or Good.
Note that the 3. Load Voltages : Load voltages are easily checked by configuring the machine to test a 6L6 do not insert a tube. Using a test socket in socket 1, measure the AC voltage between pins 5 and 8 while depressing the Quality button. Be sure to check the voltage at each Load switch position from A through J.
Of course, actual AC voltages will differ based upon input line voltage to the transformer primary. Load Resistances : Notice that the load resistances are in series, therefore each previous load affects all of the following loads except for Load C, which has the same load as Load B.
Hence, it is easy to see that if any counter-clockwise load resistor is faulty, all clockwise loads will be incorrect. Therefore, any tweaking or recalibration of these load resistances must be performed starting at Load A and working clockwise. With unit powered off, using the same test socket as explained above, set the Load switch to position J and measure the resistance between pins 5 and 8 while depressing the Quality button.
This is not a precision test because you are testing through the test entire circuit not just the load string , but the resistance will be reasonably close to the load posted at Position J which, as explained, includes all subsequent loads, thereby testing the entire string in one fast step.
As the electron emitting material of cathode is worn away from age, some of the cathode material that has evaporated gets deposited on the grid. This causes the grid to draw current and be more positive than it should be, thereby negatively affecting the circuit. The tube under test has positive DC voltage approx This DC voltage is measured in reference to - of electrolytic capacitor. If grid leakage is present, current will flow from grid to plate of the tube through the 8.
This voltage is amplified by the solid-state amp, causing a deflection of the panel meter. The user then reads the Grid Emission scale on the meter to see whether the leakage is within the green-acceptable range or the white reject range. The megohms reject sensitivity is super-sensitive.
Most tube testers will not measure leakage anywhere near this sensitive. The Sencore TC manual does a good job of explaining this benefit. This is not the level of performance that most serious tube enthusiasts are satisfied with — they want to know when the tube is negatively impacting performance.
The grid leakage is calibrated by connecting megohms between pins 1 and 9 of socket 6. At selector positions 1 or 9, push the leakage button and adjusting R16 10k trimpot so that the meter reads on the division between Green and White on the grid emission scale. On models where R16 is blue, the other pot will be white. In other words, the opposite of the color scheme explained in the manual.
It is important to note that Grid Leakage adjustments will significantly affect the emission test; therefore grid leakage adjustments must be made before any Emission adjustments. The Shorts Test selects one tube element, and all other tube elements are tied together. The test places approx VAC between this one selected element and all other elements. If the selected element has a short or extreme leakage up to 1 megohm to any of the other elements, the Neon lamp will light. The Neon lamp will just barely glow at 1 megohm, and will glow brighter as the condition approaches solid-short.
As your rotate the Selector switch from 1 to 12, all tube elements are compared to each other. The Shorts test is calibrated by connecting 1 meg resistor between any two pins and the Neon lamp should glow when the Selector switch is rotated to the number of those pins. Per the manual, connect to pins 1 and 9 of socket 6, and at Selector positions 1 and 9 the Neon lamp should just barely glow. Photo below uses pins 6 and 7 as another example. Adjustment is made by the turning the R1 2Meg trimpot on the circuit board.
The manual refers to this as a blue trimpot, but I have found models where this pot is white, so just look for R1 regardless of color. Both models can share the same tube setup data book, and therefore both models are functionally equivalent. Model will be example shown in this article. Electrical voltages are present, repairs should only be attempted by a qualified technician. Copyrighted TubeSound, all rights reserved. Model shares the same color scheme with Model , and model shares the same color scheme as model The wood case construction is similar also.
Testing metal tube without shell pin locked-out. Example c explains why there is a small chart difference between configuring a 6L6 and a The original 6L6 has a metal envelope, and like most metal shell tubes, the shell is connected to pin 1. A tube is glass version of 6L6 and has nothing connected to pin 1.
Hence, the chart configures the machine to isolate lockout the metal envelope. Selects the control grid for Emission Quality test and grid leakage test. As always, start with the fundamentals. Check and replace all non-spec resistors. Check or replace all capacitors. The electrolytic capacitor should always be replaced.
Polish and clean every socket pin. Treat all switches with Deoxit. Treat the calibration pots for Shorts test and grid leakage test with Deoxit. Treat the Sensitivity pot with Deoxit. Failure to clean the calibration pots and Sensitivity pot is one of the most common source of inaccuracy in any tube tester.
Clean neon lamp bulb and socket. Check the 6BN8 tube or replace with known good tube. Check secondary transformer voltages. If meter has suspicion of being defective, remove and test full scale 1 mA dc, ohms. Make sure that panel meter and Sensitivity pot are each mechanically indexed at zero.
The Shorts test is started automatically when a tube is inserted into a testing socket. The user rotates switch C to all positions. The neon lamp will automatically light when a short or internal connection is detected.
Use an octal test socket in Socket 3 and connect a 1-meg resistor between pins 5 and 8. Power on tester. This sets the shorts test to 1-meg sensitivity, which is the factory default. This Shorts test is not perfect because the tester does not have a full selection of lockout switches. When testing tubes that have multiple tube elements with multiple pin connections, actual shorts can be missed.
The first thing to notice is that Pin 9 is locked-out Pos 10 locks out pin 9, as explained above , therefore there will be no false shorts to the control grid. That is good.
But the chart also explains that regardless of what is going on at pins , the Shorts lamp will light. This can be a problem. In the left photo, pin 4 shows short with a perfectly good 36KD6 tube because pin 4 is internally connected to pin 10, and the chart explains that you can disregard this as correct behavior which is correct in this example. In the right photo, I have connected a jumper wire between Pins 3 and 4, thereby shorting the Screen Grid and Suppressor Grid. While the Shorts lamp is still lit at Pin 4, the problem is that the chart tells you to disregard this short because of the example in the left-side photo.
Therefore, since the tube tester behaves the same in both instances, you cannot know whether the tube has a short between those elements are not. They will not detect all shorts in tubes that have multiple elements connected to multiple pins, primarily suppressor grid shorts. This is a minor limitation, and one that is shared with most tube testers regardless of brand or cost.
0コメント