This article will discuss repair and calibration of the Knight 600 tube tester. High voltages are present, repairs should only be attempted by a qualified technician. Copyrighted, all rights reserved.

(Knight 600 testing Sovtek 5881/6L6 audio tube)

Introduction

The Knight 600 series (600, 600A, 600B) was a self-assembled tube tester kit offered by Allied Radio Corporation. The copyright date of the Knight 600 manual is 1955. This tester, along with its brothers Eico 625, Triplett 3413, and Heathkit TC1/TC2, was popular among servicemen because they could buy a “name-brand” tester and save money by building the kit themselves.

The size is 15 x 11.5 x 8 inches and weighs 11-lbs. The case is wood.

The Knight 600 uses a traditional Emission test circuit. In fact, it shares the exact-same circuit as the Triplett 3413 and Heathkit TC-1/TC-2, and the Eico 625 is also the same (except for minor circuit differences resulting from a 6H6 tube rectifier instead of solid-state rectifier.) The Knight, Triplett, and Heathkit schematics are identical (except for trivial component value changes.) Since the test circuit for all these models is identical, you can interchange the Tube Test Data charts with each other. This is a great benefit, since between all of those models, the number of tubes that have test data available is quite large. I will discuss the data charts below in this article.

In addition to the Emission test, it can check for Shorts, Leakage, and Gas.

The Knight 600 has the following sockets: 4-pin, 5-pin, 6-pin, 7-pin large, 7-pin miniature, Octal, Loktal, 9-pin-miniature (aka - noval). There is one blank spot to accommodate adding an additional socket.

A new SS9 socket saver has been installed. Original Knight supplement data book and manual is present. This is a beautiful test instrument with excellent original cosmetic condition.

Service

As always, start by removing the chassis from the case, remove the roll chart, give the unit a thorough cleaning (inside and out), and dry with a powerful garage air compressor. Do not allow either the meter or transformers to get wet. Clean the roll chart window. If the roll chart does not roll smoothly, very carefully lubricate. Check that the meter needle is indexing at zero (adjust setscrew accordingly).

There are two rubber grommets that hold the roll chart bulbs in place. Replace both grommets. The original grommets are always brittle, which allows the bulbs to flop against the roll chart assembly and create shorts. After replacing the grommets, install new #44 bulbs, and clean and work the bulb sockets with Deoxit.

Next, when servicing any kit-supplied tube tester (Knight, Eico, Heathkit…), you need to (1) check ALL solder joints, including the tube socket pins, and (2) follow the schematic and check all wiring (do not assume that the original owner wired the unit properly). These extra steps do take time, but the circuit is simple and straightforward, and can be double-checked in a reasonable amount of time.

Inspect the AC line cord and rubber feet. Replace if necessary.

Replace the 0.1 capacitor. Check all resistors for accurate values. The CR-1 rectifier is usually fine, but if it needs to be replaced, a standard 1N4007 diode will work fine, although you will need to adjust the value of R11 resistor to account for the new silicon diode (otherwise, you will not be able to calibrate the unit).

All switches, all potentiometers, and tube sockets must be meticulously cleaned and treated with Deoxit. See my article on repairing and calibrating a Jackson 648 for a detailed explanation of this process. Once every pin of every socket is cleaned, you may want to take this opportunity to install a socket saver (here, I have installed a 9-pin-miniature socket saver).

The Load control potentiometer can be opened by removing the plastic knob, remove the nut and drop the pot from the chassis, then use a small screwdriver to pry up the four finger tabs on the back side pot cover. Treat the inside of the Load control with Deoxit, work the pot back and forth, then reattach the metal cover, reinstall to chassis. It is also critically important that R10, the Calibration control, is also be treated with Deoxit and worked back and forth. R12 Line rheostat is likewise treated and worked.

In this photo, you can see (on the left) R10 Calibration trimmer, and to its right, the Load potentiometer with its back cover removed (far right).

Calibration

Now that you have double-checked all wiring, cleaned all sockets, switches, pots…checked every resistor for accuracy…and replaced the capacitor, you can now begin the rest of the calibration. Put in an octal test socket, and connect your Fluke meter to pins 2 and 7 and set for AC scale. Leave all “Tube Elements” switches in the middle position, except move #2 down. Set Filament to 6.3. Set Line control approx halfway. Plug in tester. Observe whether meter needle is moving towards the Line position. (If not, then your CR1 rectifier is defective.) Turn the Line control until your Fluke reads exactly 6.3 vac. At this point, adjust R10 trimmer until meter needle reads perfectly centered on the “Line” mark. Next, turn the filament control to each setting, and observe that the voltages closely correspond to how they are marked. Note that each filament setting will NOT be “exact.” In a few positions, the voltage will be a tiny bit high, some a tiny bit low. This is normal, and the setup charts adjust for this fact. If you have calibrated at 6.3, you have done so correctly. Recheck calibration several times, including after the unit has been on for a while. If you see any deviation, and your resistors are accurate, replace CR1. Calibration should last a while, since any deviation would be caused by resistors changing value, or the diode.

Shorts and Leakage Test

Using a test socket and jumper wire to short two pins, you can easily check proper Shorts Test functionality.

Using a resistor instead of jumper wire, you can verify Leakage Test functionality. Notice more dim response from the Neon Light, which is proper. The more leakage, the brighter the Neon will illuminate. [More leakage = closer to Short].

Tube Setup Data Charts

As previously explained, you can interchangeably use the tube setup data from any Eico 625, Heathkit TC, Triplett 3413, or of course Knight 600 itself. In some cases, the “Tube Elements” are indexed with letters (A, B, C…) instead of numbers (1,2,3…), and likewise the “A-Circuit” is indexed with letters A,B,C,D instead of 1,2,3,4. Common sense allows you to equate A=1, B=2, etc. Very simple.

Having test data from 4 different manufacturers is a great benefit because (1) the tube coverage is extensive, and (2) you get alternative test settings for the same tube. Yes, that is correct — even though the test circuit is identical, the tube setup data is not identical among each manufacturer, and different data gives different test scores. In fact, it is often quite different. Hence, each company’s engineers apparently created their own data charts by themselves, although the Triplett and Heathkit charts track close.

Witness the different configurations for a 6L6:

• from Knight chart = Circuit 4, Load 36, Up 3-4, Down 2
• from Eico 625 chart = Circuit 3, Load 24, Up 3-4-5, Down 2,8
• from Heathkit TC2 chart = Circuit 3, Load 27, Up C-D-E, Down G-H
• from Triplett 3413A chart = Circuit 3, Load 27, Up 3-4-5, Down 7-8 (same as Heathkit TC2)

In this 6L6 example, in my opinion the Eico chart data gives the most accurate condition of the 6L6 being tested. There is approx 15% test score difference between highest and lowest.

Here is another example, 6N6 (showing only first line of chart):

• from Knight chart = Circuit 2, Load 68, Up 4-5, Down 2
• from Eico 625 chart = Circuit 3, Load 67, Up 4-5, Down 2,3,8
• from Heathkit TC2 chart = Circuit 3, Load 61, Up D-E, Down C,G,H
• from Triplett 3413A chart = Circuit 3, Load 61, Up 4-5, Down 3-7-8 (same as Heathkit TC2)

In this 6N6 example, in my opinion the Heathkit test data gives the most accurate condition of the 6N6 being tested. There is a solid 20% test score difference in this example, so the differences are not trivial. I did choose these examples because they illustrate extremes, so keep that in mind.

These examples highlight that tube testing is not an exact science, much to the chagrin of novice users who want to think that “there can only be one test score.” In fact, companies did disagree as to how to test tubes — even when using the exact-same test circuit. When you buy any of these four tube tester series (Eico 625, Triplett 3413, Heathkit TC, Knight 600), you get the benefit of a “second opinion” by simply using one of the other company’s setup chart.

Conclusion

The Knight 600 is a good choice for a variety of reasons. I notice that many tube vendors on the internet use Knight/Heathkit/Eico 625 tube testers. The circuit is easy-to-use, easy-to-maintain, easy-to-repair, and easy-to-operate. This Emission test circuit was popular enough to be used by four different manufacturers. While an Emission circuit is much less sophisticated than a Mutual Conductance test method, Emission does a good job at determining whether a tube is likely good or bad, and “good or bad” is what most end-users want to know. For people with more sophisticated needs, there are certainly more complex (and much more expensive) mutual conductance tube testers available. For everyone else, this combination of simplicity, reliability and price are among the many reasons why these testers were popular among radio/tv technicians in the “old days”, and remain popular in use today.

regards,

Bob Putnak.

eBay ID = rjputnak

Additional Photos

(Photo of the case, original manual, original supplement chart.)

(Testing a Cunningham CX-350 Globe #50 Triode tube.)

(Testing 5U4GB rectifier)

(Testing weak 6N6 tube, using test data from the Heathkit TC2 chart)