Instructions for use of our calibration set, for particular tube testers.

These are just some notes, of course it can not replace the factory calibration manual. The operating manual is not the calibration manual.

AVO CT160 only.

Note, the internal functioning and also operation method of CT160 is different from all other ones. Calibration procedure is the same, only for CT160 there is one particular precision resistor inside, which is often off-value. So recalibrating this one is part of the procedure, and you must do that first. However if you find that resistor is off value, a full calibration MUST be done. Reason is, it must be brought back to the original value, but once you do you loose the original calibration settings, if someone calibrated the tester at a moment where that resistor was already a bit unprecise. It means if you decide this resistor, you must calibrate the tester afterwards. Link to official calibration procedure. Sorry only available as djvu file, but you do need it. Load viewer here

With old testers. off value after all those years. This resistor is factory selected and made of two resistors in series. Check the restoration report, linked at the main tube testers page, to see how that looks like. The Mk2...4 don't have that resistor, and they don't need that extra step. However it is must be said you will not get your CT160 every precise if that resistor is off value.

AVO CT160, Mk2, Mk3, Mk4

The actual plate current through the tube is a very strange looking half wave, non very nice sinus signal, looking as if there is "distortion" on it. It is proportional to the plate current, BUT with a totally different RMS value, and besides the panel meter is something like an average type of meter, and a strange one, since it has quite low impedance and loads the circuit of the tester considerably. However that again is compensated with factory selected resistors inside. So trying to work around with a modern multimeter is absolutely impossible. The only meter you can use is the actual front panel meter. The official AVO recommended way to go is, you put in a tube that you KNOW FOR SURE what current it draws under specific settings. For instance 52mA under the given plate voltage, screen voltage and gird voltage. Then all you do is, set the tester for 52mA, and so .. a 52mA tube indicates now 52mA, and that is just what you wanted. Since it is proportional, a 40mA tube will now indicate 40mA also, etc.

Steps to go when using the JACMUSIC CALIBRATION SET:

IF ONE OF THE STEPS FAIL, STOP THERE, AND CLEARIFY FIRST.

1) For CT160 see special notes above

2) Calibrate the mains mark. Then so far with the uncalibrated tester, set: Anode=250V, Screen=250V, G1 = -14V and test the 6L6. Values may be off 30% or so, but when you have crazy results, you should take out the delicate calibration tube, and use a cheap experimental 6L6 until the tester is more or less ok, but just not precise.

3) Now, put in the calibrated 6L6 and let it draw plate current. Check if the filament voltage is 6.3 Volts. If not, possible causes are, calibration mark is done wrong, transformer is bad, roller switch is dirty, has contact voltage drop. (must be taken apart in that case, contact spray will not help much)

4) Let the tester run for one week without tube on it. That will normally reduce any transformer leakage, and there always is after 60 years.

5) Take the JACMUSIC "leakage and shorts" test tool, which belongs to the JACMUSIC CALIBRATION SET and perform alls tests with it. Normally that will work ok, but if the are any errors at this point, like TOO MUCH leakage, you may have to clean the roller switch (must be taken apart) Find a way to make sure all wheels go in the position the where originally. Sometimes you find a filling ring between two roller wheels. Put in THAT ring at THAT position. Best is to lay all wheels on your table, in the order you took it apart. The roller wheels work very simple but it's pain to put it back together if you mix up all wheels. Don't try to clean the lettering with any fluids, it may will wipe off. If you see NOT ENOUGH leakage the circuit may have an error, not the roller switch. Also occasionally it can be one of the tube sockets.

6) Basic preparation has been done and you can calibrate the tester by the book, which means fist do Ia and then Transconductance. Use the values as you will see in the 6L6 calibration sheet in line 1, method B. This is for all AVO's

When you are finished with the calibration procedure you can take the 6922 and the 6SN7 of the calibration set, and see if your work was successful. The values of those tubes is exactly known, and you AVO must indicate it within it's specified accuracy. Also here take the values as listed under Method B, these apply for the AVO.

Hickock Testers

With most hickoks, you have to press the "test" button for a test, and if you let go, the test stops. You have to be aware that this does not heat the plates of the tube significantly. So only the filaments are warm, but not the plates. The higher models have a "lock" option, so the tube can heat up under test. That means it has a large enough transformer or even more than on. The locked method, is Method B on our calibration data sheet, and the "push to test" method is method "A".

The Hickoks use a patented circuit, which working principle can be easily understood .However in detail it is very hard to understand exactly. When you open the tester, all you see is wire spaghetti, and the circuit diagram has so many switched in it, and they draw the diagram without a clear way what is circuit ground. So my advise is not waist time on trying to understand what each wire is for, but rather follow the calibration instructions step by step. When calibrating those, you need to use a calibration 6L6 tube, that Hickok used to sell. Then what is ends up with, you have to set the tester to read this tube exactly as it says on the tube. So if that particular tube has 5700 mMho, the tester must indicate that. However, even if you have an original calibrated tube, it is not of great use since you don't know it's background, and in 50 years no tube will keep it's values. So any historical calibration tube is probably useless.

I have asked some of the real Hickok doctors, but nobody has the factory procedure. Mostly they have a Hickock that they believe is in very good condition, and test a tube with it, and write the value on that tube. For such a tube, but it only from a (real) specialist, because all people with a "reasonable" working Hickok seem to think it is a "great" one.

Another way to specify this 6L6 is not think about "Hickok" but think about RCA. Take their data sheet, do their test with standard settings, and that is the value for that 6L6. And honestly I see no reason why a Hickok should show another value.

To be versatile, and to help you do the calibration the way you prefer, the method with a known to be good Hickok is "Method A" from our data sheet That is done as "push to test". So with only the filament warmed up. Method B is by the RCA data sheet, and with the tube heated up. It is also the value for the "locked option".

With the Hickoks, they're good testers, but they're not instruments, even though they tried to make customers think so. Let me say it again here, the transconductance of a tube greatly depends on the actual settings. So how can you test a tube WITHOUT telling the settings and then say transconductance is "5700". So don't try to put more accuracy in it as intended for the Model you have. Generally you can say the more weight the have, the better they are.

Generally you do good to check systematically everything. With almost most testers there is something that needs to be replaced, or cleaned, or whatever. It is not a good idea to calibrate around that, leave the bad parts in ( the ones that caused the wrong reading) just make the needle show the correct value again. That will work then only for that particular setting, all other settings and other tubes may read wrong. To make sure this is not the case, why supply also a calibrated NOS 6SN7 and a calibrated NOS 6922.

There is a check list in the official calibration manuals. (Very hard to get manuals, specialists are hiding those like grandmother hiding the cookies). Here is a good place though. Go systematically, that means you do not try to find the bad parts be being clever. It means you begin at the beginning of a list, and you check each and every point.

At least you need to following procedure, but this can not replace the calibration manual.

  1. Check if the wiring to the mains voltage series resistor is sloppy and/or almost broken off. If it does break off, the mains wire falls on the deck plate, which is by default not grounded.... so full mains voltage can come on the deck plate. I had that happening once. Everything survived, including myself.
  2. Remove the mains cable, and mount a GROUNDED CABLE instead. From that cable mains, ground the DECK PLATE and ground the tester chassis with two separate ground wire. If you want to trust something, then trust what I am saying here. Do not trust 50 years old isolation. Do not use this metal ring to keep the mains cable in place. Throw it away, and use a cable binder instead.
  3. Oil the series resistor with contact spray, also give the axis some. You won't believe how nice it runs now.
  4. Check the mains voltage calibration mark and set it correctly. Without that you can just forget about the rest! If the mark is at the right place, you have a specific voltage at a specific point of the circuit. It is in the calilbration manual which point that is. If the voltage is wrong, t, you can have bad rectifier diodes for the meter, of generally the meter itself needs to be set better. Mind you if the meter magnetism has lowered after 50 years, the calibration mark will make you set too high voltage to the whole meter. This will of course correct also the lower meter readings in general, but it's a stupid error. You can never calibrate it well like this. There is a magnetic shunt on the meter outside, or some have a series resistor inside, or other methods. The meter is specified for a full scale DC current. You can check that with a multimeter in series. Often the mark is ok, but not always, and people before you have tried to calibrate arund this, but never got the tester really good of course.
  5. Double check the mains calibration mark now, with a 6L6 tube. The filament voltage should be somewhere around 6.3V with the tube filament hot.
  6. Here is a point that is not in the manuals, but they probably never thought the testers would last so long. Leakage and bad isolation is an issue after 50 years. Check with the leakage adapter if the tester displays leakage correct. Then do the shorts test with the same adapter.
  7. Calibrate the leakage indicator if you have to. Use the leakage adapter for that. Now remove the leakage adapter, and the Hickok may not indicate leakage at any position of the short & leakage test. Try that for several different tube settings. If it does indicate leakage now, you have dirty tube sockets. Often the Noval or miniature sockets. You MUST fix this. It is one of the main functions of the leakage adapter
  8. Check all voltages on the transformers. Boring work, but may show a bad winding. Either an open winding (probably bad solder joint) or a shorted or partially shorted winding. If so, check if it is an important winding (who needs 127Volts heater tubes?) if is yes.... either get another transformer, or just add an additional transformer with just that voltage. DON'T skip this part!
  9. Check for bad looking resistors. The BIG size resistors are power resistors, and if carbon types, they may be 30% or more too high (sometimes). Many have functions in the circuit where that doesn't matter. Some do.
  10. Check if someone soldered on it. Try to find out WHY. Perhaps that tells you something.
  11. Check of the bias pot is not having a bad contact. Clean it with contact oil
  12. Spray lots of contact oil inside the English pot.
  13. Test a 6L6. If the tester is basically working now, remove the 6L6 and let the Hickok run for some days. That is very good for the transformer
  14. And now.... do the calibration by the book.