Internal impedance error.
The Mk4 has a small shortcoming by design, which is very difficult to cure. Perhaps you noticed for some high current tubes, the test values seem too low. This is not so for all tubes, but it is only so for low impedance tubes, if tested on the 100mA scale. Reason is, the plate current is derived from a voltage drop over a 200 Ohms resistor, which is in the test circuit. This is R36 in the circuit diagram. This resistor is placed into ANY current test you are doing. So it is used, starting from 1mA test for diodes, and so on, for ALL tests, apart for leakage. The error is very small, as long as you take not any low impedance tubes that are tested at high current. What happens? 200 Ohms is in the plate circuit, and there is a voltage drop over that resistor. This voltage drop will reduce the plate voltage you are effectively testing the tube with. You will see, in most cases that doesn't matter much. Like when testing an ECC83 with it's 1.2mA the voltage drop is only 240mV. Doing so at 250V there is no measurable error, because the tube plate impedance (Rp) is high and the tube won't react much to a change of 240mV.
Also it will give no effect when testing a pentode, as long as you use a plate voltage which is higher than the screen. Suppose you test a 6L6 at 350V plate and 250V screen, as by the book, you can try out the tube doesn't even change the reading very much when lower Ua from 350V to 300V. So the 54mA that is flowing at 350V will indeed cause a voltage drop of 10.8V, but this will not change the plate current much. You do get a problem when you test the tube with plate and screen both at 250V, because -without realizing perhaps- you are now testing the tube in triode mode. Now the 10.8V matter very much. This is why you will never see such a setting in the AVO book, though such a setting is perfectly legal, and no problem for the tube - the AVO can't do it.
This is really a shortcoming, and any information about this of course was kept silent by AVO. There are many work around's possible with well chosen settings. For some tubes, such as the 6080, the lower readings gets fully unacceptable, and such tubes are marked with a triangle sign in the data manual, meaning you can't test those.
Is there a solution? Yes there is. I have an electrical solution, which works by this formula. I think it is no business model to offer this modification for sale, but if you want to buy an AVO Mk4 from me, when I have one, I can build it in for you. It is a push button, which is effective on the 100mA scale (only this scale) and when you push it, the error is gone. This means no changes to internal guts. It just gets connected with four wires to the circuit, and when you solder those wires off, it is gone. So for a 6L6 pentode mode, you see almost no change, but for 2A3 or 300B, it can be 15mA more. Here is how it works:
These are the constants:
Ua = Plate Voltage on tube
Ra = 200Ohms = Series resistor between tube and power supply
Voltage drop over Ra
Ia = Plate Current
Rp = tube impedance
This the general formula for Rp:
Rp = (Ua2-Ua1) / (Ia2-Ia1).
We can also say: Ua2-Ua1 = Rp (Ia2-Ia1)
Whereas we generate this change: Ua2-Ua, by the Voltage drop over Ra.
This drop is Ia * 200
So: Ia * 200 = Rp (Ia2-Ia1)
whereas in this formula
Ia2 = Current the tube should have by the data sheet.
Ia1 = Too low Test result on the AVO Meter.
Now all we need to do is, rewrite this formula such that we get Ia2.
Ia2 = Ia1 + (Ia1*200) / Rp
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or express this as a correction factor
Suppose you have a 300B with 700 Ohms Rp and you measure only 45mA. That is close to replacing it! Though you are sure the tube is new, and it should be appr. 60mA.
With this formula you will find Ia2= 0.045 + (0.045*200) / 700 = 58mA
I am not sure the formula is 100% correct, but it works very well. I tried it out with 300B, it works good.
If the tube is really weak and Rp may be higher than normal, you can even test Rp before you begin.
Rp = (Ua2-Ua1) / (Ia2-Ia1). So switch between two plate voltages, and you can calculate Rp.
You can try for yourself now, why it doesn't matter for pentodes, since these have very high Rp. Except when you connect plate + screen to the same voltage, because that is triode mode, and that means low Rp. Try it out and you will see. Also you will see with the relatively high Rp of all tubes that draw little current, the error gets quite small.
IMPORTANT: Now comes the calibration... Since the tester is calibrated for zero error with a low current tube (at appr 10mA) the error in this range is zero for this tube you used tio calibrate the tester, and likely very low for any others too. Conclusion: The error will mainly appear at the 100mA scale, and only with low Rp Triodes. The 10mA scale is almost free of this error, the 25mA will show it only for very low Rpp tubes. You will understand I am a bit clearer about this, than AVO ever was, but there ar two or so hints in the original documents, which actually lead me on this trace.