About tubes: What is a reference/calibration tube? ... A special tube? ... No, simply a NOS tube issued by a manufacturer that has the reputation of maintaining its production within specs. Any tube that matches the design specification is a calibration tube, in fact, any NOS tube from any reputable European or American manufacturer serves as a calibration tube, the only condition is the chosen tube must have low to medium transconductance and don't be a miniature type in order to obtain consistent results, that is: an octal tube is better because the electrode spacing is not as critical as in a miniature tube.

There is nothing special inside a tube nor any magic, the only requirement is a good manufacture within specs, and these specs should fit well into a particular circuit within dynamic conditions, in fact, a same type of NOS octal tubes from several reputable manufacturers offer similar results. Have been said that a TV-7 is not a laboratory-grade instrument, when tubes are compared, the difference of 1 scale division (2 units, each scale division equals 2 units) from one tube to another tube is NOT significative, and if we compare readings on different TV-7s the differences could reach more scale divisions, this depends on several factors like inequality or different tolerances in bias potentiometers, in meters, and in LINE ADJUST circuit. To raise paradoxes about testing tubes: the best results are obtained when the tube is a bit used. If a tube gives a noticeable "better than specs" result, a problem with the tube tester should be feared or... a "hot" tube is being tested (e.g. a gassy tube). A "hot" or "high-rated" tube could NOT be always better in-circuit, as this "excess of gain" may cause unstability on signal peaks, and a unstable tube is worst than a tired tube. Unstability can fry an output transformer operated by a push-pull of EL34 at 350 volts if the circuit starts oscillations on signal peaks, stability always is best than performance.

My calibration tubes are two 6L6 from RCA. Also I have several 6L6 from european manufacturers for comparison purposes. All NOS 6L6 I have are within the 40 - 44 range (2 scale divisions, 4 units); conclusion: a 6L6 is an excellent tube to be used as a calibration tube. The 6L6 is used officially in the TV-7 Field and Depot Maintenance manual to adjust range C, a standard NOS 6L6 has 6000 µ℧ of transconductance (6 mA/V, = the official transconductance value at design and manufacturing parameters, not a transconductance value produced with different parameters, which normally occurs in a tube tester), in practice any 6L6 variant can be used as the differences between variants usually are not large (see demo below).

THE RCA 6L6 6,000 µmho, JAN WWII, NOS.	A 1976 6L6GB from the heap, NOS (WG dist, EG mfg).
BIG differences in time, mfg, and (in theory) quality, but operational differences between them are MINImal (1 scale division).

There is the option to buy a "calibrated tube", and even a "calibration tube set", which are simply tubes that have been tested operating a "perfect TV-7" and labeled with the resulting meter division number. Buying a set, a tube is provided for each range, which is unnecessary since all the scales are interrelated: With the base of one they are all adjusted, and by retouching one they are all retouched, as demonstrates the "Simulated Tube Test" in TM 11-6625-274-35, page 41. This option has some risk since it is doubtful that the "tube calibrator" is calibrated OK and, specially, well operated, since the particular TV-7 used to tare the calibration tube must avoid LINE ADJUST inaccuracies and, very important, BIAS potentiometer tolerances, better trust the manufacturer's calibration. The only practical use of having a tube for each range is for verification purposes; a verification kit may be made by labeling unused tubes that test at each range, no matter what model they are. Remembering again that tube testers are not lab gear, tube testers have little to do with the devices that manufacturers used to define characteristics, thus it is not necessary take things to extremes that are beyond of what the instrument really is, also, tube electronics have the characteristic of having much wider internal operating margins than solid state electronics, it should not be forgotten that tube electronics usually operate with high impedances, consequently, component values can have more tolerance margin without the results being affected.

The use of a calibrator tube avoids the variac and isolation transformer procedure, that gives an output of 50 VAC in series with a 10 Kohm resistor: this network introduces an AC current of 5 mA (0.005 A = 50 VOLTS / 10,000 OHMS) in the plate to cathode circuit with the intention of obtaining 40 units on ranges B, D, E, F, and 20 on C. Within the NOS 6L6 reference basis, the TV-7 will remain not mathematically adjusted but within average specifications.

Then, once voltages are checked as correct, the next goal is the sensitivity adjustment based on good and known data, that is: the set must be adjusted in order to match the reference tube, and the difference will be noted on testing an used tube.
 

Unlike as described in the Field and Depot Maintenance manual, all the procedures shown here and in the next paragraphs are based on the 6L6 tube settings. NO special adjusting gear, nor "adjusted" calibration tubes required!!! Set controls: BIAS to 23, SHUNT to 0. Set selectors: FILAMENT VOLTAGE to 6.3, pin switches to HS5-3481, RANGES to D.

 

RANGES [ D - E - F - B - C ] Gm adjust:> These ranges are used for testing signal and amplifier tubes. Continue with the 6L6 selectors configuration HS5-3481, set FILAMENT VOLTAGE to 6.3 and FUNCTION SWITCH to RANGES D, BIAS to 23, SHUNT to 0.

If all tests related in the first test page were reasonably accurate, it is possible the set was already adjusted, check it out: Insert a NOS 6L6 selected as a calibration tube in the octal test socket, POWER ON, allow the tube to warm up for at least 5 minutes. Depress pushbutton 1 - LINE ADJ and set LINE ADJUST to the LINE TEST indication on the meter. Depress pushbutton 3 - MUT COND, the meter pointer should indicate 42 ±1 scale divisions. If OK check ranges E, F, B, C as stated below. If not OK, POWER OFF, pull out the 6L6, and continue.

• Set both R113 and R115 adjustable resistors to a reading value of ± 40 ohms in the multimeter, it can be checked directly in-circuit, this reading set the resistors in mid shaft-position and leaves it ready for a possible subsequent adjustment (the upper margin is 43 ohms). To do this loosen the clamping nuts. Like in the line test circuit, it is more important to check if both are matched values as this ensures balance at zero.
• Insert the 6L6 calibration tube in the octal test socket. POWER ON, allow the tube to warm up for at least 5 minutes.
• Depress pushbutton 1 - LINE ADJ and vary the LINE ADJUST control until the meter pointer indicates LINE TEST.
• Depress pushbutton 3 - MUT COND, the meter pointer should indicate 42 ±1 scale divisions.
• If the meter pointer indicates more than 44 adjust both R113 and R115 to a same LOWER value until the correct indication is obtained. If the meter pointer indicates less than 40 adjust both R113 and R115 to a same HIGHER value until the correct indication is obtained. Setting both network of resistors to its maximum matched ohmic value will increase the meter indication, otherwise the indication will decrease. The Gm meter bridge show this: pay attention that R115 and R113 are adjustable resistors that corrects the value of R137 and R136 respectively. As been said, setting both to an increased/decreased value affects the meter indication, similary, raising and lowering each other independently will affect also the meter indication but in this case balance at zero is lost.
  If R115(R137) < R113(R136) the meter pointer will be positioned forward (positive), otherwise backwards, but in order to make changes in sensivity maintaining balance at zero it is mandatory take into account that both interacts at the same time. If these two group of resistors are unbalanced, bizarre readings will be obtained. Those resistors are critical, unbalancing 3 ohms the network R115(R137) = 35.6 and R113(R136) = 38.6, will produce surprising results on testing 6L6 and 6146: 6L6 will offer a "correct" reading of 50, but 6146 goes directly to 90, when overall readings for NOS tubes are about 42 and 60 respectively. In order to check electrically the Gm meter bridge balance, remove tube and connect a 10,000 ohm 2 watt resistor between pins 3 to 8 of the OCTAL test socket (plate to cathode, remember count counter-clockwise on top). Depress pushbutton 3 - MUT COND, the meter pointer should indicate 0 ±½ scale divisions, otherwise the Gm bridge is unbalanced, that is R115(R137) not = R113(R136). Remove resistor, tighten the clamping nuts, insert the 6L6 tube again.
• Set FUNCTION SWITCH to position E, depress pushbutton 3 - MUT COND, the meter pointer should indicate ±22.
• Set FUNCTION SWITCH to position F, depress pushbutton 3 - MUT COND, the meter pointer should indicate ±28.
• Set FUNCTION SWITCH to position B, turn BIAS to 100. Depress pushbutton 3 - MUT COND and adjust the BIAS control counterclockwise until the meter pointer indicates 120.
• Set FUNCTION SWITCH to position C. Depress pushbutton 3 - MUT COND. The meter pointer should indicate 60 ±½. Otherwise adjust R114 until the correct indication is obtained.
• Set again BIAS to 23. Depress pushbutton 3 - MUT COND. The meter pointer should indicate 120 ±2.

SHUNT control test and RANGE A-shunt position: The SHUNT / RANGE A combination is used for testing rectifier tubes and diode sections.

• Pull out the 6L6, continue with the selectors configuration HS5-3481 and set the FUNCTION SWITCH to A-shunt.
• Depress pushbutton 1 - LINE ADJ and vary the LINE ADJUST control until the meter pointer rests over LINE TEST.
• Set the SHUNT control to 90.
• Connect a normalized 5,600 ohm 6 watt resistor between pins 3 to 8 of the OCTAL test socket (plate to cathode, count counter-clockwise on panel). Manual says two of 12,000 ohms in parallel, that is 6,000 ohms, but don't worry, it's not critical.
• Depress and hold pushbutton 3 - MUT COND. The test set meter pointer should indicate 0 divisions ±2. This is a balanced point: placing SHUNT slightly above/below to 90 the needle deflects upwards/downwards.

 

About meter readings and transconductance measurement.

The true transconductance value for each tube is the design Gm value, not a transconductance value under arbitrary test conditions. This value is clearly specified in tube manuals. When testing tubes with tube testers that have transconductance values on the meter, a NOS tube should give, approximately, the official Gm value specified in the manual for that tube. When transconductance tube testers do not give readings related to the transconductance value stated in the manual, a Gm reading on a meter marked with transconductance values has the same relative (not equivalent) value as a numerical reading on a scoring meter (such as the TV-7 meter). In this case, the transconductance value indicated on the meter shows a transconductance value, but not the transconductance value related to the official design-related transconductance value. We can only compare measures related to some measurement when there is an official definition of the value of the unit of measurement, but it is virtually impossible to design a transconductance tube tester with the necessary circuitry (mainly B+ voltages and A~ amperages) to match the official Gm value for all tube-types, but there are equivalences that might indicate that this was taken into account for some tubes.

The transconductance value stated on tube handbooks always refers to class A operation within average voltages (except very special tubes). The TV-7 has an unique plate voltage, then the result only can match with the micromho value in the tube manual if the rest of parameters were adapted to this voltage. This is not easy to apply, so TV-7 readings (and also the tube testers that have meters marked with micromho values) will not offer equivalency with the manufacturer's transconductance value indicated in the tube manuals, although there may be incidental similarities.

As an example the ECC82 (12AU7) tube, a lot of them tests between 90 to 100 on range B, this means an interval of 2,250 to 2,500 micromhos. The tube manual says: 2,200 micromho, roughly matched on the high side. This is a LOW transconductance and LOW amplification tube with relatively low internal resistance, this means it can accept load and give output power, so usually it is a driver tube. Within class A specs the input signal must be always below 8.5 volts, so no problem when this tube is being tested with 5 VAC input.

Similar example, the well known small signal ECC83 (12AX7) tube. This tube spreads on a wide range, many of them tests between 42 to 60 on range B, that means an interval of 1,050 to 1,500 micromhos. The tube manual says: 1,600 micromho, partially matched on the low side. This is a LOW transconductance and HIGH amplification tube (*) that has relatively high internal resistance, this means it can't accept low resistance load, so it can't give output power. This tube is used to transform low levels of input signal to high levels of output signal onto a high impedance load, so usually it is a preamp tube. Within class A specs the input signal must be always below 2 volts, this tube tests on range B, this range gives a test signal of 5 VAC on the grid which will cause strong saturation over part of the AC test cycle. This explains the wide range of meter indication when this tube is being tested; depending on the level of use, each tube will react differently when generating a highly distorted signal.

A clear example of lack of reciprocity (along with some "peculiarities") is the EL84 (6BQ5) tube, a high transconductance (high-slope) tube of 1953. It was designed by Mullard-Philips as a succesor of the Rimlock type EL41 with the initial intention of removing the preamp tube of the 5-tube receivers amplifying this way the signal directly from the detector, but that was only partially achieved, and quickly was noted the EL84 did offer a very good performance combined with a high gain on audio applications (and also on other applications). To accomplish this, it is necessary that a small signal be strongly amplified, so a high transconductance is needed, and this means benefits... and problems. The 1962 test data list indicates it must be tested on range C, but this range only can reach 6,000 micromhos of transconductance when EL84 has 11,300 micromhos in class A operation at the average plate voltage of 250 volts. EL84 tests are often between 72 to 90 on range C, this corresponds to an interval of 3,600 to 4,500 micromhos, but it does not matter, if the maximum limit of range C is 6,000 micromhos, and a NOS EL84 by mfg design has 11,300 micromhos... then... It is impossible any equivalence here!!!

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(*) This is because it has high plate resistance. Don't confuse transconductance with amplification: In both calculations the dividend is the same (grid AC voltage variation), but on transconductance (Gm, S) the variation of plate current is used as divider, on amplification (factor: Mu, µ) the variation of plate voltage is used as divider.

Plate resistance, amplification factor, and transconductance, are dynamic factors that relate the internal characteristics of the tubes to dynamic variations during operation. These concepts may be confusing because they are interrelated.

• Plate resistance implies application of Ohm's law for each variation of volts and amps between plate and cathode.
  Ohms = variation of Volts in plate / variation of Amperes between plate and cathode (not internal tube resistance).
• Amplification factor compares the efficiency of the variations of the grid voltage on plate voltage, thus varying tube current. Mu = variation of Volts in plate / variation of Volts in grid.
• Transconductance combines amplification factor with plate resistance and represents the influence of grid variations on plate current variations. Gm = variation of Amperes in plate / variation of Volts in grid.

An example is the best way to understand it. We have a tube in which the grid voltage varies from -4 volts to -2 volts, (2 volts less negative, thus the circuit is processing a positive signal cycle). In doing so, we see that plate amperage increases 5 mA, and plate voltage drops 40 volts.

• As per Ohm's law, plate resistance is = Volts / Amperes, then 40 volts / 0.005 amperes = 8,000 ohms of variation.
• Amplification (deamplification in this cycle) factor is = variation of plate voltage / variation of grid voltage, then 40 volts / 2 volts = 20 (note that amplification factor is the relationship between two voltages expressed equally, thus a factor).
• Transconductance is = variation of plate amperage / variation of grid voltage, then 5 mA / 2 volts = 2.5 mA/V in "european language", and 5,000 µA / 2 volts = 2,500 µmhos in "american language".

~ Understanding the transconductance test ~ - This circuit is based in the Hickok patent of 1948 and developed later in the Weiss & Wise patent of 1955 - Basic transconductance test circuit. a - Static bias and voltages produces static internal resistance in the tube tested: the mAm don't accuse it. b - The addition of an AC voltage in series to the static bias causes it becomes less negative in the positive cicle of the AC signal, and more negative in the negative cicle of the AC signal. c - This varies the internal resistance of the tube tested and unbalances the circuit. The meter will indicate the internal variation of this tube, which will be determined by its own characteristics and condition. d - In the TV-7 the AC modulation signal is provided from a source of 5 VAC, but the effective signal applied to tube tested varies depending on BIAS position and RANGE selection by means of the switching of R120 or R120 + R121.