This article discusses repair and my calibration procedure of the Precision model 10-12  tube tester.   Serious electrical voltages are present, repairs should only be attempted by a qualified technician. Copyrighted by Bob Putnak, all rights reserved.

Introduction

6 Paco 10-series tube testers from my collection

The Precision Apparatus Company (commonly known as PACO) manufactured some very high quality test equipment.  Among their offerings are the “10-series” of tube testers, such as model 10-12, 10-15, 10-40, 10-54.  The tube test method is the same in all, and the test data is interchangeable.

I have been collecting since 1990, and in my experience model 10-12 was the most popular unit.  I have found many 10-12 over the past 19 years, and serviced many more.    I have only found one different model for my own collection, a model 10-40 that you see in the photo.

Precision Apparatus 10-12 tube tester

It is easy to see why model 10-12 was most popular.  Four adjectives come to mind: Attractive, durable, consistent, quality.  Attractive — it has a beautiful furniture-grade hardwood case.  Durable — the entire unit is built-like-a-tank.  Consistent — I have always found these units to produce repeatable test results, year-after-year.  Quality — consistent test results from the “Electronamic” test method add up to a quality piece of test equipment.



Paco A-15 socket adapter

Paco G-140 Socket Adapter

The 10-series is also popular today because it will test a very large variety of tubes.  Model 10-12 has built-in sockets for antique tubes 4-pin, 5-pin, 6-pin, 7-pin large, and acorn.  It also has sockets for octal, loctal, 7-pin miniature, and 9-pin miniature.  Socket adapters (models A-15 and G-140) were later available, which adds the ability to test 10-pin miniature, nuvistor (5 & 7 pin), novar, and compactron tubes.  Therefore, if you have the socket adapter panel, you can effectively cover the entire range of tubes from antique 4-pin through modern 12-pin compactron.

The tester also has a NOISE JACK for connecting a set of headphones to audibly evaluate “tube noise.”

Size: This is a relatively large tester.  It measures 18 x 14 x 7 (inches) and weighs 18-lbs.

Precision called the testing method “Electronamic”, and they argued that is “more than just Mutual Conductance.”  Precision wrote a two-page article explaining the Electronamic test method, which you can download [HERE].  While it would be heresy to argue that Electronamic is “more” than Hickok’s mutual conductance, I would suggest that the test method is extremely reliable.  The Electronamic method is similar to the Jackson 648 test method.  I find that the tube test results from the Precision 10-series usually track quite close to Hickok test results.  The Electronamic test method uses all AC voltages.

Selector Switch “A” provides selectable Plate Voltage, Load, and meter sensitivity combinations.

• Position #1 — Plate Load = 2000 ohms.  Voltage = 175 v.  Meter shunts approx 45.7 ohms (R14, and R15 + R16).  Total parallel resistance (including meter) approx 35.55 ohms.  FS meter 4.5 mA
• Position #2 — Plate Load = 295 ohms.  Voltage = 50 v.  Meter shunts approx 45.7 ohms (R14, and R15 + R16).  Total parallel resistance (including meter) approx 35.55 ohms.  FS meter 4.5 mA
• Position #3 — Plate Load = 5000 ohms.  Voltage = 50 v.  Meter shunts approx 320 ohms (R15 + R16).  Total parallel resistance (including meter) approx 106.667 ohms.  FS meter 1.5 mA
• Position #4 — Plate Load = 2000 ohms.  Voltage = 175 v.  Meter shunts approx 320 ohms (R15 + R16).  Total parallel resistance (including meter) approx 106.667 ohms.  FS meter 1.5 mA
• Position #5 — Plate Load = 2000 ohms.  Voltage = 50 v.  Meter shunts approx 320 ohms (R15 + R16).  Total parallel resistance (including meter) approx 106.667 ohms.  FS meter 1.5 mA
• Position #6 — Plate Load = 2000 ohms.  Voltage = 300 v.  Meter shunts approx 45.7 ohms (R14, and R15 + R16).  Total parallel resistance (including meter) approx 35.55 ohms.  FS meter 4.5 mA
• Position #7 — Plate Load = 3000 ohms.  Voltage = 300 v.  Meter shunts approx 45.7 ohms (R14, and R15 + R16).  Total parallel resistance (including meter) approx 35.55 ohms.  FS meter 4.5 mA

Paco 10-12 meter sensitivity (shunt resistors) for Positions 1,2,6,7.

Paco 10-12 meter sensitivity (shunt resistors) for Positions 3,4,5.

The meter shunts can be quickly verified by removing the meter from the circuit (easily done by removing the nuts that attach the ring connectors to the meter), then connect your ohmmeter to each ring connector.  Press READ METER button and record the shunt resistance at each Selector Switch “A” position 1 through 7.

If you are using a modern DMM, you can test the total resistance with the meter in-circuit.  Place your DMM (+ red) lead to the panel meter (+) terminal and your DMM (- black) lead onto the panel meter (-) terminal.  When the READ METER button is not depressed, you will read the meter’s internal resistance.  It should be 160 ohms for the circuit to work perfectly and calibrate perfectly.  I recently serviced a 10-12 in which the meter correctly read 1 ma FS but the movement resistance was only 148 ohms, therefore only 148 mv was required to have the meter read full scale.  Adding an appropriate resistor (in the instance, 12 ohms) in series with the panel meter corrects this problem.  When you press the READ METER button, you can then measure the total resistance at each switch positions 1 through 7.

NOTE: The tube Screen voltage is fixed at “50V” with a 500 ohm screen load resistor (R6).  The meter reads the tube’s plate current.  The 10-12 meter circuit is designed for 1 ma FS with 160 ohms resistance.

Selector Switch “B” is the filament return selector.

Control “C” provides variable grid voltage from 0 to 48.5.

Control “D” is a meter shunt.

Selector Switch “E” sets the filament voltage.

The lever selectors work as follows: W = open circuit, X = Screen, Y= Plate, Z= Grid.  The lever numbers correspond to the actual tube pins.  For example, to setup a 6L6: level 4 is moved to X, lever 3 is moved to Y, lever 5 is moved to Z.  This corresponds to the tube elements in a 6L6: Pin 3 = Plate, Pin 4 = Screen, Pin 5 = Grid.

Repair and Calibration

I will preface this information by explaining that I am not aware of any factory calibration procedure for these testers. Therefore, this may not be the same procedure that the factory used.  Having analyzed a large number of 10-12 units over the years, from my own collection and for repair, I can say that the following procedure provides consistent results across units, and was also consistent with units that worked well as they were found.  If any former Precision Apparatus employee has factory documentation regarding calibration or other information that he would like to share, please feel welcome to contact me.

As always, repair must start by covering the fundamentals.  Check that all knobs and the meter itself are indexed at zero.  Verify that the meter resistance is 160 ohms (add a series resistor if low resistance).   Test all resistors and potentiometers for accuracy and replace where necessary.  The load resistors must be accurate or else the test results will suffer.  Therefore, there is no substitute for checking every one of them. Replace the 0.1 mfd capacitor and two 0.002 mfd (aka 2000 pf) capacitors.

Diligently clean socket pins

Clean all sockets/switches/lever-switches/pots.   Inspect all wiring (AC power cord, and also each wire connection at every tube socket pin).  The power-on lamp is either a #40 (screw base) or #47 (bayonet base), depending on production run.  Clean lamp socket and lamp.  (Do not use any substitute bulbs.)

Check the 5Y3 tube on another tube tester.

Once you have thoroughly completed the fundamentals, you can proceed to the next stage of repair.  All voltage readings below are taken with a Fluke DMM and without shunts.

Line Calibration

PACO Line Calibration (300V measured when meter reads at Line)

PACO Line Calibration (300V measured when meter reads at Line)

Set controls: A=6, B=2, C=0, D=0, E=1.  Lever 3 = Y.  Lever 4=X.  Lever 5 = Z.  Insert octal test socket.

Connect an AC digital meter to pins 3 and 8 of the octal test socket.

While pressing READ METER button, rotate Line Adjustment knob until DMM reads 300 VAC (+/- 2 volts).  Release button, adjust R5 calibration pot until meter pointer is perfectly centered on ‘Line’ mark. (Note – be sure that the tester itself is operating at its natural tabletop level, or else the meter needle positioning will not be true.)

The two photos here are interpreted together.  When you press READ METER, your DMM should read 300 vac.  When you release READ METER button, the meter should return perfectly to Line mark.

Check Voltages

• Selector Switch “A” = 1.  Press READ METER.  Verify 177 vac (+/- 2 volts).
• Selector Switch “A” = 2.  Press READ METER.  Verify 48.5 vac (+/- 1 volt).
• Selector Switch “A” = 3.  Press READ METER.  Verify 48.5 vac (+/- 1 volt).
• Selector Switch “A” = 4.  Press READ METER.  Verify 177 vac (+/- 2 volts).
• Selector Switch “A” = 5.  Press READ METER.  Verify 48.5 vac (+/- 1 volt).
• Selector Switch “A” = 6.  Press READ METER.  Verify 300 vac (+/- 2 volts).
• Selector Switch “A” = 7.  Press READ METER.  Verify 300 vac (+/- 2 volts).
• Screen Voltage Test: Move the DMM prod from Pin 3 to Pin 4.  Verify 48.5 vac (+/- 1 volt).  This voltage is present regardless of whether you press READ METER button or not.
• 

  Grid voltage (C at 24)

  Grid Voltage Test: Move the DMM prod from Pin 3 to Pin 5.  As you rotate Control “C”, at “0″ you will find 0 vac, and at “50″ you will find 48.5 vac.  Set Control “C” to 24 and verify 11.2 vac.  This voltage is very accurate, as you see in this photo with two calibrated 10-12 testers side-by-side.  This voltage may be very slightly different based upon how accurate you set the 300vac line, and also of course how your eye sees the “Line” mark as centered (parallax error).    Grid voltage is present regardless of whether you press READ METER button or not.

• Filament Voltage Test: Move the DMM prods to pins 2 and 7 of the octal socket.  Rotate Selector Switch “E” to all positions 1-18, and verify closely accurate filament voltages at each position.  Rotate back to position #1 and disconnect voltmeter. Reset lever switches.

Plate Current Calibration

This procedure requires a digital DC milliammeter, a 1N4007 diode, and a highly-selectable resistor decade substitution box.  This must be the kind of substitution box that allows for precision resistance selection.  (A random example — if your substitution box would not allow you to select a resistance of exactly 1,668 ohms, then your box will not suffice for this job. )

Likewise, study this procedure repeatedly until you understand what is being accomplished BEFORE proceeding.  One wrong connection or mistake and you could ruin the tube tester meter.  Proceed at your own risk.

Set controls: A=4, B=2, C=0, D=0, E=1.  Lever 3 = W.  Lever 6 = Y. Set a high resistance (100K ohms) in your resistor decade box.

All connections will be made to the octal test socket.  Connect the cathode (striped) end of 1N4007 diode to pin 8.  Connect diode anode to one lead from your resistor decade box. Connect the remaining lead from your resistor decade box to Pin 3.  Also connect the (-) lead of your DC milliammeter to pin 3.  Connect the (+) lead of milliammeter to pin 6.

25ma calibration

The goal is for the tube tester meter to read exactly full scale with 25ma plate current.  (It should read exactly at full scale — not below or beyond full-scale).  Double-check that you set Selector Switch A to position #4, and that Control D is set to zero.  While pressing READ METER button, and keeping an eye on both thee tube tester meter and the milliammeter, gradually reduce the resistance until the DC milliammeter reads 25ma.  At 25ma, your tube tester meter should read exactly full scale.  Adjust R15 (40-ohm calibration control) so that tube tester meter reads exactly full scale.  (This adjustment is very fine, a hair turn can make all the difference.)  (Note – be sure that the tester itself is operating at its natural tabletop level, or else the meter needle positioning will not be true.)  Release test button, then re-test again to verify accuracy.  Release button.

1.5ma verification

Reconfigure a high resistance (100K ohms) in your resistor decade box.  Press READ METER button, and adjust substitution box until milliammeter reads 1.5 ma.  Slowly rotate Control “D” clockwise while you are watching tube tester meter.  Meter should read full scale (or within one division) when D=50.  Release button, power-off tester, remove octal test socket and all connections to it.  If unable to calibrate, check the panel meter resistance as mentioned above.


Shorts and Leakage

Shorts and leakage are tested by a series of push buttons (1 – 12) individually depressed, and sensitivity is based upon resistors R9 and R10 across the neon NE-57 lamp.  When the “Shorts Selectivity Switch” is in the “Tube” position, R9 and R10 are in parallel.

Resistors R9 and R10 are matching values, but they seem to vary among the production run.  The schematic shows 1.6 megohm each, but several of my units have factory installed 1.8 meg resistors, and one has a pair of 2.2 meg.

To test the default leakage sensitivity of your tester, insert the octal test socket and connect your resistor decade box between pins 5 and 8.  Set a high resistance on your sub-box (2 meg).  Power-on tester, press Shorts button #5 (or #8).  Decrease the resistance until you can comfortably see the neon lamp light.

Leakage sensitivity

With the default 1.6 meg resistor pairing (approx 800K because they are in parallel), the neon lamp is only comfortably illuminated with a leakage of approx 500K-600K or closer to full short.  So, if you want more sensitivity, increase the value of these resistors until your leakage sensitivity needs are met.  Obviously, the neon lamp glows brighter when you get closer to a full short condition.

I might suggest eliminating R9 and increasing the value of R10 to 1.8 Meg, which will comfortably detect leakage closer to the 1 meg sensitivity that is common in other tube testers. (R10 is the resistor that always shunts the neon lamp regardless of TUBE/COND switch position.)

Conclusion

Testing a platinum-matched-pair of Sovtek 5881-6L6WGC with two calibrated Paco 10-12 tube testers. (6L6 test configuration)

Finish your repair by testing a “6L6″ tube of known quality, and enjoy your calibrated Precision tube tester.

In this photo, I am testing a “platinum-matched-pair” of Sovtek 5881-6L6WGC, matching having been done by New Sensor’s “platinum matched” service.  As you see, both testers test identical.

Comment(s)

1. Do not expect the test results to read as far up-scale as you may be accustomed to with other tube testers.  “65″ seems to be the magic number that the Precision test data was trying to achieve.  A few examples: expect a strong NOS 12AX7 to test around 65, and a new sovtek 5881 in the 65 range also.  This comment is not (in any way) meant to suggest that tubes do not test beyond 65.  It is simply an observation that many good amp tubes fall in this range.
2. The line rheostat gets quite hot, so this is not a tester that you want to leave on when you are not using it.  When you are finished testing, turn it off.
3. These Precision 10-series are very high quality tube testers.  If you own one of these, and worry that you are missing-out by not having a Hickok, I would suggest that you are in good hands.

regards,

Bob Putnak

eBay ID = rjputnak