Precision Apparatus 10-series tube tester

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.

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.

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.

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.

Paco G-140 Socket Adapter

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.


But if you think model 10-12 is large, then you ain’t seen nothing yet.  The Precision model 10-15 is a large countertop version of 10-12. Model 10-15 is operated standing up, and it makes for a quite impressive spectacle.  Approx dimensions are 26-inches tall, 18-inches wide, 11-inches deep.  The meter is huge and could easily be read across the room.  The meter is 11.5 inches diameter, with 9-inch window of visibility.  The photo to your left compares the size of model 10-15 vs model 10-12.  Of course, model 10-15 is meant to operated upright, whereas model 10-12 is propped upright only for size comparison. Model 10-15 also has an incandescent light mounted on the top of the unit, shining downward upon the meter and controls.  Electrically, both models are identical, and share the same manual and schematic.


remastered paperwork package for Precision 10-12 / 10-15, which consists of manual, schematic, theory of Electronamic tube testing, and two carefully selected supplement charts, all for $9.99 free ship USA.


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

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.

Control “F” (model 10-40) selects proper supply voltages for Gas Test or VR test.

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.


The Line Control pot is frequently defective and must be replaced.  We sell a replacement control with proper specifications for $39.99 free ship inside USA.  Pot is new and good quality.

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, or replace with a diode and appropriate series resistor to allow Line calibration.  The 5Y3 is only used as a half-wave rectifier to indicate the “Line” (Line voltage).  It serves no function during a tube’s merit test.


Need a pair of new socket savers to protect your tube tester sockets?
Buy a pair of new manufacture socket savers: 1 each SS-8 (octal) and SS-9 (9-pin-miniature).
$24.99 free ship USA.


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

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 (R11 for model 10-40) 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 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.
Grid voltage (C at 24)
  • 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

Ultimately, I use my set of calibration tubes as a final determination.

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.

The easiest way to increase the sensitivity of your shorts-leakage test is to leave the switch in the COND position.  This will double the sensitivity.

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.

GAS Test (model 10-40)

R18 is the Gas Test calibration control for model 10-40.  It is a cheap can-style pot, which tends to drift and have poor repeatability, and therefore I would suggest replacing with a new wirewound pot.  My method of calibrating this circuit is to setup controls for a 6L6 test, place a variable series resistance between pins 3 & 5, place a digital DC microammeter in series with the variable resistance, adjust the variable resistance so that 5µA of current flows, then I adjust R18 so that the panel meter accurately reads 5µA while “GAS TEST” button is depressed.  If you have a different calibration suggestion, let me know.

Voltage Regulator Tube Testing

Model 10-40 incorporates excellent VR tube testing capability.  DC voltage is supplied by 155vac secondary transformer winding, with actual AC voltage of this winding being varied by the user via the Line potentiometer and is filtered by a 100µf @ 250V electrolytic capacitor, and uses R31 (1500 ohms, approx 5W, cement) as the series resistor in the VR circuit.  When rebuilding, use a 450v capacitor in this circuit.  Also, it may be wise to install two new diodes 1N4007 (in series) to replace the original diode(s) in this circuit.

I do not have the model 10-40 VR tube setup data chart, so I created my own.  The setting of the “B” control is not important as long as you choose a setting that is higher than the number of pins of the VR tube that you are testing.  Hence, I chose position 12 as a safe choice.

Lever “V” selects the Plate (anode) pin of the VR tube.  For 0A2, 0B2, and 0C2, both pins 1 & 5 connect to the anode.  If you feel a need to check each pin 1 & 5 individually due to a suspected open circuit, then setup lever 1 to “Y” and lever 5 to “W” (open circuit) — check tube — then setup lever 5 to “Y” and lever 1 to “W” and recheck.

NOTE: for 0A2, 0B2, 0C2 — the base diagrams for these types indicate that pins 3 and 6 could have an internal connection to one of the elements.  I have not found this to be the situation with several brands of tubes that I visually inspected, but if you find a heavy load (a short) on your transformer, move levers 3 and 6 to the “W” position during testing of these types.

I believe that these settings on my chart will work good, but you are welcome to email me if you find a mistake.  My chart also contains information regarding voltage spread for the “F” control (no-load condition); this will give you an idea what range to select.  Keep in mind that part of testing a VR tube is to check its starting voltage, which is always higher than its VR drop voltage.  You will find that when testing VR tubes with model 10-40, you may need to utilize multiple positions of the “F” control in order to completely cover the entire operating range of the VR tube.  Therefore, my chart gives a suggested initial setting of the “F” control.

One tip — installing a plate current meter really speeds up VR tube testing because the “A” control does not need to be switched to “VR current” in order to see the VR tube operating current.

Schematic quirks

  • Model 10-40 — schematic dated Aug 6, 1958: R12 should be 128,500 ohms.  R6 is listed as 1200 ohms, but factory installed resistor was 1500 ohms.  This schematic also shows two diodes in series that supply DC for the VR test, part numbers X2 and X3.  Actual tester examined only had a single original diode, not two in series.  That said, I do believe that installing a pair of 1N4007 diodes in series would be a good idea.


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.

  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.  If your line pot is defective, we sell a high quality new replacement.  See above.
  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.


Bob Putnak

eBay ID = rjputnak