Technical bulletin #3

RECOMMENDED PROCEDURE FOR GRADING AND MATCHING TUBES FOR USE IN MODULUS PREAMPLIFIERS.

IT IS EXPECTED THAT THIS WORK BE PERFORMED BY AN EXPERIENCED AUDIO TECHNICIAN. PLEASE READ CAREFULLY!

It is important to understand that all preamplifiers we currently produce are Class A-Triode, No-Feedback designs. In addition, only a minimum amount of active feedback is used in our phono stage preamp equalization networks to insure low frequency RIAA stability. Therefore, it is imperative that only low-noise, graded and matched tubes be used! This is particularly true for the line stage section.

For all Modulus Series 2 and Series 3 preamplifiers the tube positions on the Main PCB are as follows: V1 & V2, Right and Left Phono Stages and V3 & V4, Right and Left Line Stages. This information is also included at the rear of your owner’s manual.

For all L1 and L2 Line Stage preamplifiers the tube positions on the Main PCB are V1 & V2, Right and Left Stages. This information is also located at the rear of your owner’s manual.

Please understand these are high-performance preamplifiers. Only premium-imported type 6922, 6H23EB or E88CC tubes are recommended for use in these designs. These tubes should be purchased only from reliable sources or an authorized Audible Illusions Tube Dealer.

When purchasing tubes you must purchase "twice as many" raw tubes as you will need. We reject at least 30-40% at the factory and you should anticipate an even higher rejection rate.

Also, be cautious in purchasing NOS tubes. While some of these tubes have excellent sonics, they also have very high microphonics/ringing and distortion. They are unreliable when used in most modern preamplifiers, particularly in no-feedback line stage designs like the Modulus.

To insure reliability, these tubes must first be visually inspected and burned-in at full plate-filament voltage for a period of 12-24 hours before testing for low noise, microphonics and distortion.

For this test only "full-function" Modulus Series 2 or 3 Preamps should be used.

Properly testing each tube with this procedure will require roughly 30 minutes per tube.

The Following Information Is For Use By A Qualified Audio Technician

The Modulus preamp must be set on an open bench with shorting plugs inserted into the preamp’s RCA phono jacks and a DC Multimeter, set in the 0-300vdc range, with the multimeter’s negative test lead securely connected to the ground lug on the rear of the preamplifier’s chassis. It is also necessary that the AC line plug has a ground pin properly connected to the ground of the AC wall outlet. Otherwise an additional ground wire must be connected from the ground lug of the preamplifier’s chassis to an external earth ground.

The headphone amplifier or even a good tape recorder playback electronics can be used providing the output level can be set high enough that excess tube noise and microphonics/ringing can be heard through the headphones.

A calibrated/matched pair of tubes must be inserted in the preamplifier’s line stage. The Modulus preamp is then turned-on in the "stand-by" position (unit plugged in, filaments on) but the Off/On switch is OFF.

Before proceeding, one must first understand that these are "dual-triode" tubes. This means, that there are two-separate active elements inside each tube.

For Modulus Series 2 and Series 3 Phono Stages the DC Voltage Difference between each active stage can be no more that 10% or +/- 10vdc.

For the Line Stage in Modulus Series 2-3 and L1-L2 preamps the DC Voltage Difference between each active stage should be no more than 2-3% or +/-3-5vdc. This tight tolerance is required to insure a closechannel-gain match in the preamps no-feedback line stage.

Simple testing and grading procedure is as follows:

1- Each raw tube must be installed in the preamp, (four at a time), and burned-in for a minimum period of 12-24 hours. This burn-in period is necessary, as the highest percentage of problems with the tube will materialize during this period.

2- Once a batch of tubes has been burned-in, we suggest that the lower side of each tube be numbered #1 thru # (the total amount) using a black ink marker. A separate sheet of paper should also be prepared as follows:

* Repeat tube numbers for additional tubes tested.

* We recommend you prepare a copy of this test results sheet with provision for a minimum batch of 8-10 tubes.

3- Tube Element A and Element B are tested in the Modulus Phono Stage Tube Sockets. Combined Elements C are later tested in the preamps V3 Line Stage Tube Socket.

4- Install the matched/reference pair of tubes in V3-4, the preamp's line stage.

5- Insert tube you have ink-marked #1 into socket V1 and tube ink-marked #2 into V2.

* Be advised that when the preamp is in standby position, with filaments on, there is approximately +25vdc present on the circuit components.

6- Push Off/On Button to ON.

* Reminder: Tube preamplifiers have high voltage B+ power supplies. Hold multimeter’s test probe as close to vertical as possible with probe tip firmly on PCB’s pad. Also, keep your other hand and arm away from preamplifier chassis or anything grounded.

First - Check the Preamplifiers Line Stage HV B+ Regulation

7- Holding the multimeter’s Positive/Red probe, measure the regulated B+ voltage at the lead-end of the preamp's line stage plate load resistor(s). This should measure approximately 240vdc +/- 3vdc for each channel.

* If the regulated B+ is out of range a replacement 723-14 pin regulator should be carefully installed in one of the line stage’s 14 pin regulator sockets. Turn OFF preamp, DISCONNECT from AC Power and install new regulator. (Please observe POLARITY.) Reconnect AC line cord and Push Off/On Button to ON. Confirm that Line Stage regulation is within +/- 3vdc.

Next - Check the Voltage Tolerance of the Preamplifiers Line Stage Tubes

* Wait approximately 5 minutes before proceeding to step #8. This allows the preamp's tube voltages to properly stabilize with minimum drift.

8- At the positive side of the preamp's line stage output coupling capacitors measure the plate voltage for each channel. This voltage can be between 110-135vdc depending on the tubes used. However, there should not be a difference of more than 3vdc between each channel. E.g.: (If the L/Ch measures 120vdc, the R/Ch should be no lower than 117 or higher than 123vdc.

* If the L/Ch & R/Ch plate voltages are out of range you should not proceed with testing new tubes until a matched pair of line stage tubes are installed in V3 and V4.

9- Assuming the line stage plate voltages are within tolerance you may proceed to measure the untested tubes #1 and #2 you have inserted in V1 and V2.

Next - Begin testing previously burned-in tubes in V1 and V2. Have a Tube Checkout Sheet ready to record data.

10- Assuming the preamplifier has been turned on for approximately 5 minutes please measure the plate voltage of the V1 tube. (Measurement is taken at the positive end of the preamplifier’s phono stage’s, V1-R/Ch, output coupling cap.) The voltage should read between 110vdc and 120vdc depending upon the tube. Record the voltage next to "Voltage" under Element A.

* The next measurements for noise and microphonics/ringing are subjective. At the factory, we have sensitive measuring equipment but, for these tests, you will have to rely on your "ears" and a good headphone amplifier connected to the preamps main output.

* Remember, you can also connect a quality tape deck to the preamps’ output.

* Before making this listening test we assume you have set the output level of the headphone amplifier or tape playback electronics so that it is sufficient to hear tube noise. It is also helpful to have a reference "quiet" tube that you have listened to for comparison.

11- With the preamplifier still ON, gently place the top cover on the preamp and turn the Right Volume Control Knob clockwise to FULLY ON. Listen carefully to the tube noise and mentally compare the level of noise to the reference tube in V1. Record under Element A "Noise" the letter H-higher than or S-same as, or L-lower than the noise of the reference tube in tube position V1.

12- Now, using the knuckle of the middle index finger of either hand, gently knock on the front of the preamplifier's Right Volume Control Knob while listening for any tube ringing. How does the tube compare with the reference tube in tube position V1? Record under Element A "Microph" the letter H-higher than or S-same as, or L-lower than the reference tube in V1.

*You have now tested and recorded data for Element A of tube inserted in V1. You must now test and record data for Element B of tube inserted in V2.

13- With the preamplifier still ON, turn the Right Volume Knob completely OFF and turn the Left Volume Knob completely ON.

Remove the top cover and proceed with measuring the plate voltage of the V2 tube. Record the voltage measured next to "Voltage" under Element B of Tube you have marked #2.

Replace the top cover and proceed to listen for tube noise as you previously did for V1 tube. Record your listening result under Element B "Noise".

Now gently knock on front of the preamplifiers Left Volume Control Knob and listen for any tube ringing. Record your listening result under Element B "Microph".

* Perhaps you have already noticed that the tube socket pin configuration is REVERSED between the Left and Right Channel Phono Stage of our Modulus preamplifiers. Primarily, this is done to insure symmetrical PCB layout. However, this configuration in tube positions V1 and V2 also allows for accurate testing of both elements A & B.

14- Turn the preamplifier OFF then remove both V1 and V2 tubes. Then re-insert V1 into socket V2 and V2 into socket V1. Turn preamplifier back ON and wait 5 minutes for the circuit and tube to stabilize before testing the tubes second element.

15- Measure the plate voltage of the first tube that in now in position V2 and record the voltage. Now perform tests for noise and microphonics and record. Likewise, measure the plate voltage of the second tube in position V1; listen for noise, microphonics, and record.

* You now have tested elements A & B in tubes #1 and #2 and recorded the measurements.

* Do not begin "combined element" testing until you have first tested all the separate elements of the subject tubes. Assuming you have now performed and recorded test results for eight tubes you can then proceed to testing both elements in the preamp circuit that have been connected in parallel.

* Again, please remember, it is important to understand that, with a high-performance preamp like the Modulus, the tubes selected must be of low noise and microphonics. If the tubes are to be installed in the preamp's no-feedback line stage the combined plate voltages measured for each tube must closely match.

* In selecting tubes for "combined element" testing please select tubes that the plate voltages measured/recorded of elements A & B are within 5 volts. Tubes that exceed this range may be considered for use in V1 or V2 Phono Stage.

16- Reinstall the reference set of tubes numbered 1 to 4 in corresponding tube sockets V1 to V4.

For purpose of the test remove the preamp’s line stage tube V3 and set aside.

Install tube #1 in V3; replace preamp's top cover and Push Off/On Button to ON and wait 3-5 minutes for preamp’s circuit and tube to stabilize.

17- Using headphones, carefully turn-up Right Volume Control to maximum and listen for excessive noise. Then turn-down Right Volume Control and turn-up the Left Volume Control and listen to the reference tube in V4. Record your results for Combined, H-high, S-same or L-low as you previously did in item #11 next to "Noise".

18- Next, remove the preamp's top cover and, using the multimeter, measure and record the plate voltage at the positive end of V3's line stage output coupling capacitor.

19- Using a wooden pencil, gently tap on the side of the tube with the eraser and carefully listen for excessive ringing/microphonics. Record your result, H-high, S-same or L-low in comparison with the reference tube in V4.

20- Proceed to complete testing and recording the results for remaining tubes.

* For final selection of tested tubes for installation in the preamplifier careful attention must be given to carefully matching the voltages and noise for use in the line stage, V3 & V4. The recommended difference in plate voltage tolerance cannot exceed 3-5vdc. In addition, the tested tubes must match the low noise & microphonics of the reference tubes.

* Once a good set of line stage, V3 & V4 tubes has been matched and selected it is hoped a good pair of tubes can be obtained from the remaining batch of tested tubes for the preamps phono stage, V1 & V2.

* There can be a higher voltage tolerance, +/-10vdc, in selecting the phono tubes. However, these tubes must also be low in noise and microphonics.

* Use tube that measured best for Element A in Phono Stage V1.

* Use tube that measured best for Element B on Phono Stage V2.