Here is the "Technical Bulletin No.5" from Audible Illusion concerning tube choices for the Modulua Model 2 and 3 preamplifiers:
"Technical bulletin #5
RECOMMENDED BRANDS OF TUBES
FOR MODULUS SERIES 2 & SERIES 3 PREAMPLIFIERS
NEW TUBES - Currently installed in M3A and L2 preamplifiers
6922 SOVTEC, RUSSIA - GREY PLATES
Currently imported from our sources in Europe, these tubes have excellent sonics, low microphonics and distortion.
6H23EB RUSSIAN MILITARY, RUSSIA - GREY PLATES
Most are NOS currently imported from our sources in Europe. Similar to 6922 Sovtec "Grey Plate" tubes. Closely resemble tubes installed in Modulus, series 2 and 3 preamplifiers between 1988 & 1996. Excellent sonics and reliability.
NEW TUBES - Recommended, but with some reservation
E88CC JJ ELECTRONIC, SLOVAK REPUBLIC
Excellent sonics but somewhat microphonic. Similar tooling to classic German Amperex installed in Audible Illusions and Audio Research preamplifiers in mid-1980s. New tubes have approximately 50% rejection rate for excess noise and microphonics. Inexpensive, worth the trouble to sort/select. Ideal line stage tube.
ECC88/6DJ8 - ELECTRONIC INDUSTRIAL, YUGOSLAV/SERBIA,
A-FRAME DIMPLED RING GETTER
Tube installed in Modulus series 2 preamplifiers in mid-1980s. Similar to tube manufactured by Phillips-Holland in the 1970s for Amperex. Sweet sonically. Most are very microphonic.
6922 SOVTEC - RUSSIA, MFG. BY REFLECTOR - PLATES PARTIALLY UNCOATED
Current production Sovtec tubes currently used by several tube preamp manufacturers. Initially this tube checks out OK but many prematurely fail due to poor construction. Has noticeable midrange glare when compared to "Grey Plate" Russian tubes.
NEW-OLD-STOCK TUBES - From our extensive tube library we recommend these NOS tubes with some reservation.
Ranking in order of reliability in Modulus preamps.
1- 6922 AMPEREX, GERMANY, SHIELD WITH RING GETTER
Installed in Audible Illusions, series 2 and Audio Research SP8 and SP10 preamps in the mid 1980s. Very open-dimensional sonics. Excellent reliability but most tubes microphonic. One of the finest sounding preamplifier tubes ever made.
2- E88CC TUNGSRAM, HUNGARY, SHIELD WITH RING GETTER
Excellent sonics but somewhat microphonic. Good tube life. Very popular tube for current production Modulus preamps.
3- 7308 SIEMENS, GERMANY, SHIELD WITH RING GETTER, GOLD PINS
Somewhat microphonic but performs well in A.I. preamps. Excellent line-stage tube.
4- E88CC SIEMENS, GERMANY, SINGLE RING GETTER, GOLD PINS
Excellent sounding tube. Sounds like a cross between a good Amperex and Tungsram. Somewhat microphonic.
5- ECC88/6DJ8 AMPEREX BUGLE BOY, HOLLAND, SHIELD WITH RING GETTER
Hard to find and expensive. Excellent sonics with sweet top end. Manufactured for Amperex in the 1960s and early 1970s. Somewhat microphonic. Probably the finest sounding preamplifier tube ever made. *DO NOT confuse with "rebranded" Sylvania 6DJ8’s currently sold by Richardson Electronic.
6- ECC88/6922/7308 AMPEREX PQ, HOLLAND, SHIELD WITH D-RING GETTER, GOLD PINS
Rare. Very open-uniform sound. Somewhat microphonic. In mid-1980s Gold Aero was branding them 7308’s and selling them for $50 each. Excellent line stage tube.
7- ECC88/6DJ8 AMPEREX, HOLLAND, A-FRAME DIMPLED RING GETTER
Manufactured for Amperex in the 1970s. Same tooling as EI-Yugoslavian tube but superior workmanship. Sweet sonics, somewhat microphonic.
7A- ECC88 MULLARD, GT.BRITAIN, A-FRAME DIMPLED RING GETTER
Same tube as above. Manufactured for Mullard by Philips-Holland in the 1970s.
8- 6DJ8/ECC88 AMPEREX, HOLLAND, SINGLE RING GETTER
Tube actually manufactured for Amperex by Matsushita/Japan during the late 1970s and early 1980s. Installed in early A.I. Modulus series 1 preamps, this tube has a very sweet-uniform sound. Somewhat microphonic.
9- 6DJ8 MULLARD, ENGLAND, SINGLE RING GETTER
Similar tooling compared with Matsushita/Japan tube but manufactured by Philips-Holland for Mullard. Sweet sounding tube but most have excessive microphonics.
10- ECC88/6DJ8 ZAEREX, RUSSIA SINGLE RING DISH GETTER
Tube used almost exclusively by Counterpoint Electronics in the 1980s. Quiet-smooth, low microphonics but somewhat fat bass response. Has a tendency to oscillate in early Modulus series 1 and 2 preamps. Do not use these in older A.I. models.
Tubes with questionable sonics and unacceptable technical performance
NOT RECOMMENDED for Audible Illusions preamplifiers
6DJ8H CHINA, SHIELD WITH FLAT-SQUARE GETTER
Plates only partially coated. Tube has glassy sound and unacceptable microphonics.
6DJ8 SYLVANIA-USA, SHIELD WITH RING GETTER
Tube has uneven sonics and midrange glare. Most are microphonic. Currently being re-labeled with Amperex "Bugle-Boy" logo and sold for high price by Richardson Electronics.
6DJ8 PHILIPS, USA, JAN/JOINT ARMY NAVY
*Same as above." <
-The reason I paid attention to the AI factory recommendations is that I have an Audio Research SP10 which is similar to the Modulus in that the tubes need to be carefully chosen- especially for ruggedness. In the SP10 my plan is to use 4- E188CC for the first two gain tubes of phono and line stage- (or: all 4- phono) and NOS Tungsram red label E88CC for the other 8 or 6 positions.
