AI Modulus Preamps Are Not Hard On Tubes... To date we have mfg.many thousands of these units and, to the best of our knowledge, premature tube failure has not been a problem. The problem is some "tube rollers" continue to substitute old-style 6DJ8 type tubes in our preamps. These tubes were OK for TV sets and RF applications but not designed for audio purposes. Thus the small plates and weak mica insulators resulting in excess microphonics and noise. However there do exist some NOS brands that are acceptable and exhibit good life; Tungsram is one brand that comes to mind.
Be advised that our tube preamps are open-loop/no-feedback designs. The advantage of eliminating negative feedback is well known. The downside is that the tube we select must be well constructed, have low-noise and good life. In our 30 years experience the best tubes we found that meet our criteria are those mfg.in Russia. In the early 1980's we did use some 6DJ8 tubes produced in the 70's that were well made but, when the supply of good tubes ran out, in 1987 we discovered Russian tubes. Primarily used for their military communications, including transmitters in MIG jets, these tubes were designed for low-noise spec.and reliability. All of our preamps since 1988 have Russian tubes installed.
Regarding our filament-on design, based on his experience with tube equipment in the recording industry, my boss had learned that the main contribution to tube failure was thermal shock to a cold tube. For that reason most recording studios left their tube equipment on. Of course, leaving tubes on does cause premature plate degeneration but he experimented with leaving the filaments on but with some current flowing on the plates. This design has proven to be a big success in prolonging tube life in our circuits.
Unfortunately this circuit design will not prolong the life of old-style, poorly made tubes. Suggest you tube rollers experiment with preamps employing negative feedback and cathode followers. Regards, Jerry/AI's Tech,
Be advised that our tube preamps are open-loop/no-feedback designs. The advantage of eliminating negative feedback is well known. The downside is that the tube we select must be well constructed, have low-noise and good life. In our 30 years experience the best tubes we found that meet our criteria are those mfg.in Russia. In the early 1980's we did use some 6DJ8 tubes produced in the 70's that were well made but, when the supply of good tubes ran out, in 1987 we discovered Russian tubes. Primarily used for their military communications, including transmitters in MIG jets, these tubes were designed for low-noise spec.and reliability. All of our preamps since 1988 have Russian tubes installed.
Regarding our filament-on design, based on his experience with tube equipment in the recording industry, my boss had learned that the main contribution to tube failure was thermal shock to a cold tube. For that reason most recording studios left their tube equipment on. Of course, leaving tubes on does cause premature plate degeneration but he experimented with leaving the filaments on but with some current flowing on the plates. This design has proven to be a big success in prolonging tube life in our circuits.
Unfortunately this circuit design will not prolong the life of old-style, poorly made tubes. Suggest you tube rollers experiment with preamps employing negative feedback and cathode followers. Regards, Jerry/AI's Tech,