Testing & Selecting Procedures
Noise and Microphonics Testing at V.T.S.
Because of the extremely high amplification factor and wide bandwidth of Hi-Fi audio systems, tubes used in such systems must be selected and matched to a far higher degree than almost any other use for which they were intended. The average, off the shelf tube is more often than not, useless for operation in the earlier stages of an audio system. With many of the later production tubes (when dies and maintenance of tolerances got looser and looser) an entire case of 100 tubes may only have 10 or 20 good non-microphonic tubes. The only way to properly test a tube for microphonics is by ear. At VTS rather than put noise and microphonics together as a general rating they are separated. As there is no place for any type of electrical noise in an audio system, any tube with any electrical noise above its thermal noise floor is immediately rejected. That leaves microphonics, the tendency of a tube to pick up mechanical vibration and act as a microphone.
Microphonics Selection of Small Signal Tubes
Small signal tubes (12AX7, 6DJ8, 6SN7, ect.) are selected by myself using a combination of:
(A) Very special, custom made, ultra low noise floor, auditioning and measuring units.
(B) Both vintage and current, commercial Hi – End gear.
Using this equipment small signal tubes are separated into four categories reflecting the job they do in a system. In a system using a low output moving coil cartridge the amplification factor can run in excess of 100.000 by the time the signal reaches the output of the power amp! It is very important to get this selection process right and this is one of the reasons this company was founded. (Because no one was/is doing it right). I know that no one else is doing any serious noise measurements on glass mini tubes because you CAN’T until you clean the pins! You cannot get a good enough connection to the tube with all that scale in the way to even check the noise floor!
Each small signal tube that is distributed by VTS has a colored dot on the base or vacuum tip. This is enamel paint with an estimated 100 year life expectancy on the colorfastness so it won’t rub off! The system is:
|Color||Recommended Circuit Type Use||Recommended Max. Circuit Gain Use|
|Red||=||LOW Mic. LINE level tube||100 X|
|Red-Green||=||VERY LOW Mic. SUPER-LINE level tube||1,000 X|
|Green||=||VERY VERY LOW Mic. Moving Magnet PHONO input||10,000 X|
|Blue||=||ULTRA LOW Mic. Moving Coil PHONO input tube||100,000 X|
NOTE: The colors do not designate the quality of a tube. They are only an indicator of its intrinsic Mic. level.
Power & Output Tubes
Power & Output tubes are tested a minimum of two times at VTS. The test consists of the following:
(A) The first test is at the regular rated testing voltages using fully calibrated consoles (See Laboratory)
(B) The second test is a full ratings, metered torture test at full current and voltage, done with amplifiers to make sure the tubes actually “work” properly (as compared to “measure” properly because tube testers in general do not run the tubes at anywhere near the voltage and current that actual amplifiers do). Microphonics, noise, and other parameters such as plate and grid heating are also monitored continuously during this test.