The Tungsol 12AX7 is a high grade Pre amp valve manufactured in Russia and similar in build to the Electro Harmonix 12AX7 manufactured under the same supervision but using different materials and on a different line.
The valve is very well engineered and ruggedized using thick glass and firm internal supports. On test we very rarely see failures after burn in indicating good long term reliability. The gain factor of the valve is slightly lower than the average 12AX7 coming it at around 97 compared with the average 12AX7 valves tested which test out with a gain of around 100. We found on audio test that Headroom was greater than other 12AX7's compared against and also that Noise floor was very low coming in at an average of 83dB compared with normal average noise figures for 12AX7 coming in at around 79dB. The valves sonic signature leans towards a slightly scooped mid range but very detailed and extended high frequency response and extended bass also. In Guitar application when the tubes are overdriven they respond with oodles of harmonics and really cut through the mix.
HOW DOES IT GLOW
Like it's Electro Harmonix 12AX7 brother, the heater cannot be seen glowing in the latest generation Tungsol 12AX7 so it doesn't. The designers have somewhat enclosed the heaters so they are not so visible when in operation as in some valves. This is not so good if you have a high end Hi Fi amp and like to show off your tubes glowing away. The Tungsol actually shows no exciting sexy soft red glow commonly expected from valve kit. The fact that the valve does not give off any light does not affect tone or performance and is completely irrelevant when used in a guitar combo or amp where the valves are not visible.
All audio tests carried out in Rivera 100 combo and Leak 20 20 stereo hifi amp.
Hope you found the review informative, Regards, Nic at http://stores.ebay.co.uk/Voc-Rock-Guitars/Small-valves-ECC83-12AX7-etc-/_i.html?_fsub=2596489016&_sid=186748086&_trksid=p4634.c0.m322