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August 26, 2010

12AX7 Valve

The 12AX7 (or ECC83) is the backbone of most modern guitar and bass valve amp preamp stages. We currently have stock of Tesla/JJ, GT, EH, Tung Sol, Mullard, EI, Sino and Svetlana 12AX7's IN STOCK. Contact Tym guitars for more info and prices.
The 12AX7 is a miniature dual triode vacuum tube with high voltage gain. It was developed around 1946 by RCA engineers[1] in Harrison, New Jersey, under developmental number A-4522. It was released for public sale under the 12AX7 identifier on September 15, 1947. The 12AX7 was originally intended as replacement for the 6SL7 family of dual-triode amplifier tubes for audio applications. The tube is praised for its distinctive valve sound, and its ongoing wide use in guitar amplifiers has caused it to be one of the very few small-signal vacuum tubes to continue in production since it was introduced.

The 12AX7 is basically two 6AV6 triodes in one package. The 6AV6 was a repackaging of the triode from the octal 6SQ7, which was very similar to the older type 75 triode-diode, dating from 1930.
Currently, the 12AX7 is made in various versions by two factories in Russia (Winged C, formerly Svetlana, and New Sensor, which makes current production tubes under the Sovtek, Electro-Harmonix, Svetlana, Tung-Sol, and other brands for which the firm has acquired trademark rights), one in China (Shuguang), one in Slovakia (JJ), and one in Serbia (EI Niš), for a total annual production figure of 2 million units (estimated). The vast majority are used in new-production guitar amplifiers or for replacement purposes in guitar or audio equipment.

The 12AX7 is a high-gain (typical gain factor 100), low plate current triode, and is therefore best suited for low-level audio amplification. In this role it is widely used for the preamplifier (input and mid-level) stages of audio amplifiers. With its high Miller capacitance, it is not suitable for radio-frequency use.
Typically a 12AX7 triode is configured with a high-value plate resistor, 100k ohms in most guitar amps and 220k ohms or more in high-fidelity equipment. Grid bias is most often provided by a cathode resistor. If the cathode resistor is unbypassed, negative feedback is introduced and each half of a 12AX7 provides a typical voltage gain of about 60. The cathode resistor can be bypassed to reduce or eliminate AC negative feedback and thereby increase gain.
The initial "12" in the designator implies a 12-volt heater requirement; however, the tube has a center tapped filament so it can be used in either 6.3V or 12.6V heater circuits.

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