Editing 12AX7 (section)

==Application==
{{more citations needed section|date=March 2022}}
[[File:TriodeECC83Characteristic1.png|thumb|Anode current characteristic with grid voltage as parameter]]
The 12AX7 is a high-gain (typical amplification factor 100), low-plate-current [[triode]] best suited for low-level audio voltage amplification.  In this role it is widely used for the preamplifier (input and mid-level) stages of audio amplifiers. It has relatively high [[Miller effect|Miller]] capacitance, making it unsuitable for radio-frequency use.

Typically a 12AX7 triode is configured with a high-value plate resistor, 100 kohms in most guitar amps and 220 kΩ 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 30; the amplification factor is basically twice the maximum stage gain, as the plate impedance must be matched.  Thus half the voltage is across the tube at rest, half across the load resistor. The cathode resistor can be bypassed to reduce or eliminate AC negative feedback and thereby increase gain; maximum gain is about 60 times with a 100k plate load, and a center biased and bypassed cathode, and higher with a larger plate load.<ref>{{cite web |url=https://www.ampbooks.com/mobile/amplifier-calculators/output-impedance/calculator/ |title = Preamp Gain and Output Impedance Calculator}}</ref>

<math>A_v = \mu \times R_{tot} /(r_P + R_{tot} + (R_K \times (\mu + 1))</math>

Where <math>A_v</math> = voltage gain, <math>\mu</math> is the amplification factor of the valve, <math>r_P</math> is the internal plate resistance, <math>R_K</math> is the cathode resistor and <math>R_{tot}</math> is the parallel combination of <math>R_P</math> (external plate resistor) and <math>R_{load}</math>. If the cathode resistor is bypassed, use <math>R_K = 0</math>.

The initial “12” in the designator implies a 12-volt heater requirement; however, the tube has a center-tapped heater so it can be used in either 6.3-V or 12.6-V heater circuits.