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Questions about tubes and gear that glows. FAQ

RE: 12AX7 Replacement

There is an apocryphal notion that inexorably refuses to die, particularly since it is promulgated primarily by the guitar community who are not concerned with fidelity to the audio signal, that all the 12A*7 tubes are the same with the exception of the amplification factor. A true statement is that all these tubes use the 9A base. When the original engineers proposed using the same base across a range of 9 pin tubes to aid in manufacturing, they never envisioned consumers 70 years later indiscriminately replacing one tube type with another. The good news is that nothing will "blow up" or erupt into a conflagration as a result of this, but what will be damaged is the function of the tube in a circuit for which it was not designed.

Each of these 12A*7 9A base tubes (12AE7, 12AT7, 12AU7, 12AV7, 12AX7, 12AY7) has a different amplification factor, plate resistance and transconductance. That's precisely why there are a range of DIFFERENT tubes with the 9A base: they provide different parameters that are specific to the function of the circuit into which the tube is engineered. When one tube type is replaced by a different one, a FILTER is created. Obviously, it is a random filter that will generally create considerable 2nd harmonic distortion and noise. This may be desirable in a guitar amp where distortion is one of the components of "tone" but it is absolutely undesirable in any device that endeavors to be "high fidelity."

If you use the tube type for which the circuit was engineered you will have all the appropriate ancillary components to support that tube so that it operates within its linear range of operation. If you change the tube type to one for which the circuit was not designed you will very likely end up with a tube that has the incorrect plate load, plate voltage and bias. Nothing will burst into flames and the device might even pass a signal, but it will not be the level of performance that most of us strive to achieve.



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