A: We consulted the Sweetwater Service Department’s tube amp expert, Greg Bowers, for the lowdown on this question.
“The 12au7 (as used as the power tube in the Vox Lil’ Night Train) is sometimes used as a low-gain preamp tube. However, even though the 12au7 is the lowest gain version of the ’12A’ tube family, it has the highest current capability. The 12au7 can produce about one watt per half in a push-pull configuration. This explains the 2-watt rating of the Lil’ Night Train.
“Leo Fender used the 12at7 tube as his reverb driver for a similar reason. The 12at7 has about the same gain and current as the 12au7, but has narrower frequency response. Since Fender needed to get all the shake he could out of the circuit to drive a reverb spring, he used the 12at7. The frequency response was not a problem because reverb springs have a narrow frequency response anyway.
“Fender found that the 12at7 preamp tube could substitute for the 6V6 power tube that he used in the classic ’63 standalone reverb. The ’63 reverb is really a Champ amplifier driving a reverb tank instead of a speaker. The recent desire to create ultra-low power practice amps was inspired from this. If a preamp tube could substitute a 6V6 in a reverb, why not use it to drive a speaker?”