Fender reverb driver replacement
The EH 12AU7 replaced the V3 reverb driver 12AT7 in my blackface Super Reverb with Hammond tank. Absolute sound quality of a tube itself isn't critical in this position so expensive NOS isn't a must, it's just a matter of degree of drive (gain), how that makes the reverb tank sound both tonally and in terms of 'sproingy-ness', and how it affects the useful setting range of the reverb control knob. The 12AU7 has a lower gain factor than the stock 12AT7 and can be safely substituted in this position without modification (in fact, this the only tube type substitution I actually like in this amp). If your Fender reverb is too intensely bright and splashy, causing you to only be able to use lower knob settings in the 3's or below for most material other than instrumental surf, then this substitution is worthwhile trying in order to help tame over-the-top reverb tone and increase your control of reverb balance via a wider range of useable knob settings. Especially in those inherently bright-sounding, bigger and louder Fender models like the SR this can be a very welcome cheap tweak (and you can still play surf, just turn the reverb knob up a little higher than you did before and enjoy the slightly darker, less aggressively crashy effect). The EH tube has proven noise-free and dependable in this application.