In generic tube pre-amplifiers there are usually only 1 – 2 gain stages that contribute to any apparent ”tubeness”. Rest of the tube circuitry is basically “transparent” amplification for purposes such as gain recovery, increasing signal-to-noise ratio, “buffering”, etc. You wouldn’t notice a difference to solid-state circuit.
Given that, IMO, the hybrid preamps with just 1 – 2 tubes are not “gimmicks”. Or at least not necessarily. A single dual triode already provides 2 gain stages, enough to put the tube in all places of the signal path where it counts. If two are used we can use four gain stages, and you hardly find more used from an “all tube” preamp either. So why is this a “gimmick”? I can see gimmickness of things like Eden’s cathode follower input stage, or Carvin’s cathode follower “tube signal path” in general – tube circuits that virtually do nothing meaningful or even audible! But just employing a seemingly small amount of tubes is not a gimmick in its own right. At least there is a potential to use them meaningfully. e.g. You only need a single 12AX7 to construct the legendary Fender / Alembic preamp in its entirety!
Yes, LEDs are nowadays fitted to tube equipment for enhanced “tubeness effect”. Tube equipment nowadays needs to fulfil our expectations of “visual image” of tube equipment – as much as it needs to fulfil our expectations of “audible image” of tube equipment. There is equally much sense in using the most linear amplifying device ever (a triode) to introduce ridiculously great amounts of non-linearity (“tube sound”) as there is illuminating tubes with LEDs because their filaments are too dull to give apparent visual image of tubeness. Same thing why we need to see those glowing tubes in the first place, instead of closing all that boring stuff inside the chassis and forgetting it even exists. (Which was how tube gear was designed in the golden eare of tube gear). Let’s just face it: Today visuality counts big time in overall “tube experience”!
I remember when the LED rage started from someone discovering that Behringer had fitted yellow LEDs behind the tubes in their rack gear (why not, the tubes where aesthetically put to show in the front panel to begin with). Because it was Behringer (mic preamp I think) it was naturally initially assumed that the tubes do nothing and are there just for show. No one - of course and as is usual - bothered to test anything. This was just the initial assumption, which almost everyone believed without a second thought. Ironically, that very same Behringer gear implements one of the most clever implementations of a tube circuit I’ve seen in a generic “hybrid” preamp. Nope, not just a token common cathode amp but a complete “push-pull” circuit, which’s symmetry user could manually adjust to vary between “even harmonic” and “odd harmonic” distortions. Pretty clever idea indeed and proved to never judge a book by its cover!
Then we have that issue of employing “starved plate voltage” circuits. When tubes are used just to introduce distortion I fail to see the problem. Yes, high plate voltages are ideal if we want to operate the tube in its most linear range at highest headroom possible. Did you catch that? “Most linear”. “Highest headroom”. Sounds pretty lame for anything that relates to distortion. Yep, it’s a golden rule, which applies to “Hi-Fi” tube design but has little practice in designing distortion effects. Reduced plate voltage, on the other hand, is good for that purpose since it reduces headroom and introduces greater amounts of non-linearity in general. Given how much plate voltage reduction is utilized to day in (particularly guitar) tube amps and circuits I fail to see shy “starved plate” circuits have such a bad reputation. Anyone owning an Ampeg SVT3-PRO? Is the “Tube Gain” control a useless feature, as it is nothing but a variable “starved plate” circuit?
But, IMO, it could be argued though, if preamp tubes in general are “gimmicky”. There’s nothing they do that couldn’t be - to point of incredible accuracy - done with proper design and solid-state technology as well. Even the tube "power amp" -tone with SS is nailed to close perfection by many manufacturers. Naturally it finds less use in bass amplification than it does in guitar amps. Bass power amps still benefit from higher headroom, less distortion, and "tighter" characteristics overall.
