My First Kit Build - Trinity 18 Plexi

Personally, I like to get away from having all of a preamp being 12AX series tubes. Granted this is not anywhere near as easy as subbing in another AXes, but to me the AX7 in particular is not a standout good sounding tube. Even a 6AV6, supposed to be the functional equivalent of half an AX7, sounds a LOT better. My heart truely belongs not to dual triodes, but the combo pentode/triode tubes like the 6AN8 and 6WJ8, but those won't work in a circuit designed for an AX.

I did come across something interesting, that I think I will explore further in the future, which is yellow-jacket style adapters for preamp tubes. I recently found available from shops online that I order other things from anyway, both tube bases (that I could wire up myself as adapters) as well as some premade adapters (e.g. 12AX7 to 6EU7, which I have never used/heard, but now am curious about). They are readily available (NOS and new production). Supposedly really low noise, but I dunno if they sound any good (not common, but apparently they have been used in guitar amps).

the "halo" that your referring to is the getter. its heated and contains the stuff (barium?) that makes that chromey shiny looking coating to the top of the tube (getter flash). that itself doesn't really affect the sound but its part of the mix of differences that make certain 12ax7 tubes different from others.

Different tubes will impart some differences to the sound but it also depends on the amps circuit. typically v1 or your first stages of multichannel amps would be where people say most of the flavour comes from. as well the phase inverter can be a place to tinker with different lower gain 12a tubes to change the headroom ie a 12at7 or a 5751. in a circuit like a marshall your v2 is a cathode follower and this can be a stressful job so your main thing is to pick a tube thats rugged and will stand up to the potential abuse.

in terms of plate sizes and colours. black plates are typically earlier production and command higher prices. ditto for long places vs short plates. If microphonics are an issue you want to avoid the long plates. ultimately its up to you and your ears to decide what sounds best.

in terms of marshall specific they holy grail are mullards from england which came stock back in the day. its not uncommon for mullard tubes to be relabelled by other brands so there are often deals to be had on relabels. as well if you're looking for a budget mullard there are japanese tubes made on mullard tooling. again often can be had without spending an arm and a leg. as well later marshall like the jcms often came fitted with relabeled Ei tubes made in prewar yugoslavia. they look like similar to telefunkens with smooth plates. often labelled westinghouse.