Love what you all are sharing in this thread. I'm a tube rolling newb so this is pretty encouraging. I absolutely love the clean channel with the gain boost knob pulled but have been wishing to tame the lead channel. On other forums I've just been met with "take the MT15 for what it is or get another amp."
What I mean by "tame" is that I don't like not having more fine adjustment of the lead gain - to my ear, with 12AX7s it's already super compressed heavy crunch at 8:00 - 9:00 on the dial. I guess this is what some of you are referring to as "taper"?
I read Tube Depot's write up on the 5751 and they echo Andy474x. I'm going to try both the 12AY7 and 5751 in v2 and v3 (each slot individually and together) and share what I hear.
Hey man, good to hear there are others doing their own thing with the MT15. I'll tell the abbreviation of many of my posts, first of I think that the BIG thing many tube amps offer (compared to solid state) is the option of having many amps in one amp. There are many possible combinations using different positions and different tubes, and all of them will have a specific feel/sound. I am a big supporter of searching for "your" sound, I even changed the knobs to have a better overview (the original knobs are imo cheap, ugly and lifeless). I also unsoldered and replaced fx send and return jack plugs to get rid of an unwanted hum/hiss. And finally I started to play with tubes, preamp and poweramp. Power tubes change the character in a more subtle way (kind of like choosing the surface on which you will draw and will you use crayon, charcoal or pastel to make a drawing) while preamp tubes do all sorts of magic with the sound of your guitar and affect the main character of the sound (kind of like they determine what will be drawn).
I've gone trough X, T, Y and U versions of 12a, tried 5751, some vintage ones, tried Ruby and TAD power tubes, even JJ KT66 (but they are just too thick and are virtually touching when installed, so do not buy these). The biggest difference is in swapping position V1 and becomes less significant the further down you go the tube positions, in the end they all impact the sound if you really want to sniff corks. It just depends on if it is worth buying a tube, will it make enough a difference for you to justify the price of it or to be able to appreciate the sound you get.
Then there is option of using different kinds of 12ax7 tubes, large plate models are for example more "musical" but maybe more susceptible to vibrations, like Mullard for example, the ECC83S models are very hard core/rough, you have 7025s that ought to be more resistant to vibrations, you have "frame grid" models (in a normal 12AX7, the grid wire is wound around two posts, on a frame grid tube, the grid wire is wound on a rigid frame, frame grid tubes have very low microphony and tight tolerance) ... and so on.
You decide where you want to start, but go and buy a few different tubes and go to work - invest 100 bucks/euros and buy a few reasonably priced tubes. Start with V1 tube and then move elsewhere - but do your homework and first check which tubes can be used in your amp, but there are many. Your sound will also be determined by the speaker, guitar, cables and so on. I have found that it opened my ears the most when swapping X to T to Y ... it is like having 3 different amps in a way. Try it and see where it takes you, but be patient. Cheers
