Hey all. I've now made it through this thread twice. The first time through I ended up buying a mark iii just because I was tormented to know what it could actually sound like in comparison to the mark v. Well that was a good choice and I love it, but I also have been able to find tube substitutions for V4 and V6 of the mark V that make it great in its own right, although still very different from the mark iii sound.
What I first tried based on what I found in this thread was a 5751 in the phase inverter. That came right back out, as I found that it does change the character of the amp but made it far too bright for my taste, and I know some people have commented similarly. The next thing to try was a 12at7 in v4, which I had read here was the most common replacement. I was pretty happy with how it affected ch3, but it still didn't really do it for me in terms of what I had been searching for. The next step was to try out "something" in the V6 slot, but I was out of 12at7's to try. So I tried out a Westinghouse 12ay7 that I had bought a pair of at some point. I didn't read very many people trying this tube for the mark v, but I figured that it should still be mostly ok. I was very impressed with this combo, and ultimately tried putting in both 12ay7's in v4 and v6. Then I was able to start getting the changes that were talked about here: tighter bass, smoother gain, and less grainy highs. To me, this is the magical combo. I am pretty sad though to find out that that batch was the last easy to find 12ay7's from westinghouse, as now most poeple have realized how good they are, at least i'm guessing? I know that they are rebranded from a few different actual producers, but they seem to be much more rare than when I bought the first set.
On the next run through of trying new things, I bought a genalex 12at7 and a Sylvania JHS-12AT7WA. I'm also picking up these 12at7 and 12ay7's for use in my headphone amp, so I have not had any fear of buying tubes that I didn't find useful in the guitar amps, as they are still useful to me
So this pair, the genalex and sylvania 12at7's sounded very grainy. The high-end definition was incredible, obviously superior.. but so grainy I couldn't dial it out without darkening it up way too much. So maybe that's just a bad pair, or in the wrong order, or I should try to use one 12at7 and one 12ay7 to see where it takes me. I also found that the volume difference between the 12ax7's, 12at7's are similar in volume with a slight difference, but with the 12ay7's it is a very significant difference, which lets me dial in the volume and gain in just such a nice way.
Ultimately, I think the real mojo is found with the 12ay7's and I'm surprised that wasn't tried by more people. Because of that difference, I thought it was significant enough to tell you all here. I also have to say that I'm primarily focused on getting the sound I want out of the iic+ mode on the mark v because I like its smoothness and it just sounds so nice and warm to me compared to the brighter and more modern-sounding mark iv setting. So I think partly from reading through, the reason for some people was to just get a more usable mark iv or extreme mode setting, but for me I'm mainly stuck on the iic+. What I love really is setting it all up so I have mark i for ch2, mark iic+ for ch3, then moving over to my mark iii and comparing it... I'd say the mark v has a more "ragged" sounding high-gain sound where the mark iii is smoother, but I find that the mark v reacts more in a usable way when I roll off the guitar volume than the mark iii. The mark iii seems like just all-out, all the time, super metal-y type tone. I truly love it, but it's quite a bit less versatile.
Anyways, so can anyone actually confirm for me that using the 12ay7's for this purpose is completely fine? I have read that the plate voltage for the 12ax7's is 330mv, 300mv for the 12at7's, and if i'm reading correctly the 12ay7's also use a 300mv plate voltagte for the same current. Or is there any other contributing factor in this equation? Sometime i'm also going to start trying out the 12at7's that I have in the v3 and posibly other places to find out the effect on channel 2, which I find a lot harder to set up for the most part, but have found a few very heavy tones with a humbucker-equipped guitar.