I just wanted to post this on here for JW.
Jw.. This is a diagram of your preamp. And on it, I'm showing you the way I designed it for myself and my style on my mesa, and I'll give you the explanations as well.
V1- this is the most important tube slot for your preamp. I personally haven't found anything better than an RCA for this slot. A 12ax7A is a select RCA NOS tube that kills all. When I stuck the Mullard in this slot, the distortion was way too muddy, the G.E. was just a little lacking in treble. The RCA was dead nuts perfect.
V2- The G.E. 12ax7A is strong and bold in the Bass tone area. This thing thumps. When the Mullard was put in this slot it accentuated the mids of the RCA, but just didn't seem to be the right slot to put it in with the G.E. behind it. I wanted to keep things high gain in the 1st, 2nd, and 3rd stage of the preamp. The G.E. compliments the RCA really good (kind of like a scooped mid sound) when the distortion is kicked in.
V3- This is actually the second most important slot in the preamp. This slot is where you can either drive the preamp into full on power, or powered down a little. The Mullard *7025* is a lower gain tube. It's really warm and clean. I put it here so that the clean channel wouldn't be so in your face with the Bass tones of the G.E., or high strung and high powered from the RCA. It really seems to equalize everything out to a nice smooth sound.
*** Did you know*** that you can actually put a 12at7 in this slot?? A 12at7 is considerably lower in output. Where a 12ax7 is 100% output, a 12at7 is 60% output. So if you feel like your amp is too overdriven for your style, you can put a 12at7 in this slot to tame the distortion to suit your style. This is where you could make your amp sound completely different with the right 12at7.
V4- This is your effects loop. I chose a G.E. 5751 Triple Mica for this slot because your effects can be really noisy depending on what you're using. This tube is very quiet, low in micro-phonics, lower gain, very clean. You can also put a 12at7 if you want the output to be less in your face.
V5- This is your output/phase splitter tube.. I really liked the Mesa here for some reason. This ecc83 really sounds like the Mullard with more gain. I did find it to be a good tube, and it works just fine in this slot.
Anyway.. I just thought I'd give you some insight on how versatile the Mesa Preamp is, and how you can come up with a plan to tailor your amp to suit your style. This particular setup sounds awesome, and it's perfect for my style. Paired up with the SED 5u4's it's just freakin killer.