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The wonders of a NOS Amprex 12AU7 in my Mesa Express


Silver Supporting Member
I was tube rolling through my Express 5:25 Plus the past few days trying to find a happy medium between the clean and burn channels for my PRS Zach Myers, Fender Strat and 50s Tele. I'm primarily a jazz (Scofield, Robben Ford), blues (Keb Mo, Mayer, Schofield, Kirk Fletcher) player. So I found an old, forgotten tube that a buddy gave me many years ago...a NOS Amprex/Zenith 12AU7. I had a Tung Sol 12AX7 RI in V2 and put the 12AU7 in V1...magic happened. The inherent, heavy low mids were reduced and clarity for the PRS was achieved on both channels. I nice, swirly bounce in cleans and more open dumbly overdrive in the burn channel appeared. The amp has almost ZERO hiss, floor noise now too.

I've tried several "lower" gain tubes in V1 and V2 of this amp (V2 is the first gain stage per Mesa's design) - GT 12AX7M (my favorite, but they go micro and buzzy in a month or two), Sovtek 12AX7 LPS (again, microphonic out of the box) and a Mesa new 12AT7 (yep...microphonic and howled) and never found anything to work. My default in V2 has been a new issue Tung Sol 12AX7.

My overdrive pedal (Keeley Red Dirt) sounds amazing on the clean channel for all the guitars - great clarity and just smooth overdrive. I did find I needed to redo my graphic eq settings on the clean channel to add some depth to the Fender single coils and my Xotic EP boost really helps out with the single too.

I'm now really thrilled to have this amp suit my playing style match better now - more tamed and less "balls out" in your face.

FWIW - the other tube slots have:

V3: TAD Tube Amp Doctor 7025WA (which is a warm, rounded sounding tube - tried it in V2, it was just too warm for the humbuckers).
V4: Ruby 12AX7 AC7 HG
V5: Mesa STR - SPAX7 (I've moved this guy around all slots, it had been in V1 most of the time)

Stock OEM EL84s and Vintage 30 speaker.

Just thought I'd share my experiences. Now to shop for some backups for that Amprex tube and maybe some non-microphonic 5751s to continue to experiment with.


Silver Supporting Member
Short lived...sound volume cut in half and thin tones...there goes that old tube.


If you like 12au7s start grabbing free organs off craigslist. Most of them have at least few 12au7s and several have 30 or so.

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