#1
I thought 12AX7s had the highest gain of the commonly used preamp tubes. I bought a used amp that the owner had replaced the stock 12AX7s with 12AT7s for "increased clean headroom." I don't care about cleans at all, so I was planning on replacing the ATs with some JJ or Sovtek 12AX7s as soon as I receive the amp (amp just shipped today).

So, I call up TubeDepot today and talk to the guy there and explain everything. I tell him I like metal, and that I'm looking for the highest preamp gain for my new (used) Mesa/Boogie DC-3 head. He says, well the ATs are actually higher gain than the AXs. Is this true? He recommended that I try out the amp with the delivered ATs first, before making my decision. Does this jibe with what everyone here knows/has experienced with 12AX7s vs. 12AT7s?
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Last edited by LEVEL4 at Apr 2, 2008,
#3
Usually, 12AT7's are added to really high gain amps in order to give more "usable" gain. But usually its only one of the preamp stages that has the 12AT7 installed, not all of them. Ive heard that it has about 80% as much gain as a 12AX7, but that might be a bit subjective. Still, try it out and see how it plays; you might like it.
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#4
I'm guessing you can make the amp work harder if you use lower gain tubes, thus more power amp distortion. I could be wrong though; ask the guy who created the 'tube thread'.
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#5
Typed word-for-word from the manual for my Ampeg J-20.

"Tube Types and Usage:

second paragraph

Preamplifier tubes are also used to drive power tubes. When used in this application, a 12AX7 will produce a more distorted tone than a 12AT7, which produces a clearer, sweeter sound. A 12AU7 is even clearer and brighter than a 12AT7, giving more definition to the sound. In some cases it is possible to change the sound by changing the type of preamp and/or driver tubes."
#6
I asked for a Phillips 5751 instead of the TungSol 12AX7 for a replacement, going to try it and see if it gets me some smoother gain.
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#7
I aggree with the "usable gain" comment. The 12AX7 technically provides more gain, but on an amp with several gain stages, a smoother and less gainy V1 can make the overall gain structure smoother.
#8
Well, I just had a nice chat with a Mesa/Boogie technician in Petaluma, and he strongly recommended I replace all six preamp tubes with 12AX7s. He said that's what the amp was designed for, and that the ATs would "kill" the tone.

Also, when asked to compare the two amps, he said the DC-3 can sound similar to a MarkIV using its graphic EQ, but that the MarkIV is more versatile. Although I was considering the JJs or the Sovteks, I'm ordering six TungSol 12AX7s from TubeDepot, based on some other reviews.
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American HM Strat | LP Studio
Soldano Avenger w/DeYoung OT | Mark IV rackmount | DC-3 rackmount | Single-Recto

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Last edited by LEVEL4 at Apr 3, 2008,
#9
I gurantee you the guy at tube depot won't stear you wrong. They have always answered any question I have had for the last 15 years. And have always been right.
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