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Groove Tubes SAG-AX7-MPI Matched Phase Inverter Tube Changing Kit

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Product 219132
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The Groove Tubes SAG-AX7-MPI Matched Phase Inverter Tube Changing Kit consists of specially selected and matched 12AX7 vacuum tubes for critical audi...Click To Read More About This Product

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The Groove Tubes SAG-AX7-MPI Matched Phase Inverter Tube Changing Kit consists of specially selected and matched 12AX7 vacuum tubes for critical audio or guitar/bass amplifier use. They're intended primarily for matching the two sides of the triode for phase inverter or driver use.

The Groove Tubes AX7 tube is a dual triode where the A and B sides are closely matched for use in critical high end audio applications or for use in the phase inverter or driver position of musical instrument amplifiers.

The benefit of this tube in a guitar or bass amplifier is a matched output section. At lower frequencies such as dropped tunings, 7 string guitars, or bass guitars, the upper and lower portions of the output waveform will be much more equal. When this waveform is not equal, differences are cancelled out in the NFB loop of class A/B amplifiers and are noticed as a lack of sustain or apparent "dead spots" in areas of the fret board.

At high levels at all frequencies, sustain will be improved. This is even more critical in Class A amplifiers such as the Vox AC-30 and Matchless amplifiers.

Use this MPI in any application where a phase inverter / driver of the ECC83/12AX7/7025 tube is specified. The phase inverter / driver position of a guitar or bass amp is a current driving position, and not part of the tone generation stage. These tubes are selected for specific characteristics, so may be a 12AX7R, 12AX7C, 12AX7M or another 12AX7 type that has been selected for specific curve traces as the objective. One can not specify the tube type desired.

If you want a specific type matched as an MPI you need to order the tube and then add the SAG labor charge on top of the tube. The stocked MPIs are generally made from 12AX7C, 12AX7R (WC and NOT WA) and the new 12AX7M. Any of these tubes may be supplied as they are matched for specs and vacuum tube curve traces and not for tube type.

Use of a 12AT7 in place of a 12AX7 will yield more clean headroom in your output section. The two tube types are interchangable without any adjustment required, so feel free to experiment!

Please contact GT SAG directly if you have any questions or specific application requirements.

Country of origin: China

  • Country of origin: China
  • Country of origin: China

Review Snapshot

by PowerReviews
Groove TubesSAG-AX7-MPI Matched Phase Inverter Tube Changing Kit

(based on 2 reviews)

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Reviewed by 2 customers

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Did Not Like

By Daver

from Ohio

About Me Experienced

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  • It Fit
  • None


  • Not Functional

Best Uses

  • NONE

Comments about Groove Tubes SAG-AX7-MPI Matched Phase Inverter Tube Changing Kit:

This tube actually made my amp sound worse. It gave me a Scratchy-like trebley breaking up sound at low volumn not clean at all. It sounds very bad in V3 and also in V1. It is not printed "matched phase inverter" either, like is shown in the picture. It is printed with GT-12AX7-C. No markings show it is really a matched PI tube.

I used other GT12AX7C tubes and also a JJ12AX7 Balanced tube and my Hot Rod Deluxe III amp sounds fine with both of those.

Who knows what this one really is.. maybe I got a defective one.

(0 of 1 customers found this review helpful)


Sounds too expensive, but it helps.

By SG specialist

from Florida

See all my reviews

Comments about Groove Tubes SAG-AX7-MPI Matched Phase Inverter Tube Changing Kit:

In today's computerized world, there are so many ways to achive similar results where guitar tone is concerned. We are very old school in that we hang on to technology that's old. If put side by side, a JCM 800 Marshall and a Line 6 Vetta sound so close that very few people could really tell the difference if blindfolded. If you play loud rock music it may not be the amp but your ability to discern the small differences between the two. Then it boils down to the image and peer preasure (am I cool enogh?). A Marshall half stack evokes all kinds of fellings in a person just looking at it. Players who can't tell the difference would gravitate to playing a tube amp just to be safe. The balanced triodes of a matched phase inverter tube is an option that is hard to hear. If you can hear the difference between non matched and matched, It's totally worth it. It is to me. What I listen for is sustain and harmonics. There are amps that will have these qualities even without a matched phase inverter.
I trust that Fender (Groove Tubes) really provides a truely balanced phase inverter. My ears tell me that it's somoother and woth it.
Generally, These tubes last about 2 years with heavy use. If you run at full volume maybe one year.
Expensive but with it.

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