published on
In vacuum tube amplifiers, the preamplifier tube closest to the output tubes is known as the phase inverter.
Without getting too technical about it, it turns out that what you put in the phase inverter, or PI, spot can substantially affect the overall sound of your amp. Many amplifiers utilizing a class A/B operating class use 12AX7/ECC38 tubes, although the 12AT7/ECC81 variety is also common.
When I first had my amp retubed due to output tube failure, I researched about phase inverters and found that a 12AT7 can also be used in place of a 12AX7 which my amp, a Laney LC50-II combo, originally had. I then bought an 12AT7, along with the EL34s the amp needed for the retube, and had that installed in place of the stock 12AX7.
Luckily, I was able to find the stock 12AX7, and it still works. Fast forward a few months, and I finally had the resources to make a comparison between the two. Above are a series of sample recordings comparing the 12AX7 to the 12AT7 in terms of phase inverter duties.
Here is how the tracks work:
Using my Ibanez RG2550E, I play through my Laney LC50-II, cycling through my guitar's pickup configurations starting from the bridge humbucker and ending up with the neck humbucker. I have cut up the tracks such that what you will hear is sequenced as:
~0:05 - bridge humbucker, 12AX7->bridge humbucker, 12AT7
~0:10 - bridge hum split + middle single coil, 12AX7->bridge hum split + middle single coil, 12AT7
~0:15 - middle single coil, 12AX7->middle single coil, 12AT7
~0:21 - . neck hum split + middle single coil, 12AX7->neck hum split + middle single coil, 12AT7
~0:26 - neck humbucker, 12AX7->neck humbucker, 12AT7
With the same EQ and volume settings for both tubes, the most noticeable difference between the 12AX7 and the 12AT7 is volume. However, some subtle sound differences can also be heard, especially with distortion.
Equipment:
Focusrite Saffire PRO 24 audio interface
Shure SM57 dynamic microphone
Ibanez RG2550E electric guitar
Laney LC50-II amplifier
MI Audio Tube Zone overdrive pedal
Joyo Vintage Overdrive pedal
- Genre
- Rock