Mesa/Boogie Triaxis - Tubes & Opamps


Nope, I do not own a Mesa/Boogie Triaxis preamp. But I did study it's schematic, have been a member of the Triaxis Yahoo Group for a long time and found some things that might be of interest for Triaxis users to get the best possible sounds.

After studying the complaints, manual and schematic of the Triaxis I think I've found some interesting points to get prevent sonic problems with the Triaxis. Many Triaxis' "suffer" from buzzy sounds. One source if this problem is the use of Russian and Chinese tubes, imo, another because people use low gain settings in combination with high drive settings. The last can easily be solved be experimenting a lot with various settings and reading the manual. Using the right tubes is also a must. The Triaxis is a high quality amplifier, why would you use the lowest quality tubes available? Using good tubes doesn't have to cost a fortune, but will breath live, dynamics and detail in the Triaxis you've never heard before.

The OPAMP problem
But there is another source of buzzy sounds in the Triaxis: the use of an opamp! Yes that's right, the beloved machine uses an opamp, a TL072 to be precise. Don't worry it's not used to make the distortion (as with most Marshalls), but it's used for the Dynamic Voice. And when the internal volume is too high, the TL072 gets too much signal and starts to overdrive.... which in this case is not nice and unwanted.

This same opamp was also used to get more distortion out of many Marshalls (JCM900, JMP1, etc.), Mesa/Boogie V-Twin and many other (pre)amps but in those cases it was tuned differently then now. So in all cases try to avoid overloading the opamp. One way is to keep the volume low and just turn up the output pots and power amp. Using a lower gain tube for V3 is going to help too. Several types of tubes are worth checking out although my main focus would lay on 12AT7/ECC81 (mediocre gain) and 12AU7/ECC82 (low gain) tubes. GE 5751's are an option too, but have in most cases less bass then the previously mentioned tubes, therefor I would go with either 12AT7 or 12AU7. I've used these type of tubes many times before and various setups and with great results.

My advice on tubes for the Triaxis
V1 = high quality tube
V2 = high quality tube / non microphonics
V3 = lower gain tube
V4 = tube which is not microphonic
V5 = balanced tube! / lower gain tube if rec. outs are used or with high volume fx.

Either V1 or V2 is used as main input tube, depending on the settings. These are the first tubes your signal passes and therefor need to be of high quality. In fact the inputs are directly wired to the tubes, with nothing between them besides a on/off switch. V1 and V2 have the biggest influence on your sound quality. V2 is also used as gain tube for Lead1, therefor it should also be resistant to microphonics.  Same thing counts for V4, which is the tube used for the Lead2 stages. I would look at NOS tubes for V1 and V2. And whatever you like for V4, as long as it is resistant to microphonics.

V3 is the tube placed between the programmable master volume and the Dynamic Voice. As mentioned before, the Triaxis uses a TL072 opamp for the Dynamic Voice and the signal always passes this opamp. Therefor I highly recommend using a lower gain tube with more headroom to prevent overdriving either the tube or the opamp. NOS 12AU7 or 12AT7 tubes would be my choice.

The FX Return go to the V5 tube, which is also the main output tube. A preamp tube has 2 sides which can be used to amplify a signal. In the case of the V5 each side is used to amplify one side of the stereo field. If both sides of the tube are not equal in quality then one side will be louder or different sounding then the other. So it's really important to use a good tested, balanced tube for this application. Phase Inverter tubes need to comply with these specs too, so in many cases a tube which was selected specifically for the use as phase inverter will do fine for V5 too. Please pay extra attention to this.

The recording outputs use a TL072 opamp as well. The programmable master volume is response for the volume present at the opamps. So to prevent overloading the opamp, keep the volume as low as possible. The front mounted output pots have no influence on the signal level at the recording outputs.

Because V5 is the fx return buffer and main output tube it's not a bad idea to look for a lower gain tube for this application as well. If you are using fx with high outputs, then the V5 tube can be distorted, using a lower gain tube, such as for V3 will prevent this.

Please check my Tube Advice page to see my view on different brands.

The signal line of the Triaxis
There is no easy way to draw the signal line so I'm just writing it down. A simple schematic would have been handy, but it's a pain in the ass to create an accurate one. The full schematics can be found at my Mesa/Boogie Schematics Page.

Here we go:
INPUT -> (V1) -> V2A -> EQ -> V2B -> (V4) -> V3A -> PRESENCE -> VOLUME -> TL072A -> DYNAMIC VOICE -> TL072B -> (FX SEND) -> (FX RETURN) -> V5A + V5B -> MASTER VOLUME -> OUTPUT

signal from V5 -> TL072A + TL072B -> RECORDING OUTPUTS

V1A = Input tube for Lead 1, it's only in the signal line when LD1 is engaged.
V2A = Main input tube for all other stages and gain tube for Lead 1
EQ = Treble (plus shift for different voicing), bass, mid
V2B = After EQ section
V4A + B  = Lead 2, it's only in the signal line when engaged.
V3A = Pre dynamic voice
programmable volume of each preset
TL072A = Opamp
DYNAMIC VOICE = programmable 6 band graphic EQ
TL072B = Opamp
FX SEND/RETURN = Only active when used
V5A + B = FX return for channel A and B, main output tube
OUTPUT + MASTER VOLUME = normal outputs, volume does not influence rec outputs
TL072A+B = Opamp (signal comes from V5