Amp Lab: Mesa Boogie Strategy

This Article Was Originally Published On: February 3rd, 2015 #Issue 15.

Referring to Fig. 1, we see that the chassis neatly laid out.  It is amazing to see so much circuitry packed into such an area, yet still preserve the things that make tube amps what they are, such as large power supply capacitors (the large blue canisters) and the large high voltage capacitors (the large orange devices seen in the upper left hand corner).

Powering up the amp, the user is greeted with a blue glow through the vents. Some users may like this detail. Other users may not like this detail. I would like an option to switch that glow on or off. [I personally would turn the glow off.] I am an old-school, jewel light fan (my first Boogie tube bass amp love was the WalkAbout). But your preferences may vary.

The Tone Stack

Let’s kick things off with a quick look at the Strategy’s frequency response. Fig. 2 shows our “all controls at noon” setting vs. the “optimally flat” setting. The all controls at noon exhibits a flat low-frequency response, with a mild treble bump of less than 2dB. Whereas, our measurements show a +/- 18.8dB 20Hz – 20kHz frequency response with all controls at noon, we see a frequency response of +/- 3.3dB in the important 80Hz – 8kHz range with all controls at noon, which we were able to improve to +/- 2.7dB as our optically flat setting 80Hz – 80kHz.

FIG. 2 All Controls at Noon vs Optimally Flat

There is one “trick” that Mesa/Boogie has mastered to perfection. I call it “Tone Control for the Masses.” What do I mean? There is an EQ option in this amp for literally everyone. If you are averse to using EQ, you can bypass, or otherwise set all equalization features to a neutral position. If you like complex EQ capability, it’s there. If you are somewhere in between, there are EQ options in there for you, as well.

I conceptually break the EQ layout of this amp down into four sections, running from simple to complex in terms of user interaction (not electrical circuitry). The four sections include input, Voice, tone stack, and graphic. It does not matter whether you like your tone “flat,” whether you like simple one-knob tone controls, subtle control or flat-out EQ craziness. Mesa has you covered. Let’s do a quick drive-by.

The Input EQ

There are times where you do not want to muck with an otherwise great tone. However, you feel that the top end needs goosed a scosh brighter or darker. Take a look at Fig. 3. We set the tone stack controls to their noon position, set the Voice flat and defeated the graphic EQ sections (more on that to come). Located just above the single ¼” input jack is an input select switch. This switch is a 3-position switch that allows a user to select between Active (10 kOhm input impedance 200Hz, 200 mVrms Sin), Normal (630 kOhm input impedance 200Hz, 200 mVrms Sin) – this is the passive setting, and Bright.  The Bright and Active inputs are the same except for a pad on the Active input, as illustrated in Fig. 3.

FIG. 3 Active v. Passive v. Bright

As Fig. 3 illustrates, the Bright setting represented by the red trace has the most extended top end, which should help passive instruments bring out the harmonics and a more “lively” response. Correspondingly, the Normal setting (passive mode) represented by the black trace provides the most top end cut. This can be used to great effect, for instance, where a darker tone is desired, to tame an otherwise bright room or bright cab, or where a fuzz or distortion pedal is to be used.  Fuzz and distortion pedals bring out strong high-order harmonics. This means there is a lot more energy in the higher frequencies. If the amp is normally bright, all the additional harmonics of the fuzz/distortion box can make the amp sound brittle, or buzzy, instead of overdriven. If your sound is buzzy, try the Normal setting.

The Active setting represented by the blue trace sits between the Normal and Bright settings, as far as EQ goes. The Active input also pads the input signal. As a result, the Active setting should provide the highest signal input before clipping, and represents a nice compromise for basses with a strong output.

Thus, for those of you who are adverse to EQ, you get three options, all at the flick of a switch. I think this is elegant and effective.

Voice

Referring to Fig. 4, the Voice control is a single-knob control that provides “presets” for low end, mids, and treble. The single knob provides five discrete steps. Voice 1 (fully CCW) represented by the black trace provides boosted low end and a mid scoop. Voice 2 represented by the blue trace provides a subtle low end boost with a mild dip in the mids (around 300Hz). Voice 3 represented by the red trace is designated a “flat” setting. Use this setting to bypass the Voice control. Voice 4 represented by the green trace exhibits less low end compared to the previous Voice settings, but provides a gentle treble boost relative to the previous Voice settings. Voice 5 (fully CW) represented by the orange trace provides a similar low end response to Voice 4. However, Voice 5 also includes the greatest treble boost of the Voice settings.

FIG. 4 Voice

Notably, the Voice control affects the lows, mids, and highs differently than the EQ characteristics of the Input control switch. As such, these controls can be used alone or together with great effect, in as simple a user-interface as one could hope for.

Tone Stack

Taking a look at Figs. 5-7, we illustrate the tone control sweeps for Bass, Mid, and Treble, respectively. The Bass and Treble are “active” controls, whereas the Mid is “passive.”  Look for a future From the Bench column to delve into the details of active/passive designs.

FIG. 5 Bass sweep

FIG. 6 Mid sweep

FIG. 7 Treble sweep

Graphic EQ

A 3-way switch is provided just to the right and above the Master volume knob. This switch provides settings for Graphic EQ in, Graphic EQ bypass, and Graphic EQ assigned to a footswitch.

EQ sweeps of the graphic equalizer section are illustrated in Figs. 8-16 for. The EQ points appear to be thoughtfully positioned. For instance, the Graphic EQ provides low frequency controls at 33Hz and 80Hz, which nicely bookend the (approximately) 55Hz control provided by the Bass knob. The low mid controls at 150Hz and 300Hz and mid controls at 600Hz and 900Hz sit nicely with the 300-400Hz range of mid scoops of the Voice control and the Mid tone control. The high end controls at 2kHz, 5kHz and 8kHz round out and compliment the Treble tone control, which is centered just north of 3kHz.

FIG. 8 Graphic sweep 33Hz

FIG. 9 Graphic sweep 80Hz

FIG. 10 Graphic sweep 150Hz

FIG. 11 Graphic sweep 300Hz

FIG. 12 Graphic sweep 600Hz

FIG. 13 Graphic sweep 900Hz

FIG. 14 Graphic sweep 2kHz

FIG. 15 Graphic sweep 5kHz

FIG. 16 Graphic sweep 5kHz

Output Power

Sit tight with me on this one folks. Many amps start showing signs of distortion and clipping when the signal hits “rails” – i.e., minimum and maximum voltage levels that the signal can swing. Maximum power is thus often selected by finding the point where the peaks just start to clip and flatten off.

The Strategy output starts altering the shape of the wave from a pure sine wave to something less symmetrical before the peaks are affected. There is a slight “kink” at the zero crossing. This is not the “wave shaping” that other manufacturers use to limit the hard rails in some class-D amps. Rather, it is something quite different. From my observation, this kink, which is only really apparent at high output levels, appears to be a result of the bias setting, which tends to run a bit cold on these amps. Apparently, the bias was determined by ear for the richest tone. See the review of the Mesa/Boogie Prodigy in this issue, as the behavior of the power section is similar.

Take a look at Fig. 17. This is the Strategy at 7.5 % THD+N at 4 ohms. Note that right around the zero crossing, the onset of distortion becomes ever so visually evident. See the Prodigy review for a more detailed illustration of the onset of distortion in the output of these amps, as the behavior is similar.

FIG. 17 Scope Monitor 1kHz 7.5 percent THD+N

In our continuous power testing, we observed that the THD+N spikes, due to a “kink” that appears in the output signal wave shape. This kink appears before the signal level reaches its maximum amplitude set by the power supply rails. Unfortunately, the automated test module that is built into our Audio Precision (“AP”) test software latched onto this early “kink,” causing the test to terminate before an accurate output power measurement could be recorded. As such, for both the Strategy and the Prodigy, we tested “old school” – with an oscilloscope, meter, and hand calculator – for the continuous power. The issues with the continuous power module did not seem to affect our burst power module, so we were able to effectively utilize our standard AP tests for burst power.

At 8 Ohm, we measured continuous output of 517 watts 7.5% THD+N filtered 20Hz – 20kHz; 315 mVrms 1kHz sin Input, with a burst power of 655 watts 7.5% THD+N filtered 20Hz – 20kHz, 1kHz CEA 2006.

At 4 Ohm, we measured continuous output of 495 watts 7.5% THD+N filtered 20Hz – 20kHz; 315 mVrms 1kHz sin Input, a burst with a burst power of 622 watts 7.5% THD+N filtered 20Hz – 20kHz; 1kHz CEA 2006.

At 2 Ohm, we measured continuous output of 561 watts 7.5% THD+N filtered 20Hz – 20kHz; 250 mVrms 1kHz sin Input, with a burst power of 708 watts 7.5% THD+N filtered 20Hz – 20kHz; 1kHz CEA 2006.

To see how the plot of Fig. 17 translates to harmonic content, refer to Figs. 18, 19, and 20. We drove the input with a stepped input signal, stopping when the output reached 1% THD+N through 8% THD+N. We show 6%, 7% and 8% herein. Notably, the 6% THD+N exhibits a strong 5^th harmonic. This feature is indicative of cold tube biasing. The second-fourth harmonics are relatively weaker than the 5^th harmonic, and generally the same output level. Pushing to 7% THD+N, the 5^th harmonic still dominates. However, there is not a clear trend of odd-order harmonics exceeding the preceding counterpart even-order harmonic. At 8% THD+N, we see even stronger separation between the odd-order harmonics and the corresponding even-order harmonics that exceed the preceding counterpart even-order harmonic.

FIG. 18 Distortion product ratio 6 percent THD+N

FIG. 18 Distortion product ratio 7 percent THD+N

FIG. 19 Distortion product ratio 8 percent THD+N

It is clear that the emphasis here is on an amp that is designed for handling transient bursts with large amounts of power. Seeing 600+ watts from a tube amp, even if it is in burst mode, is a real feat. I have no doubt that this amp has the potential to sound massive.

Bias

One of the cool things Mesa did with the Strategy is to provide a bias adjustment that is intuitive and easy to use. Mesa is known for fixing the bias in their guitar amps. This makes re-tubing plug and play. However, Mesa claims that KT88 tubes vary significantly in current draw. My own experimentation with KT88s bears out the same conclusion. KT88s can vary all over the ballpark.

Referring to Fig. 21, fortunately, despite eight tubes, the bias is set by two potentiometers, each potentiometer allocated to a bank of four KT88 tubes. The potentiometers can be set with a jeweler’s screwdriver. Simply set the bias switch to bias, and turn the potentiometer until an LED in the bias section goes black, then twist the potentiometer exactly ½ turn clockwise. Bias for that bank is set. Repeat the above procedure for the other bank.

FIG. 21 Bias

Tube Care

As mentioned, this amp uses KT88 output tubes. These tubes can provide rich tone and extended service life, if properly cared for. In this regard, it is important to follow a couple of “best practice” procedures.

1.) Always allow several minutes for the tubes to warm up before flipping the Standby switch. Power on the amp, then start unpacking your instruments. By the time you unpack your bass, set up your stand, and tune, you will be good to go.

2.) DO NOT flip the Standby switch while you are playing. This can cause a good amount of current to flow through the amp, which could ultimately damage the Standby switch and cause other electrical problems that you do not want to deal with, especially before a gig. Got it? Good.

Multi-Watt ^TM

The Strategy features a “multi-watt” feature implemented as a Full/Half/Low Power switch that allows the user to select a desired power range. Referring to Fig. 22, the gain of the Strategy is illustrated for each switch position. In the Full position, all eight KT88 power tubes are utilized. In the Half power mode, only four of the KT88 tubes are utilized. When using the Half power position switch, you can actually gain a bit more headroom by switching your cab to the next lowest impedance setting on the back of the amp. That is, plug an 8-ohm speaker cabinet into the 4-ohm jack, or plug a 4-ohm speaker cabinet into the 2-ohm jack. The Lo setting utilizes only two of the KT88s. The Multi-Watt feature is discussed more fully in the Prodigy review also appearing in this issue.

FIG. 22 Gain Full, Half and Lo Power

Mute/Play/Silent Record

Mute is a “been there, done that” feature that we all know. However, the Strategy features a 3-way toggle switch that performs Mute, Play and Silent Record. Silent Record mode enables using the pre-amp and DI without using the power amp. This allows the Strategy to be used without an attached speaker cabinet. This is an incredibly nice feature that leaves you scratching your head wondering why every tube amp does not have this capability.

Solo

The Strategy stacks on top of all of its features, a Solo capability. The Solo control provides a pre-selectable amount of boost. The Solo control may be tied to a footswitchable control for additional flexibility. The Solo feature is discussed in greater detail in the Prodigy review.

Conclusion

I cannot escape this review without a mention of the manual. What a treat! The manual is well-written, well-organized, informative, yet concise. I think in a moment of temporary insanity, the good folks at Mesa/Boogie forgot that amp manufacturers have historically put out no manual, or worthless manuals. Respect.

The variable-speed fan, no-load auto-sensing on the speaker jack outputs, the Solo feature, Input Select switch, and a host of other features make this amp a solid contender for the gigging musician, for the studio owner, for the bedroom player, and just about everyone in-between.

1-5 (unacceptable to impeccable)

On-bench

SONIC PROFILE:

Low: Powerful, warm and rich; great dynamic headroom
Mids: Wide range of tones available; warmth with articulation
Highs: Rich and full; harmonically complex; can dial in a little or a lot