Dual Rectifier Channel 3 Modern Tone Stack

Posted: January 7, 2016 in Amp Settings, Mesa Boogie Dual Rectifier, Tube Amps
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EQ Schematic

Schematic of tone stacks for Channel 2 and 3.  Note the Presence control circuit and the value differences for its resistors

When Modern Mode is selected on Channel 3 of a Dual Rectifier, an additional capacitor is added to the circuit.  This changes the capacitance value from 500 pf to 680 pf.  By doing this, it moves the frequency from which the treble swings on axis from 1.27 kHz down to 936 Hz.

The effect of this change is twofold: 1) The upper mids receive a greater emphasis in the sound; 2) The overlap of range between the Mid control and the Treble control forces the Mid frequencies to move up in dB, effectively giving a mid-boost throughout the range of the Mid control.  The change also prevents the Mid control from being as scooped at the minimum setting by about a maximum of 2 dB between 600 Hz and 800 Hz.

Channel 3 set to Modern has no negative feedback in the power amp section.  At performance volumes (loud), the power amp contributes relatively clean harmonic distortion.  Imagine it as lightly shaving the sound into a greater curve (“clean” tube distortions).  I believe Mesa changed the tone stack to accent the upper middle voice of the guitar to help it to come up forward from the low, sub-octaves and the high 1st and 2nd octaves which are shredded to the point of being a searing layer of sound over the core sound, depending on Presence control settings.  At low-to-moderate Presence, the upper frequencies are being pulled down in level, leaving the boosted mids free to dominate.

Ch 3 EQ Noon comparison

Fig. 1: Channel 3, Modern EQ curve compared to Channel 2.

Fig. 1  Shows a graph of the difference between Channel 3 (red) and Channel 2 (orange) when the controls are set to Noon.  The Treble is attempting to affect frequencies down beneath 100 Hz, but at these settings, the Mid and Bass controls prevent that from happening any further down than around 350 Hz.

The other change to note is the post-EQ filter on Channel 2 allows the Modern or Raw Presence control to pivot at 1.6 kHz.  The post-EQ filter on Channel 3 Modern or Raw allows the the Presence control to pivot at 2.4 kHz.

The graphs cannot account for the Presence control, but there is a significant difference.  The combination of the changes in the Presence circuit and the tone stack, by comparison to Channel 2, makes it 3 dB louder at 2.4 kHz under most circumstances and can potentially cause a greater amount of fizz to come through if too much Treble is used.  This same effect allows for greater Bass settings to be used before the overall sound becomes overwhelmed by it.

I believe the combination of those two factors contributes to the reputation of Modern as being scooped.  Greater Bass settings will cause the majority of Mid-frequencies to be obscured when the fundamental notes and harmonics of the lower frequencies become dominant.  The harmonics from the Mid control’s frequencies will be audible due to the higher treble roll-off, even if their fundamental notes are being obscured.  However, at moderate control settings, those Mid-harmonics can never match the volume of Treble control’s fundamental frequencies.  So, as before, using too much Bass is not a great idea if you are not going for a scooped tone.

Another consideration is that Modern is the only mode without any negative feedback from the transformer to the P.I..  Since the power tubes are biased cold to prevent significant overdrive, the majority of drive would be produced by the preamp.  The power amp will add another layer of complex harmonics to the already complex, harmonically rich, preamp drive, which always has a dip somewhere in the middle range frequencies.  This last layer of harmonics will be most audible in the peaks controlled by the Bass and Treble controls, as well as the high frequency shelving controlled by the Presence, if it is set high.

Ch3 Mid Sweep

Fig. 2: Mid sweep on even numbers from 0 to 10. The Noon Channel 2 settings are still shown in orange (between blue and white lines).

The simple fix for the above is to turn up the Mid control if you use a lot of Bass, but that may require some tweaking with the Treble and Presence to control sharpness.  Perhaps, just as I suggested before, set the Mid and Treble first.  Then add in the Bass and Presence to taste, but before they become too dominant.  I find that having the Bass at Noon (5) works great almost all the time for Channel 3 Modern.

Fig. 2 is an example of a mid sweep.  Each line represents a snapshot of the curves at an even number between 0 and 10 for the Mid control. These settings are laid over Fig. 1 (red and orange mid-line curves).

Ch3 Scoop Comparison

Fig. 3: Scooped setting comparison.

Fig. 3 illustrates the difference between EQ settings for a nearly exact set of curves.  Channel 2 (orange) is set at: Bass 4.5; Mid 3; and Treble 5.7.  Channel 3 Modern (red) is set at: Bass 5; Mid 2; and Treble 5.3.  The mid-boost on Channel 3 Modern allows for a lower Mid setting to match the curve of the same setting on Channel 2.

Ch3 Bass Sweep T-55

Fig. 4: Modern Bass Response.

Where the higher frequencies diverge in the Treble range, the Presence controls would be attenuating with a shelving filter.  Modern would need a lower Presence setting to be close to Channels 2, but an exact match around 2 kHz is not possible.

The last set of figures (4 and 5) reflect the sweep of the Bass control with Treble at 5.5.  These figures provide an indication of what kinds of change the Bass control exerts on the other controls as it is adjusted.

Ch2 Bass Sweep

Fig. 5: Bass response of all other modes.


Fig. 4 is for Channel 3 Modern from 5 to 0.  The orange line represents the regular,non-Modern, tone stack with Bass at 5.  Fig. 5 is a separate Bass sweep for the regular tone stack, using the same EQ as Fig. 4.  It is included to show the contrast between the tone stacks.  Open both pictures in other new tabs and go back and forth to really see the difference.



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