June 2016- I realize the flow of this article could use some work.  I have plans to reorganize it at another time.  

We’ve looked at the input and Clean, the Mode voicing and Gain control, V2 and V3EQ and modes and Presence.  I am going to bring it all together and take a less detailed, wider, view of the system as a whole.  I’m concentrating on Channels 2 and 3, since they are the primary identity; the “Recto” sound.

Additionally, I’m going to cover a little bit about the power amp, even though I have not concentrated on it in my writing this year.  I think the intention of Mesa when designing it is misunderstood.

I’m going to be making certain assumptions based on the overall design and historical precedents in guitar amplifiers and Mesa designs.  I could be wrong, but I am presenting it the way I best understand it, based on how everything interacts, and considering the differences between this amp and others much like it.  Mesa tends to tune their amps by ear, so some of the effects from changes may be unintentional or accidental.  That doesn’t negate the actual effects from being valid points innovation on the design.

For filter analysis, I’m using the Okawa website.

The Stages of the Preamp

DR stages illustration

An interpretation of the 4 stages in a Dual Rectifier preamp.

As you can see, I have compared the stages to superheroes.  Stage 1 is really clean, flat, and strong.  Despite this strength, the voicing circuit and Gain control circuit dumps most of the signal to get it down to a more manageable level.  It seems they wanted to give the amp a clean boost in front, but then wanted the level to come down immediately.

Stage 2 is makeup gain with a greater emphasis in the upper mids and more compression and harmonics than Stage 1.  Stage 3 is has a dark tonality and shreds the crap out of half of the signal.  The other half of the signal is cleaner and clearer until it reaches limiting, at which point it abruptly becomes clipped (distorted).

Stage 4 first boosts the result of stage 3 and adds a greater range of frequencies back in.  The other half of stage 4 reduces the impedance and contributes a warmer distortion to the cold clipped part of the signal.

It’s pretty clever.  Mike Soldano designed the original preamp circuit, based on designs used by Marshall, but Mesa found several ways to make their own mark on it, including:

  • The reduction of signal after stage 1 drives stage 2 to a lesser degree.
  • This reduction also makes the cold clipping more sensitive to dynamics, as it can transition out of distortion more easily.
  • The greater B+ voltage increases the headroom across all stages, reducing overdrive and compression.
Boosts

Stage 1 and Stage 4 are nearly identical and actually share the same duty as boosting stages.  Stage 1 is to get the input voltage brought up, while Stage 4 is giving a thrust to the cold clipping.

Either of these could have been made with lower anode resistors and performed about the same function with very little difference in the final volume.  The difference is harmonics and headroom/compression, especially in regards to Stage 4.  I speculate that an amplifier with this level of massive, beastly, distortion benefited by having some clarity in just the right places.

In particular, Stage 4 would need to reintroduce upper frequencies to offset the shelving in Stage 3.  The cathode bypass also adds firmness to the bass after it was just shredded.

Distortion Generation

Stage 2 cascades into Stage 3 to establish conditions for the cold clipping to be effective in the circuit.  There really isn’t another area in the amplifier in which cold clipping would be as effective as a distortion generator.  Whereas Stage 2 is bypassed to create a large gain increase, Stage 3 has extremely low gain and relies on a massive signal on its grid to be effective.

The Cold Clipper
Cold Clipper Frequency and phase

Cold clipping treble shelving.

The anode of the cold clipping circuit has a filter made up of a 100 k resistor in parallel with a 1 nf cap.  This cuts off frequencies above 1.59 kHz.  It is important to realize that no matter what the user does with the controls, treble frequencies are being filtered out of the largest part of the distortion generation.

Treble Boosting

All of the Gain controls have a treble boosting capacitor on them.  It performs its job best when the Gain control is moderately used and is most noticeable from about 11:00 and back.  If a person finds their tone to be too dark, observing the range of this treble boost could be a solution.

Voicing
ModernVintage Voice

Modern/Vintage voicing filter response.

Modern and Vintage use the same voice in the voicing circuit .  The filter cuts off frequencies below 148 Hz.  However, the tone control changes give Modern a brighter timbre on Channel 3.

The differences between Vintage and Raw are subtle.  Switching to Raw changes the cutoff frequency to 882 Hz and increases the amount of signal being dumped to ground by the Gain control.  This imparts the biggest tonal change: a slight, narrow, emphasis between 900 Hz and 1.6 kHz.

Raw Voice

Raw Voicing filter response.

In the preamp, Raw is barely less distorted than Vintage and the volume decrease is much less radical.  When monitored from the FX Send, Raw has a sharper edge than Vintage, due to the attenuating Presence circuit.

The difference in distortion characteristics a person hears through a speaker cabinet is most affected by the power amp.  Since they use the same tone control circuits, Raw and Vintage end up being fairly similar in character up until the P.I..

Out Into The Beyond

Beyond amplifier stages, other major contributions to the personality of this amp are:

  • “Spongy” can drop the B+ voltage level, increasing compression and reducing headroom throughout the amp.
  • There is a choice between a tube rectifier or silicon rectifier.
  • The tube rectifier reduces the B+ voltage, producing an effect similar to “Spongy”, albeit more subtle, and also smooths the lowest and highest of the guitar’s frequencies.
  • The cathode follower will be overdriving at lower voltages.
  • The negative feedback can be switched to different levels (off, variable, or full on).
Power Amp Negative Feedback

Modern is mildly overdriven and has a mid emphasis, because of the lack of negative feedback.  Vintage has variable negative feedback and is illustrated with the Presence completely off.  Raw is the flattest, most filtered mode.  The lowest and highest frequencies seem to be rounded off.

NFB illustration

Illustration of the effects of negative feedback on the power amp’s response.  Not literal.

My illustration isn’t literal or meant to be completely accurate.  It is meant to give a person the basic idea of what each power amp response is when adjusting the negative feedback.

Modern

The Modern power amp response is mid-humped and more easily overdriven, but lacks dynamics.  In the illustration, the top of the hump would be moving up into the threshold of overdrive.  The preamp drive could receive another layer of audible harmonics from the power amp, if driven hard enough, but the power tube bias is keeping the power tubes from creating significant drive.  Any distortion present would be generated by the P.I..

The power amp is filtered by capacitors in the P.I. circuit to keep it stable by reducing high frequencies.  There are no direct, user adjustable, controls for the power amp in this Mode.  Even the tubes’ bias are preset and are not adjustable unless modified.

One benefit of Modern on Channel 3 is the change in the tone stack.  It receives a generous boost in the midrange and the Presence control’s cutoff frequency is shifted up to 2.4 kHz.  This allows Modern to cut through in a mix or performance, but it can become quite harsh, sharp, and thin when the Presence set is too high.  The greater emphasis in the midrange throughout this mode allows for greater amounts of low frequencies to be used before becoming dominant or boomy.  The Presence and/or Treble can be increased to retain the edge and definition in the sound when the Bass is increased, but increasing the lows and highs may shift the tone into sounding scooped, despite all the mid-emphasis.

Modern is great for Metal and modern Hard Rock.  It has an aggressive edge not present in the other Modes.  With low Presence settings a tube-fuzz kind of sound can also be achieved for Doom, Stoner, and other fuzzy styles.

Vintage

Compared to Modern, Vintage has quite a bit of filtering, more headroom, and has a flatter response.  The lack of midrange emphasis in combination with the way the treble is rolled off after the tone stack gives the impression of an increase, or looseness, in the bass.  This is where the treble boost on the Gain comes in handy to retain definition in the overall sound.  The increase in headroom makes the power amp more difficult to push into overdrive.

The Vintage Presence control opens up the low frequencies first.  As it is increased it works its way up to treble frequencies, but this is late in the taper.  However, introducing treble frequencies makes the tone become harsh and can permanently damage a person’s hearing.  When the control is all the way on, it is removed from the circuit.  That forces the feedback filter to switch to working as a high pass filter at 34 Hz and the entire frequency range of a guitar is unfiltered.

I think Vintage works great for single notes or for chords above the 5th fret.  Single notes benefit from the beefy, rolled off, tone.  Chords can sometimes be a bit “squishy” or blurred by this same effect at the lower end of the guitar’s range.  For various Modern Rock styles, Classic Rock, or other styles that do not need super-heavy distortion, the area surrounding 11:00 on the Gain control is a great place to keep an edge on the distortion.

Raw

With Raw, the negative feedback is at the maximum the amplifier can produce.  It isn’t adjustable.  The response is flatter and wider than Vintage.  One drawback of this mode is that the increased negative feedback muffles the sound, but this can be easily overcome with EQ settings.  The Raw Presence control is not part of the power amp circuit.  It works as an attenuator, exactly as Modern does.

Were it not for the amount of negative feedback, the Raw Mode might be comparable to a JCM 800.  The combination of negative feedback and P.I. treble-shelving smooths a large amount of the distortion harmonics out of the signal and Raw has more touch sensitivity than the other Modes.  The Treble and Presence controls can be pushed into higher settings to offset the smoothing without succumbing to the brittleness which occurs in the other modes at higher settings.

Though it is meant to be used for Classic Rock, Blues, and other less aggressive styles of music, turning the Gain up gives a heavily distorted sound.

The Power Tubes

Mesa is famous for their innovations regarding cascading preamp distortion.  Randall Smith has stated he was looking for a solution to getting a cranked sound at any volume when he developed the Mark Series.  I think this same principle was put into play with the Rectifier Series.

It is known that the Rectifier Series has a fixed bias which is operating below 50% dissipation and it is quite a cold bias.  I believe this consideration was made for at least a couple of reasons: the amplifier’s distortion characteristics are (largely) independent of power tubes and the power reductions made by using Spongy or Tube do not cause excessive power amp overdrive and compression, like a warmer bias could.  It also prolongs the life of the tubes.

Distorting an already heavily distorted signal with the power tubes reduces clarity, but it also changes with level and depends on loud volume to be produced.  The cold bias assists in removing the Master Volume (Output control) from the process of creating significant drive.

People mod their amps and seem perfectly happy with higher bias settings.  My comments are not a case of good or bad, just what I think the designer intended.

Phase Inverter

There is one area in the power section which might contribute overdrive when pushed hard: the phase inverter.  The second anode resistor is 91 k, instead of a standard 100 k value.  It’s going to be slightly more compressed than a JCM 800 or a SLO 100 and headroom is reduced.  That said, the negative feedback from the output transformer will constantly be contributing linearity and stability to the P.I. and power tubes when Raw is used or when Vintage has a low Presence position.  In essence, the P.I. would have to be pushed quite hard to distort.

The tubes in the power amp are in parallel and working in a push-pull configuration, 180° out of phase with each other.  This will cancel most of the 2nd Harmonic and emphasize 3rd Harmonics, but the P.I. is not perfectly balanced (and is not meant to be in a guitar amp).  The canceling effect on the 2nd Harmonic is subtly defeated by a very small amount, and is slightly asymmetrical by comparison to conditions if the power tubes were allowed to distort.  The choice of driving a 12AX7 instead of 6L6s or EL34s also allows for the power amp overdrive to be better controlled by the Output control (particularly with Modern mode), but distortion is really an unwanted effect of this design.

P.I. + Power Tubes

Since the P.I. is coupled to the power tubes, the anode resistance reduction to 91 k, as stated above, is a conscious design choice to prevent bias excursion by the power tubes.  Between this, the negative feedback value, and the cold bias, they would need to be pushed much harder to produce a significant amount of distortion.  Again, it appears that the role of the entire power section is to amplify the preamp distortion, though some P.I. distortion could be present at high volume or when the preamp gain and volume are making the signal extraordinarily large.

This topic needs further evaluation to determine the details of harmonics and overdrive conditions.

The Output Control

The Output control has a fixed treble boost on it to allow high frequencies to pass, but at a range limit.  The thing with treble boosts on a volume control is that they have to be turned up nearer the middle of the taper before the low and low-mid frequencies are balanced with the higher ones.

On my Dual Rectifier, the Output has to be past 9:00 before the bass begins to come in very well and evens out the enhanced treble response.  It sounds most balanced between 11:00 and 1:00.

Using The FX Send For Tone Shaping

A work-around for a person who finds that Output range to be too loud is to adjust the FX Send control to bring the volume down a little, but not enough that it makes the signal sound weak or hollow.

Alternatively, increasing the FX Send can defeat the treble boosting to a certain extent, because the volume of the filtered frequencies increases and becomes too large for the filter to cut it as effectively.  Ideally, the Output would need to be between 9:00 and 1:00 for this to work correctly and the FX Send should not be used past 3:00.

When doing this trick, I set the Output first and then use the FX Send like a volume control.  Afterward, the Output can be adjusted, but the overall tonality is already established.  Also, when using higher FX Send settings, lower your Channel Volume controls.

Using The Solo Control For Tone Shaping

The Solo function can be used for tone shaping.  The pot is wired as a variable resistor.  As it is turned up, resistance increases and forces the Output control to raise to a different resistance value.  The value of the Solo resistance also combines with Output resistance below the wiper, causing more volume to pass by.

There is a change in the sound when using the Solo control.  I am not 100% sure why, but this is my best, educated, guess:

Since the Solo is not part of the treble boosting circuit on the Output, increasing it introduces non-treble boosted signal and it also shifts the high pass filtering created by the V4b coupling cap and the load (Output and Solo pots, in this case).  These effects partially defeat the treble boosting and darken the sound.

If a person wishes to practice at a low volume, turning off the Output and using the Solo control as the volume control will give a less fizzy, hissy response with more punch.  It isn’t perfect, since this amp requires performance volume to sound its best, but it can provide a subjectively better tone.

Final Words

I’ve spent this year focusing on the Dual Rectifier.  I hope it has been helpful.  There are still topics about the design I didn’t touch on and I may continue to write about it from time to time, but all the details I wanted to discuss are finished for now.  Good luck to you.

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Comments
  1. Kenny says:

    This is and the other pages on your site are an awesome ressource, really helpful for understanding the interaction of different aspects of my Recto. Thanks!!!

    Like

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