Channels 2 and 3 Part 2: Mesa Rectifier Design Concepts

Posted: December 1, 2015 in Mesa Boogie Dual Rectifier
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So far, we’ve looked at the input stage and Clean mode, and the voicing and Gain controls for Channels 2 and 3.  When we left off, the signal had been filtered, dropped down, filtered again, and was exiting the Gain pot by way of a 475 k grid resistor into Stage 2 (V2a).

Grid Resistor

V2aDRAside from controlling incoming current, the grid resistor serves to adjust the highest frequency allowed to enter the amplification stage by creating a filter due to the tube’s internal capacitance (around 1.6 pf).  Additionally, this stage has a 20 pf capacitor in parallel to the tube.  This increases the total capacitance to 21.6 pf.

The pot by itself has a maximum output impedance of 62.5 when the value of pot is at 50%.  Either way toward the end of its taper, it goes down from there.  That is added to the large 475 k grid resistor to get the total resistance for the high frequency roll off.  The frequency of the roll off is going to be between 15.5 kHz and 13.7 kHz under almost all circumstances.  The cut off is set high enough to not impact the guitar frequencies whatsoever.

The one exception is when the pot is rotated completely clockwise.  Then, the resistance comes up to account for the voicing circuit in parallel to the pot.  It still never comes down below 8 kHz, so the “guitar” frequencies are still unaffected, but some “air” will be gone.  The Gain isn’t supposed to be run that high, so it shouldn’t be an issue.

V2a

V2aDRStage 2 has a 100 k anode resistor and a 1.8 k cathode resistor.  The cathode resistor has limited bypass from a 1 μf capacitor in series with a 100 ohm resistor to create a gain boost for frequencies above 101 Hz.  The 15 μf capacitor (used in Clean mode) is disconnected.  This stage has gain of 59.3 V (35.5 dB) for bypassed frequencies and gain of 26.3 V (28.4 dB) for unbypassed frequencies.   This stage is biased at about 2.25 V.

Whereas this stage has a stepped boost for the Clean mode, the dirty channels have an emphasis in the mid range and V2a progressively reduces the lowest lows and highest highs that it is capable of reproducing.

When the signal exits Stage 2, it is met by the 475 k Stage 3 grid resistor and the 1 M Stage 2 load resistor.  These resistors create a potential divider.  67.8% of the signal passes to Stage 3.  The .022 coupling cap cuts off frequencies below 15 Hz.

Small Tangent

The 100 ohm resistor sums with the 1.8 k resistor for AC signals.  Modifications, such as the Pre-500 Mod, remove it.  If it is removed, the bypass frequency shifts up slightly, to 105.9 Hz, and the unbypassed gain increases slightly to 27.1 V (28.7 dB).  So, it would lose just a little bit of bass, but the level change is slight.  Changing the cathode cap along with, or instead of, this resistor will make more of a change to the sound.

V2b

V2bDRI believe this circuit is the heart of the preamp for the Rectifier Series and the Soldano SLO 100 that preceded it.  It’s a unique, simple arrangement to get a lot of distortion with massive harmonics.

V2b has a combination of an extremely cold bias, treble shelving, and a somewhat undersized load.  It is not bypassed with a capacitor, unlike the last two stages.  The function of this stage is to asymmetrically overdrive and/or distort any signal coming in.  It is really difficult to get a “clean” tone from this stage, because one half of the signal coming in the grid is less than .25 volts (3 milliamps) from cutoff, which occurs upon the current reaching zero.

Cutoff causes a softer, more compressed, distortion compared to grid current limiting (when the negative going cycle reaches the limit allowed by the grid).  The island effect allows the cutoff signal to continue trying to exceed the limits, in turn distorting more of the positive going signal without the anode actually moving further positive.

Stage 2b load line

V2b load line.

The line where the bias sits is curved and illustrates that this part of the load line will not produce a high fidelity reproduction (it is not linear).  Guitar amps are low fidelity, so that is not a bad thing.  In fact, since it’s forcing most of one half of the input to distort, while providing a lot of headroom to the other side, there is 49% 2nd harmonic content when about 5 volts of signal is input to the stage and drives it into cutoff.  That is a substantial amount.  It’s kind of like an octave effect at that magnitude.

Where some amps (or pedals) will keep clipping and limiting the signal to the point of (basically) shredding the signal, V2b is capable of creating a 3 dimensional sound, which combines a harsher distortion and a lighter, more defined tone, with a minimum of parts.

Treble Shelving The Harmonics

The cap in parallel to the anode resistor is electrically equivalent to the bottom lead being at ground.  The two create a passive shelving filter, which cuts off frequencies above 1.6 kHz.  (This cut off point shows up a few times throughout the amplifier.)  The shelving is necessary to prevent the harmonics above this treble region being produced.  They’re harsh sounding; a guitar’s higher pitched notes would be deafening.

Asymmetry produces a louder amount of even-order harmonics and reduces the odd-order ones.  At the very least, any frequencies below about 800 Hz can potentially produce the full effect of maximum amount of 2nd harmonic, if nothing else.  Above that point, signal frequencies will have less harmonics, because the treble shelving will kill it.

I think the above information is important.  There is a lot of discussion about what a great rhythm amp a Recto is, but some feel it is less suited for soloing.  Simply put, the signal is less distorted as the frequency increases in the upper mids.

An open E5 power chord will have harmonic content up to the 6th Harmonic.  E5 at the 7th position will have contain up to the the 3rd Harmonic.  The 12th fret, first string, E is going to have 2nd harmonics, but the 3rd harmonic and higher will be rolling off.

A Preamp Tube As A Distortion Generator

At the same time, thicker strings at a longer length have the potential to create the largest voltage and that potential decreases as the string diameter or length become less.  If the signal voltage reduces, the harmonic content will reduce and the amount of signal being overdriven is not as apparent, because the stress being produced is not as great.

Under moderate Gain settings, a decaying note will transition from having both sides heavily distorted, to ever softer overdrive on the negative cycle as the signal leaves cutoff and returns to being under the headroom limit, and then, eventually, only the positive going cycle is producing heavy distortion.

There are other factors beyond this circuit, but the dirt this amp creates on its own begins here, with V2a pushing V2b.  When a person boosts the amp with a pedal, they are trying to keep this stage working as a distortion generator on the negative going cycle.

Load

The last part of this circuit is the load.  At 330 k, it is kind of a low value.  Most loads are closer to 475 to 1 M or more, with 1 M being the “traditional” value.

A text book might say to make the load at least 10 times higher than the output impedance of the stage.  I believe it is undersized on purpose to use a small amount of loading from the next stage to keep the gain down and for the impact on tone.  The 1.6 volts gain is indeed very low.  Since the distortion can occur without pushing the tube hard, keeping gain low helps retain headroom for the side of the signal with less (or no) distortion.

Thanks for your time.  I hope this was informative and good luck too you.

 

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  1. […] Channels 2 and 3 Part 2: Mesa Rectifier Design Concepts […]

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