Channels 2 and 3 Part 2: Mesa Rectifier Design Concepts

Posted: December 1, 2015 in Mesa Boogie Dual Rectifier
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Where We’ve Been

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.

Either pot for channel 2 or channel 3 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 easily clip half of the signal. Since it’s forcing most of one half of the input to clip, while providing more 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.

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 harsh harmonics above this treble region being produced. Considering the information in the previous section regarding harmonics, harmonics produced above the fundamental notes will be shunted by a greater degree as the frequency rises.

For instance, a 220 Hz note will produce 2nd and 4th order harmonics (440 Hz and 880 Hz) normally.  A 440 Hz note will produce the 2nd harmonic (880 Hz) normally, but the 4th order harmonic (1760 Hz) is being attenuated. An 880 Hz note will not even produce the full amount of 2nd harmonic. It produces none of the 4th harmonic.

 

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.  At the same time, as the signal voltage reduces when the note decays, the harmonic content will reduce and the amount of signal being overdriven is not as apparent. A decaying note will transition from having both sides clipping heavily, to ever softer clipping on the negative cycle as the signal leaves cutoff. Then the negative cycle returns to being under the headroom limit and only the positive going cycle is clipping.

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 textbook would 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.  Indeed, 1.6 volts gain is indeed very low.

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

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