Channels 2 and 3 Part 1: Mesa Rectifier Design Concepts

Posted: November 25, 2015 in Mesa Boogie Dual Rectifier, Tube Amps
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I already covered the input stage and clean channel.  I’m not going to repeat the information about the input, except for applications to the dirty tones.

The input stage is followed by a coupling cap and a 2.2 M load resistor.  Most amps made prior to the Recto place the grid resistor between the coupling cap and the load.  The load is usually a potentiometer to control gain.  Mesa eschewed this and it’s one of the innovative ideas that distinguishes the amp from others like it.  I can only speculate the exact reason they made this change, but I suspect stability is the main one.  The resistor to ground also gives a path for stray electrons to prevent popping when changing mode relays.

(Note: There is an omission which will be fixed soon.  The 2.2M load resistor is in parallel to the mode voicing circuit and the gain pot, which changes the load resistor value for Vintage/Modern or Raw.  When my health is better, I will update the article with graphs and plots.)

2.2 M is quite large; an amp generally uses between 470 k and 1 M for a load.  2.2 M would provide a tiny increase to the level of the input stage (less than 1 dB).  It also lowers the overall low cutoff from the coupling cap, but it would already be so low not to really make much difference.  The negative result is there will be noise in the circuit from the resistors being a large value.

By moving the gain control from being directly coupled to the input stage, performance remains solid while switching between modes.  Since a 2.2 M grid resistor is present for Raw (and was present in older models as well), a 2.2 M load may have been needed to prevent stage 2 from loading the input stage when that grid value is selected.  If an early production design had the grid resistors before the load, this makes even more sense.

Voicing For Modes

High Gain - RawDirectly after the load is the first part of the filtering for the modes.  Each resistor has a capacitor in parallel to create a treble boost (high pass).  When Vintage or Modern is selected, the individual resistor/cap combinations are put in parallel to each other.  This makes the resistance reduce to about 519 k and the capacitance adds to 2.082 n.  The cutoff is 147 Hz and all frequencies below that do not receive a boost.

When Raw is selected, relay 6a opens and the 2.2 M/82 pf combination is the only filter in the signal path here.  The treble boost cutoff is moved to 882 Hz.  This reduces the lows and mids considerably to give the amp more of a “Classic Rock” or “Blues” voice.

In combination with the Presence circuit for Raw, all frequencies and harmonics are reduced and the preamp is no longer “High Gain” by the modern definition.  It becomes a vintage high gain akin to a Plexi or JTM-45, Bassman, AC30, or Hiwatt JP-100 with one major difference.

Those old amps were distorting by overdriving the entire amplifier, particularly the power section.  Raw relies on preamp distortion and has the cleanest power amp of all modes.  It still has a great sound for vintage styles and can produce the overdrive without deafening sound levels.

Gain Control

 

Both channels use 250 k pots for the Gain control.  There’s a 1 nf cap from lug 3 to the wiper of each to give a treble boost similar to the circuits above.  The resistance that interacts with the cap becomes smaller as the pot is turned up and it increases the cutoff.

Function of a Treble Boost Cap

When a pot is used without a treble boost cap, the lower regions of the taper will greatly reduce the high frequencies and can sound really dull, because treble has a harder time getting past the resistance.  The cap alleviates that.  The effect is less pronounced as the pot is turned up.  This is the reason behind the information regarding sweet spots in the manual.  Lower settings will appear to be brighter.  Since the high frequencies are coming through, the Gain doesn’t have to be cranked to have saturated upper harmonics and will sound balanced somewhere in the middle regions on the dial.

Treble Boost

The cutoff of the treble boost increases as the pot is turned up: 9:00 is 656 Hz; Noon is 707 Hz; 1:30 is 795 Hz; and 3:00 is 1.4 kHz.  From 9:00 to Noon and from Noon to 1:30, the differences are not that great.  The overall amount of signal coming in is making a bigger difference than the cutoff frequency, as long as the cutoff is in a good spot to boost treble and stays relatively stable, which it does.  As the Gain is increased, the overall amount of signal is increasing and the cap contributes brightness until around 2:30.  At greater Gain settings, the cap makes little difference as a treble boost, because the overall signal is now large and the cut off becomes higher.

The cap contributes to the grainy, spitting sound as the control rounds 2:00 to 3:00.  As shown above, at 3:00 the cutoff is 1.4 kHz.  At that point, the upper harmonics are freely passing right through at a great volume, and the lows and low-mids are still passing more easily than the upper-mids.  I see it in my mind as a small notch just below 1.4 kHz.  Some of the upper-mid is going to be engulfed because of its relative size compared to the frequencies from 1.4 kHz to 6 kHz and the increasing amount of lower frequencies coming through, dominating the sound.  Removing the cap would decrease these upper harmonics, but also makes lower Gain settings muddy.  This is one reason a relay to disable the treble cap would be really useful for greater gain settings.

There is a lot of filtering after this section to control the highs, to defeat them, and to distort the frequencies below 1.6 kHz, but this huge passing of harmonics at higher Gain settings leaves its mark on the tone right at this spot.  To a large extent, it can be dialed out by the EQ and Presence controls, but, to me, it sounds better if the imbalance isn’t being produced at all by keeping the Gain below 2:00.  Well, unless you do extreme metal.  It works great for those genres.

Voltage Division

The grid resistance and the total value of the pot create an initial voltage division, which is the maximum amount of signal allowed to pass the gain control.  When Modern or Vintage are used, the maximum amount of signal allowed to pass is about 32% (519 k and 250 k).  When Raw is selected, the maximum amount of signal allowed to pass is about 10%.  From here, the pot wiper will create a further logarithmic division of the signal.  At Noon, Modern or Vintage is passing 3.2% and Raw is passing only 1%.

(Some modifications being done to the Rectos involves increasing the value of the Gain pot to 1 M.  This allows more signal to pass.  Vintage and Modern would now allow 65% (6.5% at Noon) and Raw would pass 31% (3.1% at Noon).  Some care might be taken to not turn the pot up too high and damage V2a.)

Connected between the pot wiper and the grid to V2a is a 475 K resistor.  This resistor interacts with the internal capacitance of the tube to create a treble cut to prevent Miller capacitance.  This will be covered in more detail in Part 2 when I discuss V2a and V2b.

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

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  4. Shemham says:

    I’ve been recently toying with Mark series circuit design for kicks. I’ve made some interesting observations which relates to SLO and Recto designs as well I believe.
    As is well known, Mark series employs pre-gain tonestack right after the input triode which is derived straight from earlier Fender designs. Some have argued that the pre-gain tonestack is redundant for overdriven tones for two reasons. First, tonestack introduces high signal loss which essentially requires using one triode to restore the signal. Second, as the pre-gain tonestack practically bears only gain shaping function on overdriven tones, it tends to be set within particular narrow setting: bass very low, treble reasonably high, and mids somewhere above noon.
    I wondered that If the tone is only set to one particular setting and kept there, why would I need the tonestack at all? Removing the tonestack would allow removing the recovery stage as well which would simplify the circuit while still having enough gain to drive the lead stages in Mark type lead circuit.
    I played with LTSpice to compare frequency responses and signal levels between my preferred settings with different circuit alternatives to match the curve. I found that the most simple way to create very similar curve was 22nF coupling cap in series with 1Mohm/1nF RC-circuit, followed by 1Meg gain pot. Does this ring bells at all? It is identical to Soldano Avenger treble peaking circuit. If you play with the values, you can turn the values to 470k/2.2nF with 500k Gain pot which is straight from Soldano SLO. Rectifier series effectively has the same circuit with the 2M2 resistor to ground and 250k gain pot.
    This reminded me about discussion of Soldano SLO being influenced by Mesa Mark. While the circuits are very different, I think the first stage in SLO essentially represents one fixed setting from Mark tonestack. While circuitwise, SLO has much, much more common with Marshall 2203/2204, making it rather a hot-rodded Marshall, the first stage of SLO could be tonally influenced by the Mark series in this aspect – or, possibly teaching Soldano there’s no use for pre-gain tonestack in high gainers.

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    • Warpedpig#1 says:

      Hi. Long time, no see.

      I’ve come to believe the information about Soldano being influenced by the Mark series was a troll at work. The SLO is a modded Marshall design.

      In reference to the tone stack between stages 1 and 2, the second stage is indeed for makeup gain. Removing the tone stack would require stage 2 to be altered.

      I think you are correct with the assumption that what actually was the classic Marshall design was a evolution of the Fender design. I believe it occurred after the Blues Breaker. Moving the tone stack and changing stage 2 led them to use the coupled cathode follower to maintain as much gain as possible going into the tone stack at the end.

      Initially, it was two separate inputs using V1’s stages separately. Then they were altered by guitarists during the Plexi years by either jumping the inputs or what’s called the “One Wire Mod”, so as to use both sides in series-parallel. This was further modified by Marshall to become the modern JCM series when stage 1 was fed to stage 2 and stage 3 had the cold clipping introduced. That increased the gain high enough that when the ccf fed into the tone stack the loss was mitigated.

      Really, some loss of gain is not a bad idea. The rc circuits after each stage limits the gain between them to a manageable amount and a narrow frequency range, so the valves aren’t overloaded or unstable.

      I think Soldano recognized the need for attenuation in his design. The second stage loses about half going into 3; stage 3 has less than a gain of 2; and the parallel resistance out of stage 3 reduces the accumulated gain by more than half. Mesa played around with that preamp to achieve their desired tones. There is still loss at the input of the tone stack, as mentioned above.

      The point is: which design is used is a question of the desired tone. Yes, the loss up front is great and requires whatever number of additional gain stages to get the desired drive, but each design has a particular tone. The tone stack will also react differently in each location.

      Moving the tone stack to the end may need a buffer. The ccf does that, but also clips one side of the signal nearly right off at high gain settings and has a distinct effect. The cold clipping with the large resistance, ala Soldano or Mesa, helps shred the entire signal with varying degrees of clipping on each side, in concert with the ccf.

      In any case, if either design is chosen for a high gain amp, it will need about the same amount of triodes to achieve comparable clipping and volume. Even the later EVH/5150 amps use no ccf or cold clipping, but uses several stages after stage 2 to increase the gain. As a comparison, Diezel uses more stages on top of that for the VH4’s highest gain stage. (I believe, haven’t got it in front of me.)

      I’m still ill. My mind kind of aches now. I hope that made sense and was completely relevant to what you seek.

      Thanks.

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      • Shemham says:

        Thanks for sharing your thoughts.

        Yeah, I agree with you. SLO and Mark series circuits bare practically zero resemblance. If you simply add one gain more stage in front of 2203/2204 Marshall with MV, you essentially have a circuit structure identical to SLO. I think it is the most commonly adapted design. Amp builders such as Bogner, Fortin, Friedman also use that same formula, but with even lesser changes to the basic 2203/2204 part. In this aspect, it is refreshing to see some more original designs as Diezels you mentioned, or ENGLs, which have their own thing going on.

        Back to the Mesa Rectifier design, I think the 250k gain pot is actually a really good design choice in this circuit. It seems to allow the 2.2nF/470k circuit to function more effectively. I also think SLO has too much gain up front and 250k pot is much more balanced. I’ve also tested the circuit without the load 2M2 resistor. The low-end became somewhat spongy from that and I quickly put the resistor back.

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      • Warpedpig#1 says:

        Not to insult your intelligence, I can explain the difference between a 1M pot and a 250k pot.

        The voicing circuit in the Rectifiers is the upper resistance in a voltage divider against the lower resistance of the 250k pot. We can look at maximum results with the pot rotated all the way up.

        In Vintage or Modern, the total voicing circuit resistance is 519k. When the pot is 250k, only a maximum of 32% of the signal passes. Under the same conditions, but using a 1M pot, about 52% of signal passes. In both cases, the capacitor on the pot is shorted and doesn’t make anything frequency dependent.

        On a SLO, the 470k grid resistor and a 1M pot would let a maximum of 68% pass. A 250k pot would reduce it to 34%. That’s closer to a Rectifier.

        The lesson, as you noted, is about controlling the signal. Mesa decided a 250k pot produced enough signal for their needs. Indeed it is and works well, even though it seems like a massive loss on paper.

        Side note: Design choices like the voicing circuit and gain pot value are among their contributions to the basic Marshall/SLO design. It’s one example of why the Rectifiers sound different and why they aren’t just SLO ripoffs.

        The Diezel VH4 design kind of takes the initial stages of a Marshall design and marry it to a Mark IV’s cascading design. Maybe not literally, but he eschewed cold clipping by using multiple cascading stages for a massive, firm crunch.

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