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
Directly 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.
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.
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.
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.