Amps, Pickups, Cables, Capacitance, and Magic Mojo Pt1

Posted: December 8, 2014 in Cork Sniffer Heresy, Tube Amps
Tags: , , , , , , , , , , , , , ,

My current main gear is a LTD AX-2E and a 2006 Mesa Dual Rectifier.  I’ll speak briefly about each:

The LTD was second-hand and had EMG 81/85 when I acquired it.  I later swapped the EMG out for Duncan’s JB/Jazz set.  This is probably the best “cheap” guitar I’ve ever owned.  It’s odd shaped, but the ebony fretboard with neck-through design is awesome.

The Mesa DR is their original three channel design.  It’s often vilified as a less desirable amp than the 2 channel originals; especially now that the new design is out.  My amp is not modified, but I may jump the effects loop to make it serial.  I use Clean, Raw, and Modern with Bold and Vacuum settings.  It’s a trade off.  In a few years, I might switch to a Reborn Recto, just to have silicon on the Modern mode and 50W on the Raw.

I get a good clean sound and anyone who doesn’t hasn’t read the booklet or doesn’t understand the intent.  The capacitor at the top of the tone stack makes the mid control work much differently than most amplifiers.  To get a normal EQ curve, the mid needs to be turned back to 7-8 o’clock.  Turning it up further begins boosting the signal and it ends up becoming an edge-of-breakup blues sound.  Is it a magical clean that takes me up the stairway to heaven? No, but it’s damn good.

Channel 3 also has a slightly different tone stack when Modern is selected.  The presence control becomes a roll-off for the upper harmonics, the frequency of mid scoop travels backward in the low-mids and raises in amplitude by about a decibel, the frequencies above the knee of the mid scoop shifts up in amplitude, and the lack of negative feedback makes the gain (signal amplification, not distortion) increase significantly, while reducing the low pass frequency of the overall sound passed on to the power amp.

I bring these up to dispel some myths about the Modern mode.  The Dual Rectifier produces a LOT of bass frequencies and the lack of negative feedback increases the amount of gain in the phase inverter, which makes it and the power tubes distort more easily.

The following paragraphs assume that BMT are set at Noon.

By changing the value of the cap at the top of tone stack from 500p to 680p, it does a couple of things: the mid scoop center shifts from 550 Hz to (roughly) 470 Hz and makes the low-bass frequencies appear to be greater even though that band hasn’t changed, and it greatly enhances the upper mids.  In fact, a slightly more upper middy emulation of the flat Vintage EQ can be set by dialing the mid down to between 9 o’clock and 10 o’clock and turning the treble up a couple hairs past noon.

However, the high frequency content is being cut much sooner due to the lack of negative feedback.  I don’t know the exact numbers, but I would be incredibly surprised if the low pass filter of Modern mode is higher than 2 or 3 kHz.  This is a good thing.  The higher gain is producing massive amounts of harmonics and the attenuation in the low treble range reigns that in a bit and also prevents the Miller effect.  The high-cut style presence control now has the role of determining how much more you want to remove, rather than how much you want to add.

The next entry will focus on various complications of adding or modifying the things between the guitar strings and the amp, and also a bit about tubes.


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