Contrasts of Filter Response For Dual Rec Vintage

I don’t have time to type right now, but here are the images from the analysis for a 2 Channel Orange, 3 Channel Vintage on ch 2, and 3 Channel Vintage on ch 3.  The responses of the 2 Channel Orange and 3 Channel Vintage on ch 2 are very close.  Other factors could influence the differences between them.

2 Ch Orange

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Mesa Engineering Dual Rectifier Presence Pot Frequency Plots

I very quickly drew the negative feedback filter controlled by the presence control when using Vintage mode on a 2001 3 Channel Dual Rectifier.  The red line represents the -3 dB half boost mark from the point of the most filtering.  I tried drawing the entire phase inverter, but it didn’t make a difference on the plots, so I removed it and kept it simple.

R2 is being instructed to act as a variable resistor with 11 points to represent each 30 degrees of travel from 0 -10 on the control.  Anytime a part of the plot goes below the red line, the feedback is lessened by quite a bit and those frequencies pass the phase inverter more easily.  The maroon line on each picture is representing the control being at maximum and a massive amount of the frequency band is passing easily.

A Few Changes To Recto Articles

In some of the articles, I made statements regarding a possibility of mild power amp drive, because I had not had time to really dig into the power amp to confirm or deny it, and due to the sound of Modern mode.  The power amp does not overdrive.  If you come across anything relating to this, any descriptions should be discarded if they contradict the following paragraphs.

Modern has some distortion present, but it isn’t clipping distortion; It’s regular ol’ harmonic distortion, which is otherwise known as “the tube sound”.  I don’t have time or energy to get into it, but the power amp is designed to stay as clean as possible, but tubes do not always amplify in a “linear” way.  This adds curvature to the sound waves, distorting them by bending (coloring the sound).

Matching Vintage To Modern

The idea was to take Vintage on Channel 2 and make it sound very similar to Modern on Channel 3.  The results ended up being very close, but I didn’t have the time to completely match the EQ (for the most anal of comparisons).  The overall response was what I was going for and that is achieved. 

Addendum To Modification Articles

The purpose of the article about changing the anode resistor value was to improve the clean channel and smooth the dirty channels.  That is exactly what it will do, but I wanted to talk about conceptual ideas regarding this change.  I’m going to use pedals for illustration and then translate that over to tube amplifiers.

Analogy

I used to build guitar pedals and I’ve spent a lot of time cascading circuits into each other.  While transistors and op amps react differently than tubes, the overall result on dynamics is similar from one component to another.

Bold, Spongy, and Rectifier Selection

On the back of every Mesa Engineering Dual Rectifier, there are 2 switches to select options for the power supply.  One of these is to select Bold and Spongy and affect overall voltage to the amplifier, with Bold being full power and Spongy reducing power by about 20%.  The other switch is used to select the rectification: Silicon or Tube.  Selecting Tube reduces power by nearly 10%, since the tube has losses when it conducts.  There are 4 options for running the power supply:

• Bold and Silicon
• Bold and Tube
• Spongy and Silicon
• Spongy and Tube

Before I get to decisions for selecting power supply options, we should cover some facts and history.  Along the way, we’ll learn basic information about the differences between these options.

Sovtek 5881WXT+/6L6WGB and Ruby Tubes

I ran across this article about Sovteks and how the rating charts for several tube brands compare.  I am needing tube replacements and this seems pretty handy for finding tubes that fall into Mesa’s bias range for their 6L6GC.

I’ve been using Ruby 6L6GCMSTR, which is an excellent tube, and I will likely stick with it, but these others have piqued my interest in differences between the sub-types.

Faux-Fuzz and Classic Rockin’ With A Dual Rectifier

I’ve been experimenting with making Channel 3 a fuzz tone for a long time, but recently got just the setting I wanted.  This kind of setting gets close to “Dazed And Confused” and sounds good with “Purple Haze”.  Being tube, instead of transistor, it’s a little softer, but the Presence  can dial in the edge a person needs for fuzz bliss.

Yellow is the Modern setting as described above with no treble and full mids. Green reduces Bass to 11:00.

The way that old, germanium transistor fuzzes would clip is awfully close to the way the cold clipping circuit works in these Marshall/Soldano types of circuits and the Recto has the bottom end to really sound huge.

I found this Channel 2 Raw setting to be a match for the fuzz tone.  It’s bright, with plenty of bottom, and crunches up quite nicely.  Both dirty channels get cleaner as guitar volume is rolled back, but, like a Tonebender, Channel 3 never gets “squeaky clean”, but it does make a nice half-clean sound, since the treble is flattened.

I hope this helps.  Enjoy.

Dual Rectifier Cold Clipping Mods

I know I previously stated I would discontinue writing about mods, but this one has been in the back of my head for a long time.  A lot of the grind produced by the cold clipping stage is due to the presence of third harmonics (H3).  By forcing the idle point to be so close to grid current limiting, the amount of information loss from the distortion is going to produce it.

Yesterday, I was playing around with the Trioda load line program and found an interesting effect.  With the parameters set about where the resistances would be for V2b, the cold clipping stage, I was adjusting the cathode value.  As the cathode resistance moves from 39k to 10k, the amount of H3 falls to an amount which would become inaudible.

Quiet Practice Or Recording With A Dual Rectifier Series FX Loop and TC Electronic Nova System

While this is written about a 3 Channel Dual Rectifier with an FX loop modification to serial, the basic idea will apply to any amplifier with a serial loop.  As well, the Nova System can be substituted for another device with a speaker simulation.  The speaker sim is in the Utility menu.