Archive for the ‘Modification’ Category

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. (more…)

This week, I’ve come across two great sources of information, ideas, and inspiration: (more…)

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.


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. (more…)

I had just a few thoughts come to mind regarding modifying amps, modifying EQ, or creating your own.

It’s a fact that Mesa Engineering uses two modified forms of the Marshall tone stack for their Dual Rectifier series.  Compared to a classic Marshall tone stack, the Vintage/Raw tone stack moves the center of the “Mid” control down in frequency and has more attenuation.  This carves out some of the meatier part of the guitar frequency to make room for the massive lows and a high-mid emphasis.  The tone stack enabled for Channel 3 Modern has boosted upper-mids, but has a very similar, low-to-low-mid frequency response. (more…)

I previously wrote about my trials of playing guitar while dealing with a serious illness.  With my 25.5″ scale length, LTD guitar, I’ve now found a medium point of balance.  When I am able to play, the guitar nearly plays itself.

After more than a decade of doing my own setups, I’d stopped measuring them and would do it mostly by feel.  I did it that way for years.  For the problems I have now, I need to be able to easily adjust anything if the guitar shifts from temperature or humidity changes.

At the time of the previous article, I had been thinking in very abstract terms regarding tension and height; trying to imagine the physics at work.  What I’ve ultimately found is that I’d made the height so low and relief so flat, the neck did begin to backbow and warp.  I corrected the truss rod, increased the tuning, and set the guitar aside to settle in.  When I came back to it, it got a setup similar to the factory specs and then some adjustments were made. (more…)

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. (more…)

Wow.  After most of one year devoted to my favorite amplifier, I don’t have a regular topic to cover, now.  Toward the end of 2015, my health declined pretty dramatically.  I ended the Dual Rectifier analysis early, because it was taking too long to put the articles together.  Aside from helping people understand the amp or tweak it, my intention was to assist DIY circuit designers, by highlighting interactions between the preamp stages and the key changes Mesa made to the basic design as established by Marshall and Soldano.

Since this version is now obsolete, I felt there would be little harm in sharing an analysis.  I haven’t done this for people to copy the design in whole.  Since the schematic is available in dozens of places, that could already be done by anyone who would want to.  I want people who are making their own amplifiers to see what this example does and decide if anything from it should, or should not, be a part of their own design, or if a modified form could be used.

When my health became even worse than it already was, I was stopped dead in my tracks.  By the time I could sit at the computer again, the inspiration was waning; not by lack of my want to write it out, but by the effects of the disorders I have to contend with.  It’s a very difficult hobby and I don’t want to make mistakes if I’m trying to help others.

Left Undone

I had not originally planned to write about the power amp, but some of it was relevant to analyzing the preamp modes.  I put that into the “Overview” article, but there is more that could be said.  I’m not sure I’m going to be able to do it.  In particular, the negative feedback could use a more detailed analysis to determine exactly how much stability is being added.

The other thing is the power supply.  The effects from combinations of different settings of the power and rectifiers would help with understanding the compression and headroom designed into the stages.  In fact, with only a very quick look, I noticed that V1 and V2 are probably more compressed and act like a limiter compared to tubes upstream, but it would need an analysis to be sure of that and also the extent of it.


I’ve had a request to analyze the Mesa Boogie Mark V.  I might analyze select parts of it.  At this time, I don’t have the energy to go into massive detail and I don’t own a Mark to test the results.  I could post it as “just hypothesis”.

If I run into my own problems or cool, quirky tones, I’ll post about solutions or tweaks.  I also have one my old pedal designs to tinker with and some DIY ideas to look through, but I am not able to be in a hurry with any of it.

I have never had a lot of money.  My only major investment toward playing music has been my Dual Rectifier.  Guitars costing over $800 have been out of my budget and I figure the same thing holds true for many other people.  Fortunately, mid-priced, mass produced guitars are made with higher quality than just about ever before.

When it comes to upgrading inexpensive guitars, the most important thing is to make sure the neck is well made and the frets are properly installed.  Any hardware or electronics can be easily replaced, but the cost of a major fret job or replacement defeats the purpose of upgrading an otherwise good guitar to be great.

For many years now, I’ve preferred Gibson-style guitars, or at least humbucker equipped guitars, at a more affordable price.  Epiphone G Series, LTD, Schecter C Series, and the like are along the lines of what I could afford and like.  I’m going to speak mainly about these types of guitars, since that’s where my most recent experience is.


This is in regards to the Presence circuits for a 3 Channel Dual/Triple Rectifier and the Roadster/Road King.  I do not have information on the Reborn or miniature versions, though I suspect they kept it pretty much the same.

Mesa came up with a clever Presence circuit for the 2001 3 Channel Rectos, which carried over to the Road King and Roadster on their relative channels.  For the sake of flexibility, the Presence pot is flanked by relays to swap the function between modes.  The modes are used to determine power amp response due to negative feedback and the amount of distortion it is capable of producing.  From most to least: Modern, Vintage, and then Raw.

Using Modern, the negative feedback is disconnected and treble frequencies are being attenuated by an RC filter connected between the the Presence pot and the wiper of the Treble pot.  Using Vintage, negative feedback is being fed from the transformer to the PI.  The amount of feedback is controlled by the Presence pot, which also sets the frequency cutoff; it has dual functions as a resistor divider and a filter.   Raw is a special case and is discussed on its own below.  I will be using Channel 2 for the examples, but I will contrast the circuit differences with channel 3. (more…)

In any tube amplifier, the cathode resistor can be bypassed with a capacitor to increase the gain.  Aside from the benefit of wringing extra gain from each stage, a shelving filter is created by this RC combination.  Since the resistor sets the bias of the stage and would normally need to remain at a particular value, the capacitor can be be chosen, or changed, to set the cut off of the filter.

In most modern guitar amps, the cut off is set to voice the stage for a particular bass response.  While there are many filtering options in an amp, the cathode cap is  powerful.  The frequencies below cut off are amplified much less, which means that frequency also gets driven less during overdrive or clipping.  For low frequencies, this is important. (more…)