Posts Tagged ‘Mod’

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…)



Basics of the Signal Path

The input to the Boss GE-7 Graphic EQ has a buffer which is active at all times.  There is a slight emphasis around 2 kHz to help the guitar sound stand out.  Immediately after the buffer is the amplifier for the level control.  It’s a differential type with only one source voltage split with the level’s sliding potentiometer.  This slider will either boost or cut the whole signal when it is moved from the zero line.  From here, the signal is split; one path goes to the EQ section and one part goes to the positive input of another differential amp.  The EQ section feeds the negative input of the differential amp. (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 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…)

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…)


Guitar amplifiers contain lethal levels of electricity.  If you work on your amp, it is at your own risk.  This site doesn’t recommend anyone without the necessary training and/or experience to perform work on an amplifier.  Even trained, experienced people can receive electric shocks from time to time and I don’t wish for anyone to injure themselves.  Additionally, soldering on PCBs can damage traces if care is not used.  Any potential damage or wear is the fault of the person performing the work and warpedmusician is not liable for any limitations of the materials or skill of the person performing modifications.


FX loop alternative

To the left is the previously posted alternative loop modification schematic to change a Mesa Dual Rectifier from a parallel loop to a serial loop.  It’s easy.  Most of the time required for the mod is spent taking the amp apart.  (Here’s a link to a 2 Channel Recto with the same mod.)

The following pictures are provided to show the inside of the amp, the job, and a crappy job someone did of soldering the output transformer to the daughter board.  I used a 22 gauge wire with the insulation removed for a clean, short jumper.  I’ve seen people use insulation for the job, but the space is tiny.  Note that my solder joints look a little matte.  This is from the lead-free, silver solder I used.  Almost every joint looks that way when I use it, no matter what, but it hasn’t been a problem.

Update July 2016: Verified.


I e-mailed Mesa Engineering awhile back about the loop mod for the 3 Channel DR parallel fx loop to make it serial.  I received the same schematic that I now know everyone else gets.  I can see the appeal for someone who doesn’t want their signal to be “tainted” by a pair of jfets, but they aren’t amplifying, only opening to allow the signal through.  It’s no different than having any other quality part in the path.

More recently, I saw that a company is advertising their bench fee for a different mod and the loop retains some of the functionality afterward.  I thought to myself, what would do that?  One thing that came to mind while off on one tangent is the idea to pinch the dry channel off. Then the light bulb flashed and blew up over the idea I’m writing.  This is not only easier than adding a part or bypassing the jfets, it is entirely reversible.  Here’s how to mod the fx loop and gain a killswitch:


Vintage humbuckers and those which emulate P.A.F.-style humbuckers have a distinct dip in the midrange, due in part to the unbalanced DC resistance of the coils and the number of winds on each bobbin.  Many modern humbuckers are over-wound, but still have an unbalanced pair of coils to produce a “P.A.F.-on-steroids sound”.  Many other modern humbuckers are fairly well matched, giving the pickup a spike in the midrange.  The Seymour Duncan SH-4 JB is one such pickup.  At a total DC resistance of 16.4k, it’s more than twice as powerful as a P.A.F..  My idea was to create a permanent offset and have a spin-a-split that doesn’t reduce to zero. (more…)