Posts Tagged ‘modification’

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.

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. (more…)

GE7signalflow

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…)

Mission Improbable

There are a few reasons I’ve been spending so much time exploring the Dual Rectifier.  Those reasons include: misinformation, biases and myths, and helping others.

The misinformation takes many forms.  Sometimes, it’s just reading about the DR having “active EQ” (false), that it isn’t a “solo” amp (false), or other equally dubious claims about the design that are ridiculous.  Biases and myths also fit in with the misinformation, but are just as much about the insecurities of the users as any misunderstanding of the amplifier.  Claims about early revisions being “superior” or the utility of the 2001 3 Channel version are as ridiculous as the circuit misinformation, because it’s all subjective.  If I was a little more cynical, I’d even think the second hand market is involved in promoting and spreading this kind of bias to increase, and retain the increases on, the obsolete designs.  Seeing as how Mesa designs them, builds them, markets them, receives feedback, and profits most from the sales, if the old amps were superior, they’d swap directly back to it due to customer demands and complaints. (more…)

Schematic of Input circuit in 3 Ch Recto.

Schematic of Input circuit in 3 Ch Recto.

 

(Note: The 2.2M load resistor is in parallel to the voicing circuit and gain pot, which are serial to each other.  This has some major ramifications regarding anode resistor values, as the output impedance of V1 will change when the resistor is changed.  When my health improves a little, I will update this article with graphs and more information.)

Nov. 2016: While the objective facts of this article are good, one thing I didn’t address was the harmonic distortion content and I think my initial assessment about the early distortion was wrong.  I absolutely do not recommend this as an improvement to the dirty channels.

As the resistance goes lower, the harmonic distortion increases.  This isn’t clipping, but non-linear tears in the waveform.  If the resistance is lowered to 150k or 100k, it would go a long way toward adding character to the clean channel.  After tests and simulations, I don’t think it will have a major impact on the dirty channels, aside from a further decrease in clarity when the gain control is past 2:00.

As far as the fizzy nature of some of the 3 Channel Rectos, V2b has the biggest impact on it in the preamp and the power supply filtering is another source.  You can test the latter by beginning at Silicon/Bold power options and working down to Tube/Spongy.  As the power decreases, clarity decreases from the midrange and the harmonic overtones increase.

So, this article represents a good idea for improving Channel 1, but other assertions I drew were incorrect.  I apologize.  I’m leaving the article up for critique and transparency.

 


 

Original Article

Some owners or users of Mesa Dual and Triple Rectifiers make complaints about the harshness of the clipping at higher settings on the gain control.  The opinion is that getting super-sustained sound is hindered by grinding distortion (which for me is part of the draw).  Some users also complain about the clean channel not being clean enough.

There is a way to affect both of these issues with the change of a single resistor.  The input stage has a 220k resistor on the anode.  Compared to a 100k resistor found in most vintage amps, 220k presents an increase in voltage gain, while reducing harmonic distortion.  This mod reduces the value of the resistor to 150k.

Almost all iterations of the Pre-Reborn Dual Rectifier and Triple Rectifier have the same input stage, with one main difference.  Early 2 Channel versions do not have the 100 ohm cathode bypass limiting resistor, which is present in the 3 Channel version.  Later 2 Channels have LDRs with resistance at about 100 ohms, which is essentially the same thing as the resistor being present in the 3 Channel version.  The following content applies to any Rectifier with LDRs or resistors from the cathode bypass cap to ground.

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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.

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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:

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This graph would not be possible without Duncan’s TSC  @ http://www.duncanamps.com/tsc/

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