Archive for September, 2015

To Be Continued

Posted: September 30, 2015 in Uncategorized

A situation has arisen which requires some attention.  I expect to be unable to make new posts for the immediate future.  I’m hoping to be back to this by December.  When I return, I’ll be analyzing the dirty channels and looking at ways to tweak all three channels.  You can still comment or ask questions.  I’ll respond when I can.  Don’t be alarmed if it takes a few days before a reply.

Thank you,

Ted.

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…)

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…)

ACDCPlexi VintageCleanThese settings are for an approach to getting some vintage tones from a Mesa Boogie Dual Rectifier.  The Clean uses the ideas I’ve been developing about a higher gain setting against a lower Volume setting.  It also has less Bass dialed in.  This made the moderate use of Mid and Treble possible to control sizzle and distortion.  The Presence is just an extra treble control, because Channel 1 has no negative feedback in the power amp.  This setting is very full through the mids and has enough bass and treble to sound good, without either dominating.

The Dirty setting is copping a nice late-70s AC/DC tone.  It sounded really good for many Classic Rock tunes that aren’t fuzzed up.  Raw has a lot of top and bottom rolled off.  It only needs a little nudge to move into Plexi territory, though it could take some more Mid if a person wished to do so.  The Solo is engaged and dialed-in just enough to change the top frequency response.   (more…)

  1. Input Stage
  2. Stage 2: Go With The Flow
  3. Stage 3: Clean It Up
  4. Skip The Nerd Talk: Putting It All Together
Introduction

I’ve been thinking a lot of about modifications recently and I’ve been going over the schematic for the 3 Channel Dual Rectifier on a regular basis for quite awhile.  I was recently thinking back to when I was starting out and how it was difficult to understand why or what a part of the amp was doing.  I don’t consider myself an expert.  I was taught solid state theories in school and had to buy books and read websites regarding tubes and how they apply to guitar amps.  I wished there was more information about the design concepts.  So, I thought I’d break down the preamp for the Dual Rectifier into the stage circuits and just talk about the general, overall concept of what each does on its own and together with its neighbors.  This might help the users with tones and circuit designers and hobbyists with ideas and insight. (more…)

Disclaimer

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.

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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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(11/15/15: Edited information about the Presence and Treble)

Modern, high gain, guitar amplifiers generate much larger signals than vintage amps, hence high gain (high increase).  Compared to a vintage amp, modern amps can produce much more distortion and saturation.  In addition, the low frequency content of a Recto is fairly great.  Stages 1, 2, and 4 of the dirty channels are shunting the lows at about 88 Hz, -3 dB.  While the amp voicing is another topic, it’s important to have an idea of what is coming into the tone stack if you wish to shape it.  Since I own a Dual Rectifier, I’m using it as the example, but these concepts and this tone stack are applicable to many amplifiers.  I’m focusing on the Vintage mode of the Orange Channel.

Our topics for this discussion are: target frequencies, response of individual controls, the interaction of the controls, and dialing in useful tones more easily.  Some of the information is simplified for clarity.

(more…)

I am very ill.  In January of 2014, I began having pain in my shoulder.  By the next week, both shoulders, my wrists and hands were affected, more significantly on the right side.  By the end of February, I couldn’t walk very well and I felt very sick.  Since that time, I have had periods of being restricted to bed, short periods of relative wellness, and situations in between, but the pain and arthritis continue.  I was diagnosed with Seronegative Rheumatoid Arthritis and chronic pain (Edit June 2016: this was misdiagnosed Lupus).  Among the things it affected was my ability to play music. (more…)