Posts Tagged ‘overdrive’

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). (more…)

June 2016- I realize the flow of this article could use some work.  I have plans to reorganize it at another time.  

We’ve looked at the input and Clean, the Mode voicing and Gain control, V2 and V3EQ and modes and Presence.  I am going to bring it all together and take a less detailed, wider, view of the system as a whole.  I’m concentrating on Channels 2 and 3, since they are the primary identity; the “Recto” sound.

Additionally, I’m going to cover a little bit about the power amp, even though I have not concentrated on it in my writing this year.  I think the intention of Mesa when designing it is misunderstood.

I’m going to be making certain assumptions based on the overall design and historical precedents in guitar amplifiers and Mesa designs.  I could be wrong, but I am presenting it the way I best understand it, based on how everything interacts, and considering the differences between this amp and others much like it.  Mesa tends to tune their amps by ear, so some of the effects from changes may be unintentional or accidental.  That doesn’t negate the actual effects from being valid points innovation on the design.

For filter analysis, I’m using the Okawa website.

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We’ve looked at the input and Clean, the voicing and Gain control, and V2.  Now, it is time to examine V3, the coupled cathode follower.  I urge you to read about cathode followers if you do not understand how they work and wish to learn more.

The biggest things for a novice to know are:

  • A cathode follower is usually used to lower impedance to drive a tone stack or an effects loop.
  •  The cathode of the second tube follows the voltage on its grid closely (hence “cathode follower”).
  • The first tube provides amplification of the signal and stability to the cathode follower.
  • When coupled (wired together), the two tubes act like one (mostly). (more…)

So far, we’ve looked at the input stage and Clean mode, and the voicing and Gain controls for Channels 2 and 3.  When we left off, the signal had been filtered, dropped down, filtered again, and was exiting the Gain pot by way of a 475 k grid resistor into Stage 2 (V2a).

Grid Resistor

V2aDRAside from controlling incoming current, the grid resistor serves to adjust the highest frequency allowed to enter the amplification stage by creating a filter due to the tube’s internal capacitance (around 1.6 pf).  Additionally, this stage has a 20 pf capacitor in parallel to the tube.  This increases the total capacitance to 21.6 pf. (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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