Posts Tagged ‘engineering’

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…)

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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…)

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…)

(Note: During the initial 5 year period of warranty, using tubes other than those branded by Mesa will void the warranty of your amp.  This article is meant for people who have amps which are not under warranty.)

Mesa’s 6L6GC STR tubes are manufactured in China for Ruby Tubes.  It is a reproduction of the classic Sylvania 6L6GC STR.  Mesa buys a big batch from Ruby, tests them, keeps the tubes that meet their requirements, and sends back the tubes which fall out of range.  Ruby’s matched version of this same tube is the 6L6GCMSTR.  It costs considerably less than Mesa’s tubes. (more…)

I was spying a schematic for a Mesa Engineering Stiletto and noticed some very interesting details regarding the signal path.  When the Crunch mode is selected for either channel, the relays pull the cold clipping stage out of the circuit.  This creates a modified copy of a Model 1959 JCM 800.   (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…)

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…)

Edit: Since writing this post, I’ve found increasing the Send to be more effective when the bias is colder.  Tubes in the lower range will make the amp run warmer and improves the sound without adjusting the Send.

In the manual, Mesa recommends bypassing the loop to get the best tone for recording or other situations where the fidelity is needed (isn’t that always?).  The loop works by cutting down the signal to send it out (like a volume control) and then re-amplifying it when it comes back in through the Return.  This is great for inserting effects, but people often struggle with the tone, because it’s always going through those amplification stages when engaged.  I found an effective way to get a better sound with the FX loop engaged. (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…)