Archive for January, 2016

(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…)

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(Edit: I confused some 6L6 sub-types in the original article, which I don’t think is hard to do).

I ran across this article about Sovteks and how the rating charts for several tube brands compare.  I am needing tube replacements and this seems pretty handy for finding tubes that fall into Mesa’s bias range for their 6L6GC.

I’ve been using Ruby 6L6GCMSTR, which is an excellent tube, and I will likely stick with it, but these others have piqued my interest in differences between the sub-types. (more…)

Fuzz-Crunch-CleanI’ve been experimenting with making Channel 3 a fuzz tone for a long time, but recently got just the setting I wanted.  This kind of setting gets close to “Dazed And Confused” and sounds good with “Purple Haze”.  Being tube, instead of transistor, it’s a little softer, but the Presence  can dial in the edge a person needs for fuzz bliss.

Faux-Fuzz EQ Setting

Yellow is the Modern setting as described above with no treble and full mids. Green reduces Bass to 11:00.

The way that old, germanium transistor fuzzes would clip is awfully close to the way the cold clipping circuit works in these Marshall/Soldano types of circuits and the Recto has the bottom end to really sound huge.

I found this Channel 2 Raw setting to be a match for the fuzz tone.  It’s bright, with plenty of bottom, and crunches up quite nicely.  Both dirty channels get cleaner as guitar volume is rolled back, but, like a Tonebender, Channel 3 never gets “squeaky clean”, but it does make a nice half-clean sound, since the treble is flattened.

I hope this helps.  Enjoy.

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…)

While this is written about a 3 Channel Dual Rectifier with an FX loop modification to serial, the basic idea will apply to any amplifier with a serial loop.  As well, the Nova System can be substituted for another device with a speaker simulation.  The speaker sim is in the Utility menu. (more…)

EQ Schematic

Schematic of tone stacks for Channel 2 and 3.  Note the Presence control circuit and the value differences for its resistors

When Modern Mode is selected on Channel 3 of a Dual Rectifier, an additional capacitor is added to the circuit.  This changes the capacitance value from 500 pf to 680 pf.  By doing this, it moves the frequency from which the treble swings on axis from 1.27 kHz down to 936 Hz.

The effect of this change is twofold: 1) The upper mids receive a greater emphasis in the sound; 2) The overlap of range between the Mid control and the Treble control forces the Mid frequencies to move up in dB, effectively giving a mid-boost throughout the range of the Mid control.  The change also prevents the Mid control from being as scooped at the minimum setting by about a maximum of 2 dB between 600 Hz and 800 Hz.

(more…)

Wow.  After most of one year devoted to my favorite amplifier, I don’t have a regular topic to cover, now.  Toward the end of 2015, my health declined pretty dramatically.  I ended the Dual Rectifier analysis early, because it was taking too long to put the articles together.  Aside from helping people understand the amp or tweak it, my intention was to assist DIY circuit designers, by highlighting interactions between the preamp stages and the key changes Mesa made to the basic design as established by Marshall and Soldano.

Since this version is now obsolete, I felt there would be little harm in sharing an analysis.  I haven’t done this for people to copy the design in whole.  Since the schematic is available in dozens of places, that could already be done by anyone who would want to.  I want people who are making their own amplifiers to see what this example does and decide if anything from it should, or should not, be a part of their own design, or if a modified form could be used.

When my health became even worse than it already was, I was stopped dead in my tracks.  By the time I could sit at the computer again, the inspiration was waning; not by lack of my want to write it out, but by the effects of the disorders I have to contend with.  It’s a very difficult hobby and I don’t want to make mistakes if I’m trying to help others.

Left Undone

I had not originally planned to write about the power amp, but some of it was relevant to analyzing the preamp modes.  I put that into the “Overview” article, but there is more that could be said.  I’m not sure I’m going to be able to do it.  In particular, the negative feedback could use a more detailed analysis to determine exactly how much stability is being added.

The other thing is the power supply.  The effects from combinations of different settings of the power and rectifiers would help with understanding the compression and headroom designed into the stages.  In fact, with only a very quick look, I noticed that V1 and V2 are probably more compressed and act like a limiter compared to tubes upstream, but it would need an analysis to be sure of that and also the extent of it.

So….

I’ve had a request to analyze the Mesa Boogie Mark V.  I might analyze select parts of it.  At this time, I don’t have the energy to go into massive detail and I don’t own a Mark to test the results.  I could post it as “just hypothesis”.

If I run into my own problems or cool, quirky tones, I’ll post about solutions or tweaks.  I also have one my old pedal designs to tinker with and some DIY ideas to look through, but I am not able to be in a hurry with any of it.