Considerations For EQ and Amp Mods

Posted: February 25, 2016 in DIY, Modification, Tube Amps, Vanity and other sins
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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.

Major considerations for using the component values of either tone stack are:

  • What is the size of the frequency band spread in the preamp?
  • What tubes are being used in the power amp?

A Dual Rectifier reproduces frequencies from around 95 Hz to 1.6 kHz (2.4 kHz for Modern).  The result of the preamp and tone stack are meant to be pushed by a power amp with 6L6GC tubes.  6L6GC has a big low end and clear mids and highs.  Carving out the low-mids and emphasizing the surrounding areas is a big part of the Recto sound.

Taking another brand of amp and building a modified tone stack into it is only going to emulate a Recto to a small extent.  The preamp stages need to be configured to amplify the correct band of frequencies.  Likewise, if the output tubes are not emphasizing the low end needed to get that big Recto bottom or they have a major mid-emphasis, it isn’t a worthwhile pursuit to only modify the tone stack.

At the very least, I would consider the preamp’s cathode component values to set the gain and cutoff frequency for each stage.  Combined with the tone stack, this can get an amp at least half the way, if not further.

I feel that modifications to make one amp act like another is a waste of time in many cases; Buy the other amp instead.  Alternatively, using switches to enable and disable mods can be worthwhile, but may reduce value or introduce noise.  I would consider if a particular amp could be tuned differently, rather than attempting alchemy.


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