Classic Rock, Power Tube Overdrive, And The Mesa 3 Channel Dual Rectifier

Posted: June 20, 2015 in Mesa Boogie Dual Rectifier, Tube Amps
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Classic RockBefore the mass application of  post-preamp master volume controls, old amps had to be cranked up to get distortion.  The distortion of the power amp is what gave those old amps their defining overdrive characteristics.  Modern, master volume, amps rely on preamp distortion more than the power amp to create crunch and chunk without a person’s ears bleeding from the high SPL.

The 3 Channel Dual Rectifier (non-multiwatt) has an extremely cold biased power amp at full power.  This allows it to amplify the preamp distortion with a minimum of additional overdrive being created, if any is created at all.

To work around this, reducing the headroom and limiting the preamp distortion will force the power amp to overdrive when it is driven hard enough.  By pulling two power tubes and a 5u4 rectifier, using the tube rectifier, and selecting Spongy, the power is reduced from about 100 watts to about 32-34 watts.  It will sound a little less loud, but the main idea is to reduce the amount of “clean headroom” in the power amp.  Using SS rectifiers will increase the wattage to closer to 40 and will be more firm.  It’s a matter of taste, but the idea is to have a less preamp-saturated tone.  The tube rectifier helps with softening the sound and giving everything a rounder, vintage-like vibe.

EDIT 7/10/15 I didn’t think about the amp bias.  As the voltage is lowered, the bias becomes colder, up to ice cold.  Using matching EL34s would be biased warmer and produce better power amp distortion.

The amp becomes darker in tone and can be mushy and some of the high frequency fizz inherent in this amp may still be present.  To correct the fizz, I use the Solo control to adjust overall volume and the output control to adjust the high frequencies.  The mushiness is relieved by reducing the bass and/or gain and giving a slight increase in a channel’s volume.

In the above picture, Channel 1 is set for a slightly unclean tone.  Channel 2 is an attempt for a Marshall sort of high-mid tone.  Channel 3 is for a rolled off, fuzz tone (try Purple Haze with it).

One note: When the Bass is reduced, the Mid control seems to attenuate the slope toward the high frequencies more than emphasizing the lows.

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