Cathode Mods: Mesa Dual Rectifier 3 Channel

Posted: September 18, 2015 in Mesa Boogie Dual Rectifier, Modification, Tube Amps
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In any tube amplifier, the cathode resistor can be bypassed with a capacitor to increase the gain.  Aside from the benefit of wringing extra gain from each stage, a shelving filter is created by this RC combination.  Since the resistor sets the bias of the stage and would normally need to remain at a particular value, the capacitor can be be chosen, or changed, to set the cut off of the filter.

In most modern guitar amps, the cut off is set to voice the stage for a particular bass response.  While there are many filtering options in an amp, the cathode cap is  powerful.  The frequencies below cut off are amplified much less, which means that frequency also gets driven less during overdrive or clipping.  For low frequencies, this is important.

The 3 Channel Mesa Engineering Rectifier Series has cathode bypassing on 3 stages.  Stage 1 and 2 are shared by every channel.  Additionally, stage 4 of the dirty channels (the DC coupled cathode follower’s amplifying stage) is bypassed.  Each of the cathode resistors are 1.8 kΩ and the caps are 1 µf.  Ignoring the 100 Ω resistors, this sets the frequency cut off at 88.4 Hz.  The 100 Ω resistor moves the cut off to about 83 Hz for the AC signals.

User Third Age Amplifiers from The Boogie Board recommends the following:

….Change the first two cathode bypass caps the .47u. It brings out the mids and highs a bit more and gets rid of the flubby low end.

This cap change sets the cut off at 178 Hz.  This definitely removes flub.  Since each section gets passed to another and then another, and so on, amplifying the bass early on can produce a softened low end.  Leaving the cap unchanged on stage 4 allows the amp to produce one low bass response in a stage and does it late enough in the preamp to keep the bottom tight.

The other cap value often used is .68 µf, setting the cut off at 123 Hz for AC.  Whether one is better than the other, or even better than stock, is a matter of opinion.  Alternately, a steady increase could be used to introduce more lows from stage to stage.  One scenario could be:

  • Stage 1: .47µf
  • Stage 2: .68µf
  • Stage 4: 1µf

The many possibilities are outside of the scope of this article.  Good luck to you and thanks for reading.

(Much appreciation and thanks to Dave H. and pdf64 over at The Music Electronics Amplifier Forum for proof-reading and suggesting changes to this article and others.)

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Comments
  1. […] just a little bit of bass, but the level change is too slight to notice on its own.  Changing the cathode cap along with, or instead of, this resistor will make more of a change to the […]

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