Mesa Engineering Dual Rectifier Presence Pot Frequency Plots

Posted: July 12, 2016 in Amp Settings, Mesa Boogie Dual Rectifier, Modification, Tube Amps
Tags: , , , , , , , ,

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.

Aside from the stock values of 25k and 100k, I tried 10k and 50k as replacement values.  10k actually gives a lot of flexibility, but doesn’t end up being able to make a flat response when the control is turned all the way down.  That may, or may not, be a problem for a person, but it might make getting more treble a little easier.

I haven’t yet plotted the Modern/Raw attenuator circuit to see what the differences are, or what changes would do;  I’ll try to get to that soon.

I hope this helps.

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Comments
  1. Denis says:

    Anybody ever experienced a HUM on CH2/CH3 any mode… could it be normal?

    Like

    • Warpedpig#1 says:

      Check out the V2 and V3 tubes. One of those might have to be changed.

      Like

      • Denis says:

        Thanks, I already did but nothing changed.

        Like

      • Warpedpig#1 says:

        Check cables, pedals, and other things. I had a similar problem and a faulty cable was the culprit. Something may have broken it’s ground connection.

        I’d also highly recommend getting contact cleaner. Even if it’s not the problem, cleaning the jacks and tube sockets every 6 to 12 months is good for the amp. You can also use it on dirty pots (volume, tone, gain, etc.) on your amps and guitars; that gets you back up and running, trouble free in 20 minutes. It’s also good for breaking up junk on non-electronic, metal, objects, like door hinges.

        You can Google “troubleshoot amp hum” for more ideas on hunting down the hum. You might only be hearing it on the dirty channels, because the distortion is making it more apparent. That tends to indicate a problem prior to the input.

        Like

  2. Denis Novello says:

    I usually connect just the amp for trouble shooting, so there are less ???, btw, in the meantime my tech found the problem and modified the amp to resolve it, in case you need details, pm me I will send you his contact. I am completely happy!

    Like

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