Mesa Nomad Ch3 useless. Needs Mod

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by Tommy_G, Mar 3, 2012.

  1. Tommy_G

    Tommy_G Member

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    Channel 3 is nothing but glissy harmonics amounting to white noise and no TONE. Things do improve by turning the volume pot on the guitar a few notches, but why would that be an advantage if the fundamental circuit doesn't sound great with full on guitar volume? (BTW, Channel 2 is just fine).

    Looking at the schematic, ( www.webphix.com/schematic%20heaven/www.schematicheaven.com/boogieamps/boogie_nomad100.pdf )

    I'm guessing the source of the lack of fundamental tone and all the high frequency content is because of the 680k resistor right in front of the gain pot which forces most of the signal through a 0.002 cap (see pg 4/10 in the pdf). Sorry don't know how to calculate corner frequencies to verify.

    The rest of the Ch3 circuit looks kind of Marshall-ish, leading to a common tonestack at the back end of the preamp, with a 47k slope resistor.

    Any suggestions on exactly how to mod this channel to something that has tone? A classic marshall tone would be a good place to start.
     
  2. Jerry Glass

    Jerry Glass Member

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    I think your Mesa has a problem. The issue is most like around V4a. I'd try a new tube in V4 first...if that doesn't do the trick you may want to involve a tech. The JFET that kicks in the 25uF cathode bypass cap on V4a is a likely culprit. If you have ever "hot swapped" this tube, a failed JFET is a definite possibility.
     
  3. Tommy_G

    Tommy_G Member

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    Thanks for your interest and response.

    Gauging from the circuit, what kind of channel tone is this based on or should be sounding like?

    It has always been hollow and glissy since I bought it new. It has soooo much saturation that I can't stand to put it past 9:30 on the gain dial, otherwise its simply nothing but overdrive to the nth degree, and even at 9:30 has too much gain. - Like I said, lacks fundamental tones - from the viewpoint that it sounds far too scooped.

    Channel 2 sounds good, and it shares the tubes with Ch3 - but I'll change out v4 to make sure that tube is ok.

    Any way to test myself if the jfet is working correctly? Can I put a voltmeter across it and hit the footswitch on and off? Is there a way to indirectly tell by switching in and out of Ch2/Ch3 in terms of the tones?
     
  4. oldhousescott

    oldhousescott Silver Supporting Member

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    You can remove the .002 and see if you like it. Just unsolder one leg to get it out of circuit. If that sounds better, completely remove the cap.

    The .001 as a bright cap may be too large for you, as well. If the channel is still too bright with the .002 removed, try reducing the .001 to 470pF or 250pF.
     
  5. Tommy_G

    Tommy_G Member

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    Thx oldhousescott. I'll give it a try.


    There may also be something to that 25uF cap. The bass levels only come up to snuff with the treble turned wayyyy back. I don't remember this channel to be like this, so maybe this is a problem that recently happened. There was a reason I stopped using the amp....I could no longer dial in a proper tone on that channel.

    How much risk would I be taking to test that jfet myself?
     
    Last edited: Mar 4, 2012
  6. stanmuso1

    stanmuso1 Member

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    Tommy G, I agree with you on the treble frequencies of Channel 3. I have the Nomad 100 1x12 with graphic EQ. To be honest in general I find it a great overall workhorse amp. Im not a metal player but more of a Classic Rock, Bluesy, Country Jazzy player! I had a Nomad 55 for a couple of years and it sounded the same. I upgraded to the 100 for the big headroom and the EQ. I had an F50 and used it for 2 months. It was WAY to bright and harsh IMO. I could never get a decent sound out of the F50. The Nomad is a much warmer amp and I think was desinged to be a darker sounding amp in the Mesa range. A Bluesy sounding amp with an overbright third channel! Like yourself Im wondering if anything can be done with Channel 3. I could be so useable. Is it that big 680k resistor and caps as you described? The only way I use it is with the Neck pickup. Also Is Channel 2 based on any particular tonestack? Is Channel 1 based on any model in particular? I know the paralleled V1 makes a difference. In general Im wondering if its metal players and people that like super bright sounds that want to do all these mud mods to the Nomad? The Nomad gets a lot of slating on the forum's but I think this "black sheep" is a "dark horse"!
     

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