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 Post subject: Re: nomad mods
PostPosted: Wed May 25, 2011 8:45 pm 
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Mark I

Joined: Thu Mar 26, 2009 7:20 am
Posts: 46
Uh, my brain just exploded, but Koreth I do believe you have found the culprit. It does to me sound like a bad distortion pedal playing faintly in the background when I set the reverb/gain to extreme settings, and disappears entirely as I roll off the reverb control.

It really sucks that we have to re-engineer something to work properly that was released as a tested, finished product. Why put a beautiful long-box three spring tank in the amp and drive it with a flawed circuit? I'm dreading the idea of buying 6 germanium diodes (if they as rare, expensive and thermally sensitive as their transistor cousins). What info do I need to source the appropriate diodes? (ratings, tolerances etc)

Note to prospective Nomad buyers: Prepare to compromise on the reverb, buy a good pedal (any suggestions??? ;) , or mod the amp to work properly.

Hopefully we can get to the bottom of this,

Thanks for all your help guys, much appreciated.

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USA Strat, Les Paul Studio, My 2204 clone, Weber 30w Alnico 12".


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 Post subject: Re: nomad mods
PostPosted: Thu May 26, 2011 1:06 pm 
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Mark III

Joined: Sat Aug 11, 2007 9:14 am
Posts: 215
Another thought. Keep the 1N4007s, add two more of em, and replace the 47k resistor with a 56k or 51k resistor to pull the gain of the opamp back a bit to keep the increased signal voltage in from overdriving the opamp. Probably easier than sourcing germanium diodes and would involve replacing less parts.


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 Post subject: Re: nomad mods
PostPosted: Sat May 28, 2011 9:16 am 
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Mark II
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Joined: Sat Mar 05, 2011 9:13 pm
Posts: 121
Well Pat...I tested my reverb on channel one with the gain up, and sure enough, I could hear some hashy, diode clipping like distortion under the note. You really got to tune your ears into the reverb signal but it is there. I guess I just havent noticed it because I run channel one with the gain very low. Are you going to try the mods Koreth suggested? - Pat.

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Nomad 100 2x12 combo


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 Post subject: Re: nomad mods
PostPosted: Sun May 29, 2011 3:24 pm 
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Mark I

Joined: Thu Mar 26, 2009 7:20 am
Posts: 46
Koreth

The idea you had sounds good:

(Keep the 1N4007s, add two more of em, and replace the 47k resistor with a 56k or 51k resistor to pull the gain of the opamp back a bit to keep the increased signal voltage in from overdriving the opamp)

Do you mean triple the 1N4007 diodes in parallel, or series? I take it the 47k resistor should be among the 10 or so for the reverb circuit. Should I be able to source 1N4007's at an electronic shop, or are they more rare?

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USA Strat, Les Paul Studio, My 2204 clone, Weber 30w Alnico 12".


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 Post subject: Re: nomad mods
PostPosted: Sun May 29, 2011 5:26 pm 
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Mark II
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Joined: Sat Mar 05, 2011 9:13 pm
Posts: 121
I think he is saying to put them in series so that the clipping action is higher. Replece the 47K connected to pin 2 of the op-amp with the 51K or 56k. This with result in some lowering of the overall gain going into the reverb driver, which means the reverb level in all channels will lower slightly. I would do the diodes first and if the results are not satifactory, then do the resistor. - Pat.

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Nomad 100 2x12 combo


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 Post subject: Re: nomad mods
PostPosted: Wed Jun 01, 2011 1:43 pm 
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Mark III

Joined: Sat Aug 11, 2007 9:14 am
Posts: 215
Correct. In series, the voltage needed to clip is the total of the forward voltages of all the diodes. Thus, for two 1N4007s, the clipping voltage would be 1.4Vp. However, if you still get a distorting sound on the reverb, change the resistor in question to 51 or 56k, this will reduce the gain slightly to keep the opamp from overdriving, While in theory it would reduce the reverb level on all channels, because a larger signal is being fed into the opamp than before, you should in fact get more reverb.

Stock: 0.7Vp into the opamp, 7Vp into the reverb tank
With the diodes and gain resistor changed: 1.4Vp into the opamp, 13Vp into the reverb tank.

I've never been able to find 1N4007s at any of the local Radio Shacks, and AFAIK, they are the only 'electronics' shop in my area. However, you might have more luck. Failing that, they are commonplace enough that you could probably yank them out of the power supply of some busted electronic gizmo that runs on mains power, or order them online if you're not in the mood to try to salvage some.


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 Post subject: Re: nomad mods
PostPosted: Thu Jun 09, 2011 1:57 pm 
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Mark II
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Joined: Thu Jun 09, 2011 1:05 pm
Posts: 57
Reading through all ten pages of this thread, I was curious about a few things.

When talking about general muddiness (which is something I haven't noticed) I assume this is with various guitars? I have two tele's that are super-bright in general - could this be counteracting the supposed muddiness?

With effects loop muddiness while using an external preamp - could this be the result of impedence mis-matching? If the mesa pre out matched the level of the other poweramp it would sound ok, but if the device you were feeding into the nomad pre didn't impedence wise - couldn't that account for the mud?

I saw some mentions of delay not working well in a parallel loop. Currently I slave all of my modulation an time effects through a second amp driven by the nomad. I blend the volume of that second amp in to reach unity with the nomad when the effects are on, but isn't this what the parallel loop does internally? I'm trying to scale down my gigging rig to the 1X12 combo, and it would be nice to not need the second amp.


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 Post subject: Re: nomad mods
PostPosted: Thu Aug 11, 2011 11:28 am 
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Bottle Rocket

Joined: Tue Apr 26, 2011 4:54 pm
Posts: 17
I do not know if I missed it, but has anyone posted some audio files of a Nomad with the Mid/High cap removal mod? I would like to hear a few samples before I go digging into my amplifier.


Also, would it be possible to put these caps on a toggle switch?


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 Post subject: Re: nomad mods
PostPosted: Thu Aug 25, 2011 1:25 am 
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Mark III

Joined: Fri Dec 11, 2009 9:54 pm
Posts: 250
If you look up the diagrams and images Nomad100HD put on the first couple of pages, that's what my Dad and I used to help perform the mod. My Dad is an electrical engineer, so that definitely helped me out. It cleaned up the low-mid to low region; it's a subtle but noticeable difference.

_________________
Past Amps:
2004 Mesa Triple Rectifier
2000 Mesa Nomad 100
2009 Mesa Electra Dyne
2012 Mesa RA100
2012 Mesa Mark V
2007 Mesa Stiletto Deuce II
2014 Rectoverb 25 Head
PRS Archon 50 Head
2009 Mesa Dual Rec

Current: Mesa TC50

Guitar Cab:
Mesa Road King 2x12


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 Post subject: Re: nomad mods
PostPosted: Fri Aug 26, 2011 11:10 pm 
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Bottle Rocket

Joined: Wed Nov 03, 2010 11:32 am
Posts: 12
Quote:
When talking about general muddiness (which is something I haven't noticed) I assume this is with various guitars? I have two tele's that are super-bright in general - could this be counteracting the supposed muddiness?


I think the "mud mods" should be considered part of so-called blueprinting, which is to say these mods are small tweaks entirely up to the user. What someone else considers mud you might consider tonal nirvana. In any case, even when you can get two guitarists to agree on tone, different guitars, strings, guitar cables, effects pedals, speakers and tubes can make all the difference in the world in the sound, so possibly most people would agree your rig sounds great as is.

If you like the tone you are getting out of your Nomad, don't change a thing!


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 Post subject: Re: nomad mods
PostPosted: Sat Oct 29, 2011 6:40 pm 
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Mark III

Joined: Sat Nov 29, 2008 12:52 am
Posts: 340
Koreth wrote:
To be fair, I haven't done any actual physical digging into the my amp as I've needed it operational for rehearsals and shows. So figuring out how to model bits of the Nomad circuit in SPICE, or any tube circuit for that matter, has been a Godsend when it comes to learning how these mysterious glass bottles interact with the rest of the amp and make the magic sounds they do. When one considers that hitting a PC board with a soldering iron too much can kill the board, I'm especially glad I can simulate in SPICE what will happen before taking the risks of open heart surgery with the Nomad.

So, we have shown that the FX loop recovery stage is attenuating all frequencies past 100hz Well, why? And what to do about it? Let's take another look at the schematic for that stage. and it's frequency response

Image
Image
The green line shows the frequency response of the stage as it exits the valve, but before it has crossed the output coupling cap. The blue line is it taken at the knee of that voltage divider/low pass filter, formed by the 100k resistor, 56k resistor and .005µF capacitor. That capacitor is bleeding off high frequencies to ground, and due to it's size is actually bleeding off quite a lot. What were to happen if we were to make it smaller by an an order of magnitude, from .005µF/5nF to 500pF?

Image
That's a bit better. We've got our mids back now at least, but we're still loosing some of our top end. Let's see what happens if we shrink the cap again by another order of magnitude, from 500pF to 50pF.

Image
That's much better. Frequency response is nice and flat for most of the range of interest. Sure, the highs and lows are being rolled off a bit, but the lowest part of the lows were getting rolled off anyway, and at least now, freqeuncy response on the high end isn't down until past 10kHz. The rolling off of high frequencies should still prevent any instability or oscillations, especially if the phase inverter caps are still in place, unless there's a problem elsewhere in the amp.

So, here is what I propose for a De-Mud Mod - Mk II(C+? :P) change the .005µF cap on the output of the FX loop recovery stage to a 50pF cap. the nearest 5% standard value, 51pF or 10% standard value, 47pF should work fine too. This should stop the attenuation of all the midrange and high range through the FX loop while still bleeding off enough high range frequencies to prevent instability.

Nomad100hd, since you have an interest in tweaking the Ch3 tone stack, I'll run some simulations of that next.


Nicely done, Koresh, I think you've nailed it.

I'd like to trade in my Mechanical Engineering degree for your Electrical Engineering degree.


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 Post subject: Re: nomad mods
PostPosted: Sun Oct 30, 2011 2:16 pm 
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Mark III

Joined: Sat Nov 29, 2008 12:52 am
Posts: 340
I did an experiment with my Mesa Formula Pre, which also has an effects loop, and it has been advertised by Boogie that the Formula takes direction from the Heartbreaker, and I'm pretty sure that the Nomad took direction from the Heartbreaker and Formula - so, shall we say, cut of similar cloth?

I ran two identical George L's from the Formula
1) from EFX Out
2) from Main output

Thru an A/B switch and into my ADA Ampulator, which Fed a ProTools Mbox set at 96kHz sample rate.

I set the effect out level (using the Volume Masters) and then the Main Output Level (using "Output") to be identical in amplitude, at least as good as my ear can hear levels while switching back and forth between the two output sources.

I changed all manner of eq settings on the ampulator, and even listened to the signal dry, and there was a very very slight, almost unnoticable improved clarity on the EFX out vs. the Main Outputs. There was a subtle preference in 'feel' between the EFX Out vs. Main Outputs, and in evaluating the subtlety in detail, concluding most of the tonal difference was in a slight improvement of the articulation of pick attack.

So, I would suggest, in spite of my support for Koresh's theory that the EFX return coupling is a possible source of mud, I have to suggest that at least on the Formula, there isn't enough difference to worry about.

As a disclaimer to my change in opinion, I will say that I worked long hours to pick tubes for the Formula that made it acceptable sounding, and it could very well be that I have a bright tube in the EFX or PI stage that effectively countered any tonal loss in the Effects return side....

I'm going to try the same thing with the Heartbreaker and Nomad (that is - when my wife isn't around...)


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 Post subject: Re: nomad mods
PostPosted: Sun Oct 30, 2011 10:52 pm 
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Mark III

Joined: Sat Nov 29, 2008 12:52 am
Posts: 340
I setup the Heartbreaker, and ran the Ampulator from of the EFX loop send of the HB. Note that the theorized source of mud is in the capacitor coupling of the FX return tube stage (FX Recovery stage), so taking a signal from the FX "Send" is believed to eliminate that source of potential mud.

Now, I found that I could not a/b the EFX vs. Main Outs on the Heartbreaker, because the "slave out" is actually tied into the output transformer and I can't do this microscopic headphone stuff with a live and loud speaker in the room.

While tube selections are very much diffferent between the Formula (mostly NewPro) and Heartbreaker (mostly VOS), there are a couple of preliminary observations:

The Heartbreaker is very much a darker voiced amp, right from the preamp side. I had to run the Ampulator Presence and Speaker Emulation 'highs' about 25% higher to get a nice balanced sound as compared to the Formula. (That said and done, I prefer the Heartbreaker...but maybe its the VOS tubes...but the gain structure seems more manageable and in the realm of vintage rock)

Just for sake of info, I noticed the treble adjustments to be very dominant, and there was a definitive 'sweet spot' where the tone cut through the best. Set too low, there's simply not enough treble. Set a fraction above the sweet spot, and the treble starts fizzling and being masked by indistinct distortion.

On the Love channel, the sweet spot for Treble was at 1:00 and the mids sounded best at around 8:00...(which I understand is 'about neutral' on this channel. On the Lust channel, the Treble sweet spot was at 2:00 with the mids at 2:00 as well. I had the bass sitting around 9:30 for both channels.

Next UP: Nomad.


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 Post subject: Re: nomad mods
PostPosted: Fri Dec 02, 2011 10:53 pm 
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Mark II

Joined: Fri Dec 02, 2011 10:19 pm
Posts: 124
Hello.

I just joined this board to simply say Thank You! As you can guess I have (2) Mesa's. Two days ago I would have NEVER dreamed I'd be willing to go inside my amp and use my soldering iron. Last night I found this board and the various nomad mod threads. After two hours of reading I could hardly sleep (though I was exhausted). I got home from work and with in 2 hours I pulled the chassis and performed the mud mod (Eliminated the (2) 120 pf) and the FX loop mod (replaced .005 uF with 47pf ). I put it all back together and took it for a ride.

WOW, I hear a huge difference. There are sounds and tones that were not there before. It is a wilder amp now. I love it. There is considerable presence that was not there before which is tricky for me as I run presence typically at 8:00...off. I have no issue rolling back treble or increasing base. I like to miss-match the 8 ohm speaker in the 4 ohm jacks so I can push the tubes better, it softens the whole sound a little. I think this amp was missing certain key frequencies. and those two mods really ARE a must.

- I am not sure if I will remove the parallel loop, as I see value in it nor do I expect much difference.
- I do not know if the Reverb is worth doing either. Though this one looks the easiest mod to do.
- I am really considering lifting the NFB. The cut in the presence would be welcome. But I am not sure where the better "presence" is, the NFB or the tone stack.
- Really not interested in the bias mod.
I wonder what the EL-34s will sound like...

Any advice would be welcome.

Again thank you so very much for all the effort in modeling and documenting the mods. It made my expendable amp a real keeper.


I love my Mesa's so I am sure we will all get along.
G


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 Post subject: Re: nomad mods
PostPosted: Sun Dec 04, 2011 1:56 am 
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Mark II

Joined: Fri Dec 02, 2011 10:19 pm
Posts: 124
I went ahead and did the NFB mod. I posted some images and instructions on how to do it in this thread: http://forum.grailtone.com/viewtopic.php?f=4&t=56044&p=404518#p404518

I think the bottom end "boost" from the speaker resonance frequency is not an issue. The low end is full and articulate, no flubby-ness. The intense high-end appears to be tamed, I can actually hear the presence control work throughout its full range after the NFB mod. It never did much and was very subtle. Being in the tone stack, I know it has nothing to do with the mod except for some common frequency ranges.

The amp is very alive breathy with the Mud, FX and NFB mods. I think I am done modding for the moment, unless I become convinced to do the reverb mod.


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