Still loving my Nomad ...

Jimmy James said:

Does the mod for changing the cap in the Fx loop only work on the loop? Or does it work on the amp even without the loop?

Click to expand...

Nomad 45:
The caps that are changed are in the power supply, not the FX loop.

Specifically, The cap at point "C" in the power supply should be changed from 10uF to 47uF
The cap at point "D" in the power supply should be changed from 10 uF to 47 uF
The caps at point "B" in the power supply should be changed from 2X 10uF in parallel, TO 2X 22 uF in parallel.

At point A in the power supply, following the standby switch, you have 6 resistors in series.
220 ohm,365 ohm, 365 ohm, 365, 365, 365...
then you come to a 10uF filter cap. Change that cap to 22uF.
That's a wrap for stiffening the power supply.
Then at V1 input, pin 7, you should install a 68K resistor in series with the ferrite bead.

That cleans up the performance quite a lot. It's just about perfect now.
Sounds like a whole new amp. The sag and mud are gone, the highs are way clearer.
You can crank the gain much higher, without any oscillations (motor boating) occurring.

However, you should have a professional do it for you.
I do not recommend that inexperienced person attempt this update.
The circuit board must be removed to do this update. And, it must be done very carefully.

Then, there are issues:
as long as you have pulled the circuit board, carefully test ALL the mechanical 6 volt relays.
A worn out relay will cause problems. Make sure your relays are all good, before you put it back together.

soundguruman

Hi all,

for those who care about the humming in "Extreme Mode":

Mesa Boogie support recommended a change of preamp tubes. I replaced V1 (input stage) and V5 (driver). I tried different configurations with a Mesa SPAX 12AX7A, a TAD 7025 premium selected and a balanced Sovtek 12AX7LPS. None of the configurations stopped the humming.

So if you wanna try the same: Safe your money.

Regards,

rumpel

rumpel said:

Hi all,

for those who care about the humming in "Extreme Mode":

Mesa Boogie support recommended a change of preamp tubes. I replaced V1 (input stage) and V5 (driver). I tried different configurations with a Mesa SPAX 12AX7A, a TAD 7025 premium selected and a balanced Sovtek 12AX7LPS. None of the configurations stopped the humming.

So if you wanna try the same: Safe your money.

Regards,

rumpel

Click to expand...

If you do the update I described, NO MORE HUM.
That is why I designed his update.
There were 2-3 basic problems.
The amp is unstable with the stock power supply capacitors.
The power supply is not stiff enough.
The hum you are hearing is actually caused by oscillation.
After upgrading the power supply there is no more oscillation, and the hum is gone.

The design causes a low frequency oscillation in the power supply. Motor boating- Triggered by too much SAG.
When this oscillation occurs (when playing a note on guitar)
it drives the grid of V4 positive. This causes a cutoff distortion. (MUD)
As soon as V4 grid pops up positive, it causes the FET (mute circuit) to make a loud 60 cycle BUZZ.
So every time you play a note, as the note occurs and decays, you hear the HUM, Buzz noise.

IF you take out the FET (mute circuit) at the gain control, the buzz hum will stop.
BUT there is a way to retain the FET and stop the hum too.
Simply stiffening the power supply cures this oscillation, and the HUM.

It took me several hours to find this solution.
You can thank me later.

Of course, it's a bit of trouble to remove the board and change those capacitors...but worth it!
after the update, the amp sounds great.
soundguruman

The mud you are hearing is caused by oscillation in the power supply.

Hi Soundguru,

wow! That's really usefull. I'm gonna sleep one night and decide than, whether I do it myself or give it to a professional to implement your mod.

PS: Thank you for sharing knowledge.

Regards,

Rumpel

rumpel said:

Hi Soundguru,

wow! That's really usefull. I'm gonna sleep one night and decide than, whether I do it myself or give it to a professional to implement your mod.

PS: Thank you for sharing knowledge.

Regards,

Rumpel

Click to expand...

Hello Globetrotters,
From the secret rock and roll amp lab...

Get yourself a 50uF capacitor, 450 volt
Clip this capacitor (with clip leads) into the preamp power supply. At point "C"
Presto, the problem is gone.
How much easier can it be to test? In 5 minutes you have the proof.

OK, you need the schematic? These schematics are posted all over the internet.

The update I described makes this change permanent.

The capacitors you need to order, that I recommend:
450 volt, 105C temp rating, 5 MM lead spacing, 10,000 hour life rating
you can buy these from Digikey. These are about $3 each.
These are rated much better than the originals.

ALSO don't forget:
this amp uses mechanical relays for the channel switching.
If a relay is worn out, it will cause all kinds of problems.
When you take the board out, carefully test all the relays.
Replace any malfunctioning relays before you re-assemble the amp.

I install sockets for the relays. I use Machine pin sockets...
This means that any relay can be pulled and tested or replaced without removing the circuit board.
This saves many many hours of time, when doing the maintenance. Therefore saving you a huge amount of money.
Once the socket is installed, you can just about change your own relays....

An acceptable substitute for the original relay is:
Omron G5V-2

LAST issue:
Power transformer- look at where the secondary of the power transformer is plugged into the circuit board.
Take out the plastic connector. Solder the secondary wires directly to the circuit board.
This plastic connector will overheat and fail.
Remove the plastic connector, to prevent this failure from occurring.

All done. Rock on.

Hi Soundguruman,

ok I got the schematics and got the capacitors now. I think I gonna open the amp within the next few days and have a try

Thank you again.

rumpel said:

Hi Soundguruman,

ok I got the schematics and got the capacitors now. I think I gonna open the amp within the next few days and have a try

Thank you again.

Click to expand...

this amp should be worked on by experienced techs.
You need a temperature controlled soldering station.
Please do not try to work on the circuit board without professional tools.
An ordinary soldering iron is not good enough to remove the parts.
The board can be easily damaged by an inexperienced person.

I told you WHAT to do to fix the problems.
But the SKILL of the soldering is important!
The board has circuit traces on BOTH sides.
The holes are PLATED THRU.
This requires the TOOLS and the KNOWLEDGE to do the update correctly.

I just acquired a used Nomad 100 watt 1X12 after a buddy of mine had a 55 2X12 that sounded awesome. I got the 100 watt because of the 50 watt option for smaller venues. I re-tubed the power section with MB 6L6s but the preamp tubes are probably the originals. Channel 1 sounds ok, but has a constant crackling that I think might be a dirty pot. Now for the bigger problem: Channels 2 and 3 don't sound good at all. I describe the sound as muddy and cheap. I play a USA Wolfgang which should pair fairly well with this amp. I sent MB an e-mail asking what they thought and here is the response I received:

"Well- it is tough to say what the "cheap, muddy sound" may be, since it is so subjective... I can tell you that the NOMAD series by design has a very THICK (muddy) tone... not a whole lot of clarity between notes on channels 2 & 3. It wasn't designed to be as articulate as a Rectifier. Now, with that said: it may just be that the preamp tubes are old & need changing to breathe some life into your tone... but again, that is hard to call via email without even hearing the amp."

So that was not much help. Does anyone have any ideas on how to correct this? Is it possibly as simple as a new set of preamp tubes? I don't mind the "thick" or "heavy" tone, but it really sounds like a cheap solid state amp.

davolk said:

I just acquired a used Nomad 100 watt 1X12 after a buddy of mine had a 55 2X12 that sounded awesome. I got the 100 watt because of the 50 watt option for smaller venues. I re-tubed the power section with MB 6L6s but the preamp tubes are probably the originals. Channel 1 sounds ok, but has a constant crackling that I think might be a dirty pot. Now for the bigger problem: Channels 2 and 3 don't sound good at all. I describe the sound as muddy and cheap. I play a USA Wolfgang which should pair fairly well with this amp. I sent MB an e-mail asking what they thought and here is the response I received:

"Well- it is tough to say what the "cheap, muddy sound" may be, since it is so subjective... I can tell you that the NOMAD series by design has a very THICK (muddy) tone... not a whole lot of clarity between notes on channels 2 & 3. It wasn't designed to be as articulate as a Rectifier. Now, with that said: it may just be that the preamp tubes are old & need changing to breathe some life into your tone... but again, that is hard to call via email without even hearing the amp."

So that was not much help. Does anyone have any ideas on how to correct this? Is it possibly as simple as a new set of preamp tubes? I don't mind the "thick" or "heavy" tone, but it really sounds like a cheap solid state amp.

Click to expand...

They are always telling people to change tubes, but that is only solving about 50% of the real problems.
no, a new tube will not cure the muddy sound. You already guessed that.
Also, you don't need to buy "factory" tubes. I think maybe you figured that out.
The tubes are the same as any others...despite the brand that is stamped on them.

The two main issues are power supply and relays...the mechanical relays wear out...
when the relay contacts go bad, there is quite a bit of crackle and noise...or the channels won't switch properly.

So, your constant crackle may actually be a dirty pot or a bad relay. I am not surprised if a relay is bad, especially in an older amp.
The trouble being, to check or change the relay properly, you gotta take the circuit board out.
Activate the relay with an external power supply, and test the resistance of all the contacts. Both ON and OFF, it has 2 poles.
The last Nomad I checked had 3 bad relays. That tells you something....

Now I'm going to have a look at the schematic for the 100, and get back here soon.

http://www.freeinfosociety.com/electronics/schemview.php?id=289

That being said, it's similar to the 45. There are a few small differences.

davolk said:

I just acquired a used Nomad 100 watt 1X12 after a buddy of mine had a 55 2X12 that sounded awesome. I got the 100 watt because of the 50 watt option for smaller venues. I re-tubed the power section with MB 6L6s but the preamp tubes are probably the originals. Channel 1 sounds ok, but has a constant crackling that I think might be a dirty pot. Now for the bigger problem: Channels 2 and 3 don't sound good at all. I describe the sound as muddy and cheap. I play a USA Wolfgang which should pair fairly well with this amp. I sent MB an e-mail asking what they thought and here is the response I received:

"Well- it is tough to say what the "cheap, muddy sound" may be, since it is so subjective... I can tell you that the NOMAD series by design has a very THICK (muddy) tone... not a whole lot of clarity between notes on channels 2 & 3. It wasn't designed to be as articulate as a Rectifier. Now, with that said: it may just be that the preamp tubes are old & need changing to breathe some life into your tone... but again, that is hard to call via email without even hearing the amp."

So that was not much help. Does anyone have any ideas on how to correct this? Is it possibly as simple as a new set of preamp tubes? I don't mind the "thick" or "heavy" tone, but it really sounds like a cheap solid state amp.

Click to expand...

NOMAD 100:
Now get a schematic...they are posted here and there.
This update stiffens the power supply.

It is designed to eliminate a few problems associated with muddy sound, humming noises, and other things that may affect a guitar player........

At point "A" (follow along thru) there is choke, then there is 6 resistors in series, 220, 365,365,365,365,365....after that - there is a single 10uF cap, change that to 22uF.
At Point "B" leave the 33uF stock.
At point "C" there is two 10uF caps in parallel, change both those to 22 uF.
At Point "D" there is 10uF cap. Change that to 47uF.
We come to point labeled "360V" This is really point E...not labeled as such on schematic.
At point "360v" there is 10uF cap. Change that to 47uF.

And all these caps, I would like 105C, 5mm lead spacing, 450V, 10,000 hour life rating. I get them from digikey.

Then for relays, I take them all off and install sockets.
Once the sockets are installed, it's easy to change a relay, in about 5 minutes or so.
This eliminates removing the circuit board to change a relay. Instead, I just plug a new one in, or test an old one.
I can also leave a space, under the relay socket to accommodate a wire tie.

Now test your relays, and make sure that in both positions, you are getting 1 ohm or less contact closure.
Should be 0 ohms. I test them with a 6 volt power supply to switch the coil on or off.
Replace any bad relays, of course.
You need your relay contacts to be very tight, of course.

Just one last thingie....
I would like you to install a 68K metal film resistor on the input jack, in series with the ferrite bead, to pin 7 of V1.
The guitar is no longer connected to pin 7 V1, there is a resistor in series now....

The next issue is the transformer power connector.
This is a white plastic connector that connects transformer secondary to the main board.
I would like you to take that connector off the board, and solder the wires directly to the board.

So that being said,
This is not the amp to "learn on."
Have the updates installed by a qualified person, with the professional tools.
Circuit boards are easily damaged by inexperienced persons.

WTF does all this do?

FIRST, let me make it clear...if you test with a sine wave and oscilloscope, you will never find this problem.
You have to test with A GUITAR.

Yes, that is correct, test with A GUITAR. Your sine wave generator is futile.

There is actually a low frequency oscillation in the power supply.
This oscillation is triggered by playing a note or a chord on the guitar.
When this oscillation occurs, it causes the grid of V4A to be driven slightly positive, temporarily..
When this occurs it causes V4A to temporarily distort, a "cutoff distortion."
This cutoff distortion is the MUD you are hearing.

A. muddy sound, undesirable distortion
B. sometimes a "buzzing noise." when a note is played only. This is temporary ripple in the output of V4A...
C. Sometimes a "tremolo effect." sound cuts in and out.

By stiffening the power supply, the low frequency oscillation will cease to occur.
the amp sounds considerably cleaner
the highs are noticeably cleaner and brighter
much less muddiness, less undesirable distortion...
buzz sound is gone.
Can crank amp up much higher, without oscillations. Tone is coming thru unhindered.
Now, it sounds like it should sound. New Amp.

soundguruman

Hi Everybody,

I have joined this forum purely so I can comment on the Nomad Power Supply mod as detailed by soundguruman. I normally just lurk, cherry picking gems magnanimously proffered by other benevolent forum posters, but no-one has posted any post mod feedback so I decided to go for it and give soundguruman the credit he deserves.

I hope its ok to resurrect such an old thread...

Also, its a long post, but there is a lot to say...

I bought my Nomad 45 112 about 10 years ago after I played a gig where the back line was provided and to my surprise I was supplied with a Nomad 55 212 to use on stage rather than the cheesy amp I was expecting. I really liked the amp for the time I had to use it during the gig. Obviously there wasn't too much opportunity to tweak and play but I just plugged in, switched to channel 2 and just used the volume on the guitar, not knowing about the solo switch and didn't bother with channel switching as I'd only had time to set up the one second channel. I liked the way the amp cut through on stage and in the mix so decided to get one for myself (I'd always promised myself I'd get a Mesa Boogie one day and that day had come!).

I opted for the 45 112 for its size and convenience for gigging and smaller stages. I was unaware of the EL84/6L6 difference in the power stages.

I re-tubed with JJ matched quad EL84s and high gain 12AX7 preamp valves. The amp sounded ok but didn't blow me away and certainly didn't seem to have the clarity of the 55 I initially tried. I don't know if 55 amp was stock and just a lucky good sounding one. There seems to be quite a lot of variation in both sound and reliability in the Nomad range from what I've read online.

I came across this forum and the "mud mods". People were raving about how the mods brought the amp to life so I thought I'd give it a go. The mods certainly improved the darkness issue and gave the amp more "airy" breath and top end and yet it still didn't quite sound right. Kind of edgy and brittle?

I did the reverb mod too and whilst this made the reverb more audible on the overdrive channels 2 & 3 it was quite horrible and distorted on the clean channel where it had sounded lush previously.

Years later (now) I decided to reverse the reverb mod and so looked up the page I bookmarked with the mods on in case I wanted to un-mod at any time in the future. This was when I came across the mod suggested by soundguruman to replace some of the electrolytic capacitors in the power supply. I read his description of the problems caused by the power supply design. His conviction that this was indeed why the amp didn't perform as well as could be expected made me curious to try it.

I located the schematics on line to be able to interpret the instructions.

I have to concur that this mod is not for the faint hearted or inexperienced (I am an IT technician and ex wireman and comms engineer so have access to a temperature controlled soldering iron and solder sucker). It took a while finding the right diameter capacitors with the correct lead pitch but a company called Semicom in the UK and on eBay had both the 47uF & 22uF electrolytic capacitors in 12.5mm case diameter with 5mm lead pitch, 450V rating, 105C temp rating & 20,000 hours MTBF Nichicon manufacture. First of all I reversed the previous "mud mods" to put the amp back to standard

There is a lot of work required to be able to get to the underside of the board. This is necessary to be able to de-solder and re-solder the radial electrolytic capacitors.

As soundguruman suggested none of the other mods are needed after the power supply is sorted by stiffening.

The modded amp is completely different! The clean channel now has some sparkle. Its easier to hear the tone controls working. It sounds much nicer when "pushed" and overdrives warmly and is more rounded.
The overdrive channels - channel 2 is now thick and fat but warm too, much creamier; channel 3 is now usable! It has more bite and clarity rather than just fizzy mush. It can be very heavy!

When cranked the amp just seems to get nicer as the output increases. I haven't had a chance to try it in a band environment but I've found myself playing, sat in front of the amp just listening to the tone and dynamics. I don't know if anyone else has tried this mod but I just had to comment as no-one else has. Having tried both approaches I felt compelled to join up and post this for anyone else who isn't satisfied with their Nomad, but like me feels that the amp could sound so much better

Soundguruman said to another poster that they would thank him after performing this mod; I haven't seen a reply but I signed up purely to be able to do that - THANKS Soundguruman you've certainly lived up to your username in my experience 8)

Did the cap augmentation posted by soundguruman and the amp can deliver considerably more low end punch, where before it just went to mush. Checking other mesa 100 watter schematics I noticed that all of them have two or three times total capacitance on these nodes than the nomad does. It looks like mesa wanted to experiment with making the amp spongey by using smaller caps. I like dual recs in tube rec mode, but the nomad just lost too much fun as it was.
Removing the board is pretty difficult. Much easier to piggyback more caps over the nodes, or strap in the caps near the air vents and run leads.