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PostPosted: Sun Nov 09, 2014 10:29 am 
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Mark II

Joined: Tue Jan 07, 2014 10:54 pm
Posts: 70
Jonom wrote:
soundguruman wrote:
Jonom wrote:
I posted this in modern amps but didnt get a reply, so I am posting it here, too.
My Nomad 55 combo has developed a hum. Anyone know what could be causing a hum in the red channel? The other two channels are fine. Changed all my tubes thinking that would solve it.
Still there. Idea's anyone? Someone told me could be the power transformer going bad, but if it was that wouldn't the hum be present across all three channels?

Thanks in advance...


Change the 22 uF capacitors in the preamp high voltage supply to 47 uF, change them all.
This will eliminate the hum, buzzing noise, and will get rid of the muddy sound.

Hey thanks for the reply Soundguruman!
I will get to swapping out the caps soon and see how she sounds.


You probably are asking questions about this, so i am going to post some answers...

The capacitors are put in parallel, to achieve the desired values, because two caps in parallel will handle twice as much ripple current.

The little radial cap can only filter "so much" current, from the ripple of the rectifier.
There is a current limit to the power supply ripple, before the little cap starts to overheat...and break down...

But using 2 caps in parallel, the ripple current rating is doubled.
As you see, they are using little tiny radial caps, and these don't handle much current individually...
(big beefy caps are made to handle high ripple current)
So there is a compromise between cap size, and life expectancy.

The little cap is trying to kill filter out that ripple and it gets hot...but 2 in parallel shares the heat between both caps.
Since the heat is cut in half, (by using 2 instead of 1) the life expectancy is basically doubled.

But more than this, the caps are not that great quality to start with.
They are just cheap caps, usually. Not really good ones.

So when you buy new capacitors:
Check the voltage rating, the ripple current rating, the temperature rating, and the lifespan hours rating.
Also, check the lead spacing, and the diameter and height, to make sure the new caps will fit on the circuit board.

I will always buy the highest life hours rating (20,000 hours or higher), the 105C temperature, the higher ripple current rating...
all this will make my capacitor last longer. The upgrade will only cost a few cents more - per cap.

The truth is - buying these caps won't cost a lot of money.
The good caps are affordable, and not very much money! So, the whole deal is worth the cost.

You have been WARNED!
The circuit board is easily damaged. DO NOT attempt the upgrades without:
Professional tools and equipment! You need to know HOW to use them.
Hire a Technician to do the upgrades, if you are not qualified to do them yourself.


Here is the 3 basic Challenges:

1. Good Tubes. The tubes must be good or you will suffer.
2. Good Relays. The mechanical relays (channel switching relays) must all be working 100%!
LDR light dependent relays: must all be working 100%.
3. Power Supply. The filter caps must be upgraded, to make the HV supply stiff enough.
(Minimum: replace HV caps in preamp supplies with: 47uF)

And once you have those 3 verified as working...you will usually have quite a nice sounding Nomad.
And as I said, there may be no need to do any modifications.


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PostPosted: Sat Nov 14, 2015 6:22 am 
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Bottle Rocket

Joined: Sun Nov 01, 2015 7:58 am
Posts: 3
Hi Folks,

Does somebody know if a mod like this can be done on a DC3?

I've looked at my DC3 chassis but the only cap I can see is located in a completely different position...

Cheers :)


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PostPosted: Tue Aug 16, 2016 4:51 am 
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Bottle Rocket

Joined: Tue Aug 16, 2016 3:03 am
Posts: 4
Hello, an old thread but I have been following it recently. I tried one of these mods the FX loop one and it helped somewhat but what has completely opened
up the sound of this amp was getting rid of the black shadow speaker and replacing it with an EV 200 watt. Overkill I know but it was to hand. I -love- the sound of the Nomad now.

What I am really interested in now is if anyone knows if it is possible to make channel 3 a clone of channel 2 or at least make it sound somewhat like it.
I used to like channel 3 when I use EMG pickups but now its practically unusable for me using standard strat pickups.

I have the 45 watt 1X12 version.

Any thought on this greatly appreciated :-)


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PostPosted: Wed Nov 09, 2016 6:17 pm 
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Bottle Rocket

Joined: Wed Nov 09, 2016 6:10 pm
Posts: 1
I know this is an old thread, but I had a quick question if anyone can help, I've done the negative feedback mod on the 8 ohm tap, if I use my extension cab and the internal speaker together on the 4 ohm tap, won't I need to remove the purple cable attached to that tap too? This might be really obvious but thought I'd check first.

Thanks
Jimmy


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PostPosted: Fri Aug 11, 2017 5:34 am 
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Bottle Rocket

Joined: Tue Aug 08, 2017 8:05 am
Posts: 4
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)


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PostPosted: Sun Aug 13, 2017 8:11 am 
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Bottle Rocket

Joined: Tue Jul 25, 2017 5:26 am
Posts: 13
soundguruman wrote:
Then at V1 input, pin 7, you should install a 68K resistor in series with the ferrite bead.

.


Hi PlankSpanker, a very interesting feedback, but did you have made this change too?


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PostPosted: Wed Aug 16, 2017 7:28 am 
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Bottle Rocket

Joined: Tue Aug 08, 2017 8:05 am
Posts: 4
Quote:
soundguruman wrote:
Then at V1 input, pin 7, you should install a 68K resistor in series with the ferrite bead.

Frankolino wrote:
Hi PlankSpanker, a very interesting feedback, but did you have made this change too?


I hadn't added the 68k resistor but I've just been reading about it here http://www.aikenamps.com/index.php/grid-resistors-why-are-they-used. From what I've read it sounds like it should clean things up a bit more (especially at the high end frequencies) and reduce the input noise and hum on the overdrive channels.

So, I've just ordered some 68k resistors which I hope will be here for the weekend. I will post the results when I've added the resistor and had a chance to see how things have changed.

I am still well impressed with the upgrade to the pre-amp power supply and have been having a great time getting to know my new improved Nomad 8)


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PostPosted: Thu Aug 17, 2017 9:00 am 
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Bottle Rocket

Joined: Tue Jul 25, 2017 5:26 am
Posts: 13
Ok, thanx, was it difficult to de-solder and re-solder the caps?
I havn't found yet the right capacitors, Semicom in the UK did not have them anymore.
I only found someone that have the 22uf caps.


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PostPosted: Sat Aug 19, 2017 8:01 am 
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Bottle Rocket

Joined: Tue Aug 08, 2017 8:05 am
Posts: 4
As far as getting to the underside of the main circuit board goes I physically detached the front pots, toggle switches including power and standby and the input socket from the front of the chassis. I detached the three reverb pots and the extreme toggle switch from the rear of the chassis. I also disconnected the three plastic strip connectors near the fuse. In order to be able to tilt the board I de-soldered the three channel indicator LEDs (green, orange and red). It was also necessary to unscrew the two RCA scokets for the reverb in and out and also the three screws holding the power valve board. This the allowed for the front of board to be tipped upwards allowing access to the underside. I had an assistant hold the board upright while I applied heat to one of the capacitor solder pads while the assistant gently pushed the capacitor away from the pad being heated until the leg popped out and then did the same for the other leg. I used a solder sucker to clean the holes out.

Soldering the replacement capacitors was quite easy, just make sure you get a good solder flow to make a sound connection to both sides of the board. I used a temperature controlled soldering iron with a fine tip.

I checked on eBay last night and Semicom (Semicom2005 is the name of their eBay shop) had both the 22uF and 47uF in 12.5mm diameter with 5mm pitch leads made by Nichicon, although only a few left. If you are still having trouble locating them I can post the links to the ones I bought for you. It looks like they do international shipping too.

I am about to add the 68k resistor. It looks like it can go in the jumper strap which comes from the ferrite bead on the circuit board across to pin 7 of V1. From what I've read it should be located as close to the valve as possible.


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PostPosted: Sat Aug 19, 2017 9:22 am 
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Bottle Rocket

Joined: Tue Jul 25, 2017 5:26 am
Posts: 13
Thanks a lot, I've found the ebayshop and the 47uf caps, but the problem is they have only 2000 hours
service life. Soundguruman wrote it must be, when possible, at least 20.000 hours.
I did not found the 22uf caps at that site.


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PostPosted: Sun Aug 20, 2017 2:05 am 
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Bottle Rocket

Joined: Tue Jul 25, 2017 5:26 am
Posts: 13
@PlankSpanker , you have a pm


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PostPosted: Sun Oct 22, 2017 5:31 pm 
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Bottle Rocket

Joined: Fri Sep 16, 2016 8:13 am
Posts: 17
Hi! Is there a way to increase gain on channel 3? I have a Nomad 100 Head!
Thanks!


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