"Fixing" the Nomad's 3rd channel

[guitargeek6298]

Hello All! I'm new to the boards. I picked up a little while ago a Nomad 55 head and cab for $400. I wanted something with channel switching and something rather flexible, and this seemed to fit the bill, especially for the price! However, While channels 1&2 are useable to me (not great...), channel three sounds like a bad Marshall caricature.

I've poked around on the boards, and found the 11+ page Nomad mod thread, but I'm not sure if any of those mods are really going to get that channel to where I would consider it useable (although I haven't modded anything yet). For one, the bass roll-off is much too high; I understand that they wanted a tighter sound for that channel, but it's really kind of ridiculous. There's no "oomph" at all, and if I play my 2204 through the same cab, it's plenty tight, but still has plenty of "chug". The Mesa just sounds thin on that channel.

Second, I have to practically max out the treble and presence controls to get any sort of brightness, which is odd to me considering the bass roll-off issue; everything seems very narrowly focused in the mids, even more than a Marshall which tend to be very middy!

So I want to re-work the channel. I have some amp building experience, having built a couple single ended amps, plenty of mods to my Marshall, and I have a mostly built 2204 on my bench as well. I would like the channel to be a bit more "Marshally", if you will, but with more low end. I'm not going for some sort of clone of anything, but just to make it useable.

Looking at the schematic, the first thing I notice is how odd the negative feedback circuit is, at least to me coming from working on Marshalls. Even compared to other Boogie's, like the Dual Rec, it seems unusual, and the "presence" controls aren't really what I would typically call a presence; it's more of an overall filter off the tone stack and not adjustable negative feedback.

So where do I start? The biggest thing to me is the shelving of the bass, but I'm not quite sure where it's coming from. There are a few resistor/cap circuits in between the gain stages that I know are going to be affecting the frequencies, but I'm not sure whats dropping all the bass, or if it's even in the preamp where it's occurring. The other issue is the treble response; I'd like to be able to dial in some sizzle on the channel, but right now it just doesn't get there, which is why I'm looking at the negative feedback circuit.

Any input is appreciated!

 

[talltxguy]

I cannot offer any advice but would ask that you please post your progress and results as you hopefully complete the modifications. I am interested in this stuff. 8)

Thanks!

 

[TimeSignature]

Sounds like you might want a completely different amp! Is selling it and getting something you want more of an option? I had a Nomad 45. It was cool.. but weird. None of the channels were great.

 

[guitargeek6298]

Well, you're actually right; I do want a different amp! However, I couldn't get something that replaces this amp for what I paid. All my other amps are single channel, and I need the flexibility of having channel switching. Channels 1&2 are useable enough; not spectacular, but they work for what I need them to.

Plus, I like to tinker, so if I can get channel 3 to the point where I can use it, then all the better. I will probably only end up spending a few dollars on the parts at most, so it's not a very big risk.

Right now I'm just planning and thinking about it, but once I start, I'll try to document it here.

 

[deeman]

Best of luck! I tried a Nomad and I didn't like it much either.

 

[loylo69]

It seems to me that there is lot of room to improve this amp. At least, this is what I can see from the schematic. But the mods I'll suggest to you in a minute might be not so good sounding in real life conditions...

First of all, there is a mod in the 11+ page Nomad mod thread that could allow you to retrieve some brightness and life. You need to get rid of the two 120pF caps on the PI anodes (V5a and V5b), or reduce its value (see the mod thread to find some suggestion).
You can also try the "de-mud" described by Koreth on page 7 of the same thread, in order to increase general brightness.


Regarding the presence controls and the global negative feedback circuit. The nomad has a fixed global NFB circuit. It is similar to the Soldano SLO, except:

1) the "presence" pot of the SLO is replaced by a fixed 2K7 resistor. You could replace this resistor with a a 25K pot in order to have the traditionnal presence circuit. In conjonction, you may have to defeat the original presence circuit of the Nomad on channel 3 (even channel 2), by desoldering one of the wires on the presence pot. As you said, this circuit is a variable low pass filter that loads down the signal even when the pot is at 100k. By defeating it, you may get some extra punch. And with the new presence pot added in the global NFB in the traditionnal way, you may have better control on the brightness of your amp.

2) and the feedback is connected to the 8 ohms output tap while it is taken from the 4 ohms tap on the SLO. By lifting the feedback loop from the 8 ohm tap, the amp has more NFB resulting in a tighter tone. But too much NFB can make the amp sounding like a dull and lifeless solid-state amp. So I suggest you to reduce the NFB sothat it can also boost the bottom end of your amp. You have 2 possibilities: you can increase the 39K resistor in the NFB loop to 47K, 68K or even 100K (see what you prefer). Or you can desolder the NFB circuit from the 8 ohms tap and solder it to the 4 ohms tap.


Last, but not least, I feel that the first gain stage of the amp can be improved. But it will affect the 3 channels, which may not be a concern.
The first two triodes (V1A and V1B) are wired in parallel. It is a design that can be found on Matchless amps, but I haven't seen this on any other Boogie amps so far. The problem is, to me, that Mesa have kept the anode and cathode resistor values of the first gain stage of the Mark amps (150K / 1K5). But as the 2 triodes are in parallel on the Nomad, you may need to change several things in order to keep a traditionnal frequency response at the input:
1) Cut value of grid stopper in half (since input capacitance doubles with dual triodes, halving resistance keeps same freq response).
2) Cut values of plate resistor and cathode resistor in half.
3) Double value of cathode bypass cap.

So you may test with a 68K resistor in place of the 150K one, 820R in place of the 1K5, and a much bigger bypass cap (to increase the bottom end) from 10µF to 47µF. Another idea: just change the 150K to 220K, keep the 1K5 resitor and increase the bypass cap.

Please, keep us informed of your tests. :wink:

 

[guitargeek6298]

I'll have to check out the schematic on the SLO to compare the negative feedback. I've never played an SLO, but I have played some Hot Rods, and I always felt I could dial in plenty of that sizzle that I'm looking for with the presence control.

I agree that there seems to be a lot of room to improve the amp. If nothing else, it's a three channel, four gain stage amp! That alone makes it valuable and gives it a lot of potential.

I saw the mod where you clip the 120pf caps, but am hesitant to try it because I don't feel like there is a lack of brightness in channel 2, only channel 3. I'm trying to figure out what is throttling this channel in particular; it sounds like there's a blanket over the speaker when its switched on. I think disconnecting the presence control will be a good start, but there's also a filter before the gain pot - a 680k that's bypassed with a .002uf capacitor, and off of it is a .001uf with a 20pf to ground. the 20pf I imagine is there just to eliminate some potential oscillation, but the resistor thats bypassed with a cap forms some sort of filter... but I'm not well versed enough to know if it's a high pass or low pass, because that values are outside of what I'm used to seeing.

I noticed that the plate resistor was 150k on that stage, and thought about dropping it to 100k, but leaving the staged biased at 1.5k. The higher plate resistor with the bias resistor at 1.5k would make it biased a little warm, right? It might provide some extra headroom as well, but i thought I might change it just to experiment. However, I hadn't thought about how the stage being wired in parallel would effect the tone. I may have to experiment further.

Overall, I'm somewhat surprised at how little difference there is between the 2nd channel and the third; a lot is shared between them, but they sound so completely different. That actually gives me hope, because the second channel is at least useable.

 

[loylo69]

The 120p caps not only have an influence on the upper frequency range. Clipping them also clears up the bass frequencies.
You should try it by simply desoldering one leg on each of them. If you don't like the mod, it could easily be reversed.

Converting the NFB to traditionnal presence control and the 120p caps clipping are the two mods I'd surely perform if I had the Nomad.

You are right to point out the 680k / 2n filter, which is acting as a treble peaker. The Dual Rectifie has the exact same configuration between the first and second gain stage, with the 680k / 2n filter and a 1M gain pot bypassed by 1n cap (but the DR has one more gain stage than the Nomad). You can test different mods: 470k / 470p is traditionnaly used by Marshall as a treble peaker (even more treble than 680k / 2n, so not exactly what you are looking for), Soldano uses a 470k / 2n treble peaker in this position in the SLO but before a 500k gain pot, and a 1M / 1n treble peaker in the Hot Rod before a 1M gain pot (should be similar to 470k / 470p frequency wise, but less volume because of the voltage divider formed with the gain pot).
You may simply want to clip the 2n cap, and even changed the 680k resistor for a 150k resistor just like channel 2.

Clipping the 1n bright cap on the gain pot is also a good idea (or reducing it, from 470p to 47p).
The tonestack of the channel 3 is not so different from the one on channel 2. I wouldn't bother modding it, nor deal with the 20p cap to ground.

On the first gain stage, if you change the anode resistor from 150k to 100K with V1a and V1b paralleled, It would act the same as if you had 200k with a 3k cathode resistor on a single triode stage (pretty cold bias if I am correct, not warm). I would rather mod this stage with 100k anode resistor and 820R cathode resistor (equivalent to 200k / 1k6 single triode stage, similar to the first gain stage of the Dual Rectifier and the SLO).

 

[Monsta-Tone]

I think the Nomad is going to sound close to itself, no matter what you do.
On the plus side, it's not worth much, so modding it (if done well) won't really hurt resale value.

On the other hand, I might be interested in buying the head (not the cab) from you for a decent price if you decide to sell it.

 

[guitargeek6298]

I'm not really looking to radically alter the sound (yet?), but just change the frequency response. I've done similar mods in other amps, and things like just changing the value of a bypass cap can make a huge difference. However, as I mentioned before, I've mostly worked on Marshalls and similar, which are much simpler circuits. That's why I came here to get feedback.

Thanks for the responses!

 

[loylo69]

This thread made me want to buy a used Nomad amp to see how it sounds stock and test the mods I described.
Unfortunately, those amps don't come cheap on the second-hand market in France, in the 800/1000€ (1000/1200$) range. :?

According to the schematic, it seems to me that channel 2 may have a bit more gain than channel 2 because of the voltage divider formed by the 150k resistor and the 1M gain pot (instead of 680k/1n filter with 1M gain pot on channel 3).
The only thing is that a 15µ bypass cap on the cathod of V4a is activated on channel 3. And this should bring more gain, especially at lower frequencies, on channel 3.
Guitargeek6298, what is your feeling about it?

 

[guitargeek6298]

I'd say they're pretty close in gain. I might say that channel three has a touch more, but I typically play channel two in vintage mode, but channel three in modern mode, so it effects how the gain is perceived. I would think that bypass cap would have a pretty good effect, though, since it's on the second stage, and at 15uf it would effect most frequencies that we'd be concerned with for a guitar.

Actually, I'm surprised to see a cap that large with this many gain stages. Even on my Valve Junior, which is only two gain stages, a 10uf on the second stage is way too much and it gets flubby. Obviously, very different circuits, but that still seems to be a large bypass cap for an amp with this much gain. Actually, it gives some insight, because there must be a large amount of bass being rolled off prior to that stage, because that channel doesn't get flubby at all, but channel two actually can if the bass is pushed too high with too much gain.

 

[Koreth]

You do correctly note that channels 2 & 3 have very similar preamps, sharing the same signal path, with only the gain controls, tone stacks and a capacitor switched between channels. Thus, unless you start tapping into the swiching system of the amplifier, most mods you can do to affect one channel will have roughly equal effect on the other. You also correctly note that the "presence" control is not the upper-midrange boost in the negative feedback loop like in a Marshall or other amps, but a lowpass filter hanging off the end of the tone-stack. This is the same kind of "presence" control found on the red-channel of the Dual Rectifier. The Recto removes the negative feedback in the red channel in Modern mode, thus a traditional presence control wouldn't work. The values for the feedback resistors in the Nomad's feedback loop are such that they have the effect of a tradtional presence control set at noon.

I do recommend the the de-mud mod described on page 7 of the Nomad Mods thread. It is a very simple mod to do. pull out the 5nF cap, and replace it with with a 50pf cap (or 47pf, easier to find and close enough). This shifts the high frequency rolloff from 100Hz to 10kHz and helps brighten and tighten the entire amplifier. If that's too much brightness in channels 1 and 2 for you, you can always turn the high and presence controls down.

Next if you want to add more lows and low mids, there's a few places to look.

First, mods in the tone stack will only affect that channel. With all the controlls at noon in Modern mode, the Nomad's channel 3 tone stack has a 'hump' in the bass at around 80Hz, with the low mid's rolling off around 275Hz as the bass and mid controls interact. If you look at the frequency response graphs from the simulations I did in the Nomad Mods thread, you'll see what I mean. Reducing the value of the capacitor feeding the mid potentiometer shifts the low-mid roll-off up in the frequency band, widening the hump and giving you more low-mids. For example, changing the mid cap from .02µF to .01µF widens the hump in the low mids, with its peak now around 110Hz, and the low mids rolling off at around 380Hz. This also has the effect of shifting the notch in the mids controlled by the mid control from 650Hz to 900Hz.

Second, mods in the signal path of the preamp will affect both channels 2 and 3. You could try increasing the value of the cathode bypass capcitors on V4a or V3b, or reducing the value of the resistor in line with the bypass cap on V3b. The former will shift the low frequency roll off point of the filter formed from that capacitor and the cathode resistor down in the frequency range, from the high-mids, into the mids, and down into the low-mids, depending on how big you make it. But be careful here, too much could make both channels 2 and 3 muddy. The latter option will increase the effect of V3b's cathode bypass cap. With the 1k resistor in line with it right now, its effect is suble. Removing that resistor entirely and replacing it with a jumper will give you boost everything from about 275Hz on up on V3b.