nomad mods

[Koreth]

Aren't most "Presence" controls on an amp a negative feedback control anyway? If we wanted to mod the Nomad to have a more traditional Presence knob, wouldn't the place suggested for the "extreme" switch be the place to wire the presence knob?

 

[nomad100hd]

well no, cuz there a fixed presence control in the negative feedback circuit already there. So you would replace the fixed resistor with a pot. The switch would be were you would put in a pot to let you have a variable nfb, instead of just on and off.

 

[Koreth]

I guess I am confused as to how this whole presence and the NFB thing works then.

 

[Monsta-Tone]

NFB is sort of like adding exhaust back into the combustion chamber of the engine in your car.
It allows the engine to burn cleaner and waste less fuel. This is the case with NFB.
It allows the power amp to have more headroom before distorting.

Basically, Fender started adding gobs of NFB in the mid to early 60's because Leo couldn't fathom the fact that distortion might be popular one day. Basically, Michael Fox was right in Back to the Future when he said, 'Your kids are gonna love it someday!'

The Heartbreaker has a switch that is called, 'Bold/Curvaceous." It basically changes the NFB resistor from a 10k resistor (Bold) to a 1k resistor (curvaceous).

Some of the Mesa amps, like the Rectos, cut the NFB out on the distortion channel in the 'Modern' mode.
This means that there is no NFB going back into the Phase Inverter circuit.
We have a footswitchable mod that adds a pot to adjust exactly how much NFB is used at a given time.
The reason for the pot is that there is a huge increase in volume when the NFB is cut out of the picture. The pot allows you to dial in just the right amount.
The amp gets much louder and wilder. Harmonics seem to jump off the fretboard. :lol: Now, if I could just figure out how to make it remove women's undergarments at the same time..... :lol:

 

[earachemyeye]

I just read how Louis Electric Amps have this feature. It sounds interesting. I'm curious, does anybody ever use the pushed mode on ch.1? To my ears it sounds like a cranked Fender Champ but it's too harsh to be usable for me. A mod there would be worthwhile. Something to cover the distance between ch1 and 2.

 

[lerxst88]

ive found that a moderately cranked tubescreamer is great for an in between gain sound.

ive actually thought about making the pushed switch a bright switch. just to have something useful on the clean channel

 

[earachemyeye]

Your right, I just put a TS10 on my pedal board and it is very nice for that. If you can get your hands on a MT10 (Mostortion) try it with the Tube screamer in front of it. More bass and smoother breakup. Very nice.

 

[MusicManJP6]

I've been meaning to post these pics for the last two weeks! I no longer have the Nomad (traded it for a Rectoverb), but while I had it I performed the 'mud mod' (as I like to call it), and the FX loop mod as well. And, I took pics!

Mud mod BEFORE:

Mod mod AFTER:

FX loop mod BEFORE:

FX loop mod AFTER:

Both mods made a world of difference IMO!

 

[Monsta-Tone]

Here is a good explanation of what these capacitors did: http://www.tone-lizard.com/Mods_and_Odds.htm

"No phase inverter is balanced perfectly, even in Hi-Fi equipment."

The balancing of the two outputs is not a matter of life and death, seeing as the output transformer won't be perfectly balanced. The adding of a Presence control gives a little more flexibility, but even with a 100-ohm resistor in this spot a 'Presence' control is still possible (it would need to use about a 1uF capacitor). Of course you know about trying different phase inverter tubes. The standard Fender 'Blackface' phase inverter is a 12AT7. A 12AX7 will give more drive, and a 12AU7 will give less drive. Sometimes across the phase inverter plates will be seen either a 47pF or a 100pF 'snubbing' capacitor, that we were introduced to earlier. This was done over at Fender to facilitate speedy production methods. Rather than learn neat and tidy lead layout, the capacitor was added as an insurance measure to guarantee a stable amplifier. You have a 50% chance of cutting it out and not having an oscillating amplifier, but instead getting a little high end 'sparkle' or 'sheen' back. Most people don't notice the difference, though. As a final thought, I have seen snubbing capacitors through every amplifier stage and even across the output tubes as well. What ever worked for the prototype amplifier was used in the production run.

I'm thinking about removing it from my Heartbreaker and seeing if there is a huge difference.

 

[earachemyeye]

Thanks! I like learning about this stuff. I was playing through mine yesterday and I'm amazed at the snap it has added to ch.1. Hope yours turns out the same.

 

[Koreth]

Looking at the schematic, these 120 pF capacitors are wired parallel to the plate resistors on both halves of 12AX7 in the phase inverter circuit. Digging through schematics, I made up this list of Mesa Boogie amps which have these 'load' or 'snubbing' capacitors. I couldn't find schematics for the Lone Star Special, Stilletto, Mark V, Electradyne, Roadster, or Road King, though I'd guess the latter two have them, like other models of the Recto. No idea about the Mark V, Stiletto or LSS.

With PI plate bypass caps:
Blue Angel
DC-5
Dual Rectifier
Heart Breaker (common cathode side only)
Lone Star Classic (common grid side only)
Mark I reissue (common grid side only)
Mark II (common grid side only)
Mark IIB (common grid side only)
Maverick
Nomad
Rect-O-Verb
Trem-O-Verb

Without PI plate bypass caps:
50/50 Power Amp
2:90 Power Amp
DC-3
Mark I
Mark IIC+
Mark III
Mark IV
Son of Boogie
Studio .22 Caliber
Studio .22+ Caliber
Studio .50 Caliber
Studio Caliber DC-2
Subway Blues
Subway Rocket

That's almost a 50/50 split, a perfect 50/50 split if you take those two power amps out of the latter list. Are all of the amps in the upper list considered dark and muddy and all of the ones in the lower list bright and tight? I doubt it. While I don't doubt the Nomad PI cap mod has an effect, I'm not sure it's were we should be looking in the Nomad's circuit to cure a mud problem. I think we should probably look at the value of components in the tone stack or the cathode bypass caps in the preamp first.

 

[earachemyeye]

I wonder why they would leave them out of those particular models? It changed the sound of the gain on mine for sure. The clipping is brighter sounding on top, not as smooth on the trail off (FWIW I'm running el34s). I bumped the 200k,800k and 1.6k up on my eq and it sounds very nice. I'm sure there's some tweaking that could be done to the tone stack also but this was an easy one.

 

[nomad100hd]

UR list is wrong btw my schematic at least for the dual rec don't have the the pi load cap, didn't check the other amps. Any preamp i've plugged into my nomad was to muddy to use without serious preset tweaking. The tone stack in channel 2 is a marshall stack same with channel 3 but with one resistor change. also if i come out of the nomad's preamp into another amps power amp there is no mud.

 

[Koreth]

These were the schematics I referenced, and per both schematics, the Dual Rectifier does have plate bypass caps in the PI. I didn't spot them at first because of the way the circuit diagram is laid out, but as soon as you find the pate resistors, you'll see those 120pf capcitors wired parallel. I do see what you mean about the Nomad's tone stacks being the same as a marshal one. Looking at some schematics for Marshalls, I see the same layout, with the exception of the resistor and cap values for the treble knob, and slightly different pot values for all three knobs.

I'm curious at what point in the signal path you patched other preamps into the Nomad's power amp and the Nomad's preamp into other power amps. Through the FX loop? If so, isn't it possible that the tube in the V4b position (FX Return) was coloring and muddying the sound? I'm not saying that removing the plate bypass caps from the circuit doesn't brighten and clear things up, I'm sure they do. But Mesa/Boogie put those caps in there for a reason. I highly doubt it was in an effort to keep the Nomad somewhat muddy so as to keep it from taking too many sales away from their flagship lines (especially when one of those flagship lines also has plate bypass caps in the phase inverter) or because they didn't know what they were doing. When I read up about what plate bypass caps are supposed to do, (trim off excess highs, tame excess gain, prevent oscillation), I'm wondering if they made it into the amp to keep the power section clean, stable and from being unbearably bright.

 

[nomad100hd]

i see the caps on the rec. I just think ur assuming to much bout a company! the nomad after all is a blues amp we just like to crank the gain. I forget what marshall schem i was looking at that had the same values as the nomad. The recto is often complained about being muddy, and having no cut. i have heard the heartbreaker a little muddy.

 

[timowens]

I too struggle with the muddiness of my Nomad 55. Whenever I found this thread about the PI mod I was curious because a 120pF in parallel with a 100K crosses over at 13,263Hz and an 82K & 120pF at 16,174Hz. To me this would seem pretty much inaudible in a guitar amp so I figured they were to help prevent noise and oscillation but since I had to open my amp up anyway to work on one of the pots (don’t you hate the pots in the Nomad) I thought I’d try the mod. I took some pictures if anyone wants and I also recorded some before and after clips to see if I could really hear the difference. To me, it is real hard to hear the difference, on ch-3 with high-gain it is really hard but with ch-1 and ch-2 there might be a noticeable difference. I didn’t spend any time dialing in any tones and I didn’t spend any time adjusting the mic placement for the recording but for an A/B comparison the test should suffice. First I started off with ch-3, then I switched to ch-2 and ch-1 and back to ch-3. The only discrepancy would be when I backed off the volume on the guitar, I didn’t pay attention to how much I exactly backed it off between the two recordings but it should still give you an idea of the difference.

I think I will leave this mod in for now even though its effectiveness is questionable. I would like to say though that I did do another mod that seemed to help more for my setup. Since I use humbuckers I always struggle with too much bottom end, especially in high-gain settings, it just seems to really get muddy on the low notes. To tighten it up I replaced a .02 cap on the input circuit (C32 I think) with a .0022 cap and it really helps, I’m even thinking of going a bit smaller. I have pics it anyone is interested.

You can hear the test clips of the before and after PI mod at http://www.soundclick.com/timowens

 

[nomad100hd]

i noticed a big difference with mine and so did everyone at brand practice.

 

[Koreth]

I too struggle with the muddiness of my Nomad 55. Whenever I found this thread about the PI mod I was curious because a 120pF in parallel with a 100K crosses over at 13,263Hz and an 82K & 120pF at 16,174Hz. To me this would seem pretty much inaudible in a guitar amp so I figured they were to help prevent noise and oscillation but since I had to open my amp up anyway to work on one of the pots (don’t you hate the pots in the Nomad) I thought I’d try the mod. I took some pictures if anyone wants and I also recorded some before and after clips to see if I could really hear the difference. To me, it is real hard to hear the difference, on ch-3 with high-gain it is really hard but with ch-1 and ch-2 there might be a noticeable difference. I didn’t spend any time dialing in any tones and I didn’t spend any time adjusting the mic placement for the recording but for an A/B comparison the test should suffice. First I started off with ch-3, then I switched to ch-2 and ch-1 and back to ch-3. The only discrepancy would be when I backed off the volume on the guitar, I didn’t pay attention to how much I exactly backed it off between the two recordings but it should still give you an idea of the difference.

I think I will leave this mod in for now even though its effectiveness is questionable. I would like to say though that I did do another mod that seemed to help more for my setup. Since I use humbuckers I always struggle with too much bottom end, especially in high-gain settings, it just seems to really get muddy on the low notes. To tighten it up I replaced a .02 cap on the input circuit (C32 I think) with a .0022 cap and it really helps, I’m even thinking of going a bit smaller. I have pics it anyone is interested.

You can hear the test clips of the before and after PI mod at http://www.soundclick.com/timowens

The crossover points you listed are the -3 dB points if the PI bypass caps were a lowpass filter. But placing the bypass cap in that position creates a shelving filter instead. The cap's resistance at high frequencies is much lower, acting as a low value resistor in parallel with the plate resistor, presenting the tube with less load resistance at high frequencies, reducing the gain at those frequencies. Conversely, the lower the frequency, the higher the cap's resistance, which in parallel with the plate resistor, presents a greater load to the plate, increasing the gain at lower frequencies. Thus, rather than simply cut everything above a certain point, The highs are cut, and the lows are boosted. This would by why some find the Nomad muddy. Cutting highs is one thing, but cutting highs while boosting lows is going to create a muddy sound. Try it on your favorite stereo sometime. Turn up the bass while turning down the highs. Going too far will make for a muddy sound.

 

[nomad100hd]

thank you for your excellent explanation!

 

[cscotto]

The crossover points you listed are the -3 dB points if the PI bypass caps were a lowpass filter. But placing the bypass cap in that position creates a shelving filter instead. The cap's resistance at high frequencies is much lower, acting as a low value resistor in parallel with the plate resistor, presenting the tube with less load resistance at high frequencies, reducing the gain at those frequencies. Conversely, the lower the frequency, the higher the cap's resistance, which in parallel with the plate resistor, presents a greater load to the plate, increasing the gain at lower frequencies. Thus, rather than simply cut everything above a certain point, The highs are cut, and the lows are boosted. This would by why some find the Nomad muddy. Cutting highs is one thing, but cutting highs while boosting lows is going to create a muddy sound. Try it on your favorite stereo sometime. Turn up the bass while turning down the highs. Going too far will make for a muddy sound.

Hi Koreth,
Thanks for the excellent explanation. It makes me think the addition of the cap was not really about cutting highs because of the frequencies involved. That is, rather than statically raising the gain of the PI by increasing the plate load resistor (and the cathode resistor), Mesa created a dynamic circuit that only boosts the gain in the presence of low frequencies. If I am not mistaken, there already is a 75 pf "snubbing" capacitor across the power tubes a little earlier in the circuit that would take care of squealing. There are two points to this. First, could you achieve the same effect by putting a lower gain tube in the PI position? Second, perhaps the reason some people might not hear a difference in the pre/post mod sound is they are listening for the wrong thing. I mean, you might expect to hear more highs, but what you really hear is just more clarity in the bass. That is what I hear in the sound clips a few posts back. The pre mod clip sounds like someone talking with their hand blocking the air stream from their mouth. The post mod clip sounds like the hand has been removed and you can hear the words more clearly. Thanks again for the great explanation.