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eugene martone 8th January 2013 10:12 AM

Questions about Champ mods, NF loop and more
Hi everybody. Posted this in the wtong forum, so here we go again:

Going to build a champ, and as I've already played a few of those, I want to make a few changes (even before the build, cos when I've got it working I'm not going to change anything for a while)

Using the 5f1 schematic I'll try to make the overdrive a bit smoother/ more marshally (read jtm45). I'll add a cathode bypass cap on the first gain stage, either a 47 or a 25 mF cap, keeping the 1,5k resistor. Next i thought about changing the cathode resistor of the second stage to 820 ohm to run it a bit harder. Then I thought of using sozo mustard coupling caps making the first .022 and the second .01???

Any other tips would be appreciated. Or if you think any of mine are counterproductive.

Now to the most important question: Negative Feedback?
I've seen a lot of people remove the NF loop, but it sort of takes the amp the opposit way of where I want it (Yes it breaks up earlier, but sounds even less smooth).
I'll be using a bigger OT with 4 8 and 16 ohm taps and was contemplating taking the NF from the 16 ohm one (like Marshall did on the JTM). Any one try this, or anyone smart enough to predict how this might work? Should I just lower the value of the NF resistor? Just leave it alone?:confused:

PS I'll put a g12m in an oversized cab for this so that should take me closer to the marshall sound.

PPS I know it'll still be a champ and I wouldn't want it to be anything else, but just want the od to resemble early marshall a little more...

cyclecamper 8th January 2013 08:01 PM

Just got my first Champ myself...
You can always put in a hidden switch for the NFB loop.
The heck with tradition, go for perfection of your tastes and needs, as you're already using a 'better' output transformer (good move IMHO).
For a champ, some don't use standard layout & wiring techniques like twisting the heater wires because it's so quiet they don't notice the noise. It's probably worth doing, and getting rid of all noise IMHO. You're going to push it hard, but might also mic it or use it quieter. The real champ has little power so little dynamic range, a good thing for compression sustain and early breakup. It's so quiet that it doesn't seem to have much noise until you mic it, direct box it, record it, or play quietly in your bedroom, then you may notice noise. So IMHO the best things you can do either make it a little bit louder or reduce the noise, like the Weber trick of raising the heater AC with some DC offset bias, via a 10/1 resistor voltage divider across the B+ to give 1/10 the B+ DC voltage to the center tap of the the heater secondary (or to the middle of two resistors across the heater power). Supposedly, residual hum disapears. I have not tried it yet. But what I really want is that real Champ sound with a bit more power, without changing tube types or going to push-pull instead of real Champ single-ended class A. If you can afford a really big really good single-ended output transformer with a gap and that can burn off the DC, you can use parallel single-ended output tubes and half the transformer primary impedance and if your transformer doesn't spoil the sound, still sound like a Champ if you size the power supply right for similar sag. You actually reduce the transformer ratio, which may make things sound even better. That gives a real class-A Champ clean sound and the tube charactieristics come thru as it gets dirty, but the power improvement means you can have a wider choice of speakers and cabinets, and the 'clean' mode becomes usable, the 'dirty' mode is still great, and if the noise is kept low it records really well. And best of all IMHO the clean-dirty transition is amplified and expanded into a wide range of delicate intricate tonal differences. For recording, don't even bother with a tone control on the Champ. Mine just has two volume controls. I want parallel single-ended class A version of a Champ because just that little more power doesn't really make much difference in volume, it just makes the clean mode usable and expands that range between clean and dirty, yet the dirty mode isn't that much louder because twice the power is only a few more DB; changes the touch response more than the volume. But you should hear a Champ into a light-cone 15 with a small voice coil in a big open-back cabinet, that big sound and the big cone flapping in all those various modes. I also want it as a head, so I can use a variety of different speakers and cabinets, as I don't need the convenience of a combo unit.

Good luck. You are asking personal questions, my opinions are also personal and may not fit another person. That's for a swampy bluesy amp that maximizes 'touch response' as you play across the clean/dirty line for the emotive effect of tone changing with touch from sweet croon to hoarse scream.

For a Marshall sound that is all in the distortion mode, for that rhythm crunch and so that every lead note sounds like it came from the same machine gun due to compression and sustain and distortion, kind of like 'auto-correct' mode to 'fix' any picking technique problems and sound like rock and NOT sound like jazz or blues, that's the opposite of touch response, and makes hammer-on, pull-off, tapping, harmonics popping out, sweep picking, all have similar tone and attack and volume envelope...well then you want a 12" greenback and not a 15. Again, if you really want a Marchall sound put that greenback in a really big closed-back cabinet and you might also want that parallel-output single-ended class-a bastardization of a son-of-Champ. IMHO the key to Marshall sound is to keep the preamp bright, generate the distortion with all that treble, then voice the output to lose some of the treble and keep the Marshall mid peak. The speaker may help in that final treble-cut and mid-boost. But the preamp has to remain bright with plenty of treble for the distortion to generate rich overtones and for the harmonics to pop and chord complexity. If you get it voiced for the "right" frequency response but put the bass on first then the treble, the distortion won't come out right. The disotrtion has to be generated on a very treble signal, then the treble cut back later. That worked well for the Marshall effect on the Peavey Classic 50 dirty mode in the combo with 12's, though many feel they went too far in limiting the bandwidth (almost no bass). It really got impressively close to a Marshall sound but using EL84's, impressive for a cheap printed circuit mass-produced box.

So if you want Marshall rock sound, re-consider your circuit mods to keep the preamp trebly, even bright, then knock the treble off later, maybe in the feedback loop, possibly even with a crossover-type coil in the speaker line.

We really have two different goals, but I hope it's food for thought.

eugene martone 9th January 2013 05:14 PM

Thanks man, that was pretty informative, set me off thinking :D
I'll have to look into that webber mod as well.

Keeping in mind that I prefer the 60s marshall sound (JTM45 and 45/100), and I do enjoy a little blues (probably why I want a champ in the first place), would putting a 0.68 uF cap on the first cathode and (for instance) a 1nF plate bypass on the second stage (and perhaps increasing the cathode bypass on the power tube to 47uF?) get me closer? I mean I don't wanna build a class 5 :p

Also, does it seem like a good idea to put in a switch to select witch tap the feedback loop is taken from?

cyclecamper 10th January 2013 12:32 AM

I don't really know since I haven't tried those, and I don't have schematics in front of me, and I'm still kind of new to my own Champ.

All I can recommend for a Marshally sound is to keep the first stages and distorting stages bright and trebly via small caps across resistors, have plenty of gain, then put any treble-cut caps in the later stages. When you tune an amp to your taste, it's mostly adjusting those filters, but the sequence is important too, because it's not just the summed up total EQ that matters, the EQ applied at the stage where the distortion is generated makes a big difference. I like a treble-cut AFTER the distortion, but treble-boost early so that the guitar isn't dull.

bright treble-boosted but somewhat lmiited bandwidth (not too much bass, lots of treble) >>--> more gain >>--> distort >>--> low-pass to cut treble and make it sound 'creamy' and not irritating = Marshall sound.

But that's just my opinion. The amount of gain may be more important! BTW there are good books available, including some about only the Champ circuit. I haven't even read them, I just wish I could afford them.

eugene martone 11th January 2013 10:01 AM


All I can recommend for a Marshally sound is to keep the first stages and distorting stages bright and trebly via small caps across resistors, have plenty of gain, then put any treble-cut caps in the later stages
Yes, but where to put that treble-cut cap? and how to boost it (normally with the cathode bypass i presume)?

So, now I'm thinking 1uF bypass cap on first stage. both coupling caps .022. Second cathode resistor 1k (down from 1.5). Some sort of gentle treble-cut somewhere??? 15w multitap OT. NF loop switchable between 4-8-26 taps.
Anyone try any of these changes in a 5f1?

eugene martone 11th January 2013 12:41 PM

What about putting a 2k resistor on the first cathode? keeping the second at 1k(or even 820)? with a 1 uF bypass cap on the first stage. Am i right in thinking this would give me a brighter "gainy" first stage and a more crunchy second stage. Will it sound at all closer to a jtm or am I moving towards more of a stiff modern "high gain" sound here?

PS Just in case I haven't made it clear, I don't wanna build a marshall (I already have 2), I just want a champ that overdrives smoother (more grinding like a jtm) and perhaps has a little more even frequency response. My own little Champenstein..?

eugene martone 4th April 2013 07:43 AM

Ok. Finally got my parts together (those sozos took over two months to get here), and I'll see if i can't start the build sometime next week. Just assuming someone's interested in my results, I'll post some thoughts and perhaps clips when I'm done.:cool:

cyclecamper 5th April 2013 04:37 PM

I'm curious what output transformer you're using. My champ 'kit' came with what looks like a 2-watt output transformer, and I'm just now adding a 15-watt Hammond. I am really eager to compare the sound, but I have SO many projects in-progress at the moment... What a time for my test gear to break down. Now my output transformer will be bigger than my power transformer...which is probably appropriate for single-ended, but also says the 'kit' power transformer was undersized too. I just want to learn what that output transformer saturation distortion sounds like, compared to its elimination. I might even leave both transformers installed, with molex plugs to select which I'm using; after all the original one weighs almost nothing.

I'd really like a combo, but I can't decide what speaker. So I guess I'll build a head and a few single-driver cabinets. A large sealed cabinet should help get that Marshall sound for you. Sorry I'm not more help in predicting what your proposed circuits will sound like...that's the kind of learning I'm hoping to get from these projects. Please do post as you progress.

shanx 8th April 2013 03:27 AM

Please post your progress, it should be interesting. Couple of suggestions, would be going in direction cyclecamper suggested. Look at first stage bypass cap , and if you want more gain emphasis on treble start by reducing the cap value. You can calculate where you want the roll off point to be. It will also get a lot less flabby sounding on the bottom end. A bright cap across the volume will also do, but it's effect is only getting to second stage. Where do you roll off all those highs? You could do some in the NFB network. Now with negative feeback what you actually need to do is send back more of the high frequency content to that "inverting" node which will now selectively drop off more hi end gain . This rolls off the hi end fizziness. Look at a Marshall presence controls etc to get a feel. It will also be subjected of course to rolloff from speaker and transformer, so maybe not too much will be needed. You will probably find a nice balance point between preamp overdrive and poweramp breakup with the NFB. Don't need to change OT taps, you can change resistance value (and capacitance), to get there.

shanx 8th April 2013 03:28 AM

Sorry on my previous post should have said look at first stage cathode bypass cap

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