Marshall VS100 analysis & etc

[Josh X]

Longtime listener, first time caller. Most of the time I can find whatever I need to know just by searching, but today I'm not modifying my CE-2 to make UFO sounds or trying to hi-fi my Tube Screamer, so I could use a little more direct help with something.

The "something" is a 100-watt Marshall VS100 Valvestate combo. It's been my main amp for a number of years now, and I like it, and I've generally not messed with it, but like everyone else I got the jones for tones, and I'm open to opening it up. I searched for what I could find on it but didn't really find much. So I'll share what I've igured out on my own in return for any input.

The schematic for the preamp section, via Dr Tube: http://www.drtube.com/schematics/marshall/v100-60-02.pdf

What most of that stuff is doing:
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IC4a (top left) is the preamp for the clean channel and is followed by a Vox/Dumble-style tone stack. It's your basic MXR D+ kind of circuit, but set for low gain (10 or so) and excludes any clippers. SW1c/d activates a combination high boost/bass cut for a brighter/thinner sound.

IC2 controls the switching between Clean/OD. When IC2 switches to OD, IC3 switches in a Marshall-style tone stack after the tube.

IC4b (another D+-type circuit, gain around 20) is the first preamp for the two dirty channels. "OD1" constitutes IC7a and the clipping/tone network to its immmediate right, "OD2" uses IC7b and the clipping/tone network to its immediate left.

IC5c-e and IC6 control the switching between the two OD channels. IC2 sends the selected preamp's signal to an active effects loop (shown on a second schematic, link at the end of the post), then comes back to the preamp to hit the tube. From there it goes either to IC3 (if Clean) or to a tone stack and then IC3 (if OD1/2).

Controls:
Clean: VR1=Volume, VR2=Bass, VR3=Mid, VR4=Treble
OD1: VR5=Gain, VR6=Volume
OD2: VR7=Gain, VR8=Contour, VR9=Volume
OD1/2 shared tone stack: VR10=Bass, VR11=Mid, VR12=Treble

That's the major mechanics of the preamp section. Anything I missed?

Questions n' possible mods:
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What's the purpose of the diodes to +/-15V at the amp input?

What are R68/R70/C52 doing to IC7a? They're affecting gain in some kind of frequency-dependant way, right?

Clean has much less gain and volume (obviously) than OD1/2, so the volume is almost always pushed to the upper end to get the same level as OD1/2 gets on half. My first guess is to raise R53 to up the gain on IC4a. Thoughts? I want some boost, but not so much it distorts too early. I'm thinking in the 180k neighborhood.

There's a certain brightness I want to tone down in the amp. I play a Strat-style guitar, so I've always gotta keep an eye on the high-end stabby-stab. I took the bright cap (C39) out of the Clean channel. Are C64 and C65 bright caps as well? Or are they doing the opposite and decreasing the gain of the high end?

Also in that vein, IC4b has the typical TS cap/resistor combo coming off the feedback loop to ground, so gain from this stage gets applied more to frequencies above 720HZ. Raising C62 to .47uf for full-range gain (72Hz & up) sounds about right, but what are the potential downsides here beside more bottom, volume, and distortion?. And would removing R54 result in more volume going to the next stage?

The "Countour" knob on OD2 adds a mid scoop when it's fully up due to the bridged-T filter made by C58/R75/R76/C55, but I'd like a heavier scoop on this channel. I'm thinking change C55 from 22n to 33n? And when the contour knob is fully down, I don't really hear what it's doing. I can see that it hits a filter made of C56/C57, so I'm guessing it makes a dip or notch? The manual just says "Great for fusion sounds," which doesn't tell me a lot. But if I take out C56/C57, I'm just left with the variable mid scoop, right? And how close to "flat" would the mids get at the other end of the dial after taking them out? And lastly, raising R75 to 47k would widen the Q of the scoop, right?

That same exact contour circuit is mimicked above it on OD1, only without the pot. I want to take this tone-shaping network out, or at least bypass it. Would jumping R61 to VR6 work? And if I want to add soft clipping on this channel, would it be as simple as jumping R69 with a couple diodes? And are R67/R44 on IC6 also playing into the gain of this channel?

I want harder, crunchier clipping on OD2. Would LED's work better here than the 1n914's it has? And what's with connecting D8/D9 to +/- 15V anyway? More headroom?

I'm looking at the R48/R49 divider after the tube, and the R77/R78 divider after the OD1/2 tone stack. Do these look like hardwired "master volumes" to anyone else? Would tweaking the R48/R49 pair be more effective at upping the volume of the clean channel than tweaking the channel's opamp stage?

The tube! Aside from the usual tube-swap suspects, gimme some ideas for increasing the mojo 'round this guy. Decreasing R18 or jumping it with a cap? Increasing R22? I'm sure it's a starved-plate setup happening here. What would altering R16/R17 do? What about R23? I usually like to push the tube by putting a distortion pedal n' whatnot in the FX loop, but that's a whole second chain of cables and pedals I'd rather not have to lug around.

I've also got an overhaul to the Clean tone stack to allow SW1c/d to switch between "Fender/Vox" rather than "normal Vox/bright Vox" or whatever it does:

-Jump R57
-Decrease R58 to 22k
-Remove R59
-Increase C40 to 68n
-Lower C43 to 330p
-Raise C44 to 1n
-Jump C42

The mods basically counter much of the loss that comes from a passive stack like this. It also adds a little bottom end, decreases the depth of the mid dip (which is still prominent even when the mid knob is full up), and take advantage of the low-low output impedance of the TL072. The "C" side of SW1 now does nothing, and the "D" side simply takes C44 in and out of parallel with C43. This lets you select between the wide dip/thin top of the Vox EQ curve and the low-mid dip/thick top of the Fender EQ curve, roughly graphed below:



Yellow is the typical Fender curve, blue the typical Vox curve, and the green curves are my approximation of both, depending on the presence of the 1n cap. They aren't perfect matches, but better than what was there before and probably not that far off audibly.

Any other mods, ideas, anything that pops out?

Other pertinent schematics (daughterboard and power amp):

http://www.drtube.com/schematics/marshall/v100-61-02.pdf
http://www.drtube.com/schematics/marshall/v100-62-02.pdf

[shags0024]

Josh X
just ran across this post. i know it is almost 8 years old but thought i would ask anyhow.
Did you do the mods?
How well did the Clean tone stack mod work. i have always likes the Fender clean better than VOX

Thanks for the great explanation of this circuit.

[Ice-9]

I love those M5201 switching op-amps I have used them in a few projects but they are a sod to find, I usually have to buy them from a China Ebay seller. Never have found another equivalent readily available.