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First of all, make sure the Filter Caps and the Tubes are new and working well. If it has 30+ year old tubes and caps, step 1 is replace them. These amps do NOT like old caps and tubes, and invariably sound like crap if you don't do this.
This also assumes you have the post 1983 version. The pre-84 version looked the same, same model number, same control layout, but COMPLETELY different circuit, didn't sound nearly as good out of the box and works totally different in terms of what can be done to mod it.
Ok first, how to use the amp in stock form:
Clean Channel: This is a turd. Nothing you can do for it but live with it, unless you want to perform major surgery like I did.
Gain Channel:
1) I always see people saying "I want more gain". I always think "really? wth?". These things have a whole lot of gain already. Look at the schematic. Gain Channel volume performs more like a gain control than what we usually think of as a volume. Start thinking of it as "gain 2" instead of volume. Want a lot of gain? Max out the channel volume and the gain control. Now you have it. Drop the Master (which is the real volume control for the gain channel) to compensate. Clean channel has to be turned up but it sounds like ass anyway.
2) Tone knobs: This is a plate driven stack rather than a cathode driven stack like on most Marshalls. IMO this is a GOOD thing. It is a lot more versatile in terms of how it responds to the controls. Also, the Mids control is a massive 100k. That not only gives you a lot more range than the stock 25k on most Marshalls, but it responds more off the high impedance source.
Treble control remains linear, where it would make a lot more sense to have it be audio taper. It comes on like a rocket, which is not a good thing. Throw away your ideas of how a treble control on a Marshall should work, unless you replace this pot with a 220k audio taper pot, which you can get in the same form factor.
How to set it: (note: 9:00 = 9 O'clock, etc)
Metal: Presence 2:00 to max, mids 0-10:00, treble 10:00-noon, bass maxed. Max gain channel volume and gain pot. If this doesn't do it for you, I don't know what to tell you.
Classic rock: Presence 11:00-3:00, mids 10:00-2:00, treble 9:00-11:00, bass noon-maxed. Set gain/channel volume to get the gain levels you want.
Big, thick sound: Presence 0-max, mids 3:00-maxed, treble 0-9:00, bass noon-maxed. Want to cut a mix and sound huge? There you go. No other Marshall will produce this tone.
As far as what I do to them:
1) Clean channel separated from Master volume. This allows using Gain 2 (channel volume) while keeping cleans clean.
2) Channels separated from input to reverb return. No gain stages shared until the effects loop return stage.
3) Clean channel changed to blackface Fender circuit. 25k mid pot added on back. This is no-joke major surgery, not just simple component changing. Not for the squeemish.
4) Clipping diodes removed and gain channel re-built from the ground up. Note: The stock gain channel is plenty good, but I figured what the hell while I was in there. The issue with clipping diodes is that, while they sound just fine with the gain way up, the feel gets a little plastic with the gain lower. The gain channel I built in retains the tonal versatility while adding (a lot) more gain at the extreme, with better feel the whole way through the gain pot, from almost clean to melt-your-face. The gain channel has a lot more gain than I'd ever use, all the way to complete compression with 0 dynamics, but I did it to prove a point.
Why they put clipping diodes in this amp is anyone's guess. The usual guess you see on the web is wrong, which goes like this: "You can't get much gain from 3 stages without using clipping diodes". BullSh*t. You can get nutty amounts of gain from 3 stages. IMO 3 stage gain channels also sound more organic than gain channels with 4 or more stages.
5) Dialable, footswitchable "solo boost" added across both channels. Gain channel has separate mid pot on back that engages when boost is engaged, so you can set mids higher for leads if desired.
6) Reverb on clean channel only. Personal preference, but I hate verb on gain channels. They lose punch and mix cutting ability, whereas clean channels sound better with some verb.
7) Audio pot for treble on gain channel to make it much easier to dial in. Also, Master adjusted so it comes on more gradually and not off-LOUD in 1/100th of a turn.
Note: I did NOT change the power section. It's just fine. Most of the web advice you see to that end would leave you open to motor boating and oscillation, and just plain doesn't need to be done. This amp has a lot of gain in the pre-amp. If you want to push it into power tube clipping then fine, turn down the pre-amp gain and turn up the master. In fact, reading quite a few posts with 'advice' on component changes, most of the time I end up thinking "WTF". The people offering the advice either haven't spent much time in this circuit, or don't really have a solid grasp on what affects what in a tube amp circuit. Very little of it would be helpful, most of it detrimental and not necessary for the result.
This also assumes you have the post 1983 version. The pre-84 version looked the same, same model number, same control layout, but COMPLETELY different circuit, didn't sound nearly as good out of the box and works totally different in terms of what can be done to mod it.
Ok first, how to use the amp in stock form:
Clean Channel: This is a turd. Nothing you can do for it but live with it, unless you want to perform major surgery like I did.
Gain Channel:
1) I always see people saying "I want more gain". I always think "really? wth?". These things have a whole lot of gain already. Look at the schematic. Gain Channel volume performs more like a gain control than what we usually think of as a volume. Start thinking of it as "gain 2" instead of volume. Want a lot of gain? Max out the channel volume and the gain control. Now you have it. Drop the Master (which is the real volume control for the gain channel) to compensate. Clean channel has to be turned up but it sounds like ass anyway.
2) Tone knobs: This is a plate driven stack rather than a cathode driven stack like on most Marshalls. IMO this is a GOOD thing. It is a lot more versatile in terms of how it responds to the controls. Also, the Mids control is a massive 100k. That not only gives you a lot more range than the stock 25k on most Marshalls, but it responds more off the high impedance source.
Treble control remains linear, where it would make a lot more sense to have it be audio taper. It comes on like a rocket, which is not a good thing. Throw away your ideas of how a treble control on a Marshall should work, unless you replace this pot with a 220k audio taper pot, which you can get in the same form factor.
How to set it: (note: 9:00 = 9 O'clock, etc)
Metal: Presence 2:00 to max, mids 0-10:00, treble 10:00-noon, bass maxed. Max gain channel volume and gain pot. If this doesn't do it for you, I don't know what to tell you.
Classic rock: Presence 11:00-3:00, mids 10:00-2:00, treble 9:00-11:00, bass noon-maxed. Set gain/channel volume to get the gain levels you want.
Big, thick sound: Presence 0-max, mids 3:00-maxed, treble 0-9:00, bass noon-maxed. Want to cut a mix and sound huge? There you go. No other Marshall will produce this tone.
As far as what I do to them:
1) Clean channel separated from Master volume. This allows using Gain 2 (channel volume) while keeping cleans clean.
2) Channels separated from input to reverb return. No gain stages shared until the effects loop return stage.
3) Clean channel changed to blackface Fender circuit. 25k mid pot added on back. This is no-joke major surgery, not just simple component changing. Not for the squeemish.
4) Clipping diodes removed and gain channel re-built from the ground up. Note: The stock gain channel is plenty good, but I figured what the hell while I was in there. The issue with clipping diodes is that, while they sound just fine with the gain way up, the feel gets a little plastic with the gain lower. The gain channel I built in retains the tonal versatility while adding (a lot) more gain at the extreme, with better feel the whole way through the gain pot, from almost clean to melt-your-face. The gain channel has a lot more gain than I'd ever use, all the way to complete compression with 0 dynamics, but I did it to prove a point.
Why they put clipping diodes in this amp is anyone's guess. The usual guess you see on the web is wrong, which goes like this: "You can't get much gain from 3 stages without using clipping diodes". BullSh*t. You can get nutty amounts of gain from 3 stages. IMO 3 stage gain channels also sound more organic than gain channels with 4 or more stages.
5) Dialable, footswitchable "solo boost" added across both channels. Gain channel has separate mid pot on back that engages when boost is engaged, so you can set mids higher for leads if desired.
6) Reverb on clean channel only. Personal preference, but I hate verb on gain channels. They lose punch and mix cutting ability, whereas clean channels sound better with some verb.
7) Audio pot for treble on gain channel to make it much easier to dial in. Also, Master adjusted so it comes on more gradually and not off-LOUD in 1/100th of a turn.
Note: I did NOT change the power section. It's just fine. Most of the web advice you see to that end would leave you open to motor boating and oscillation, and just plain doesn't need to be done. This amp has a lot of gain in the pre-amp. If you want to push it into power tube clipping then fine, turn down the pre-amp gain and turn up the master. In fact, reading quite a few posts with 'advice' on component changes, most of the time I end up thinking "WTF". The people offering the advice either haven't spent much time in this circuit, or don't really have a solid grasp on what affects what in a tube amp circuit. Very little of it would be helpful, most of it detrimental and not necessary for the result.
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