Marshall Amp OD design.

[GraemeSP]

One way to clip a signal to give overdrive effect is to clamp diodes across the output of a gain stage - Ibanez and Boss do this with their OD pedals. I see from old Marshall 2205 schematics a diode bridge after the boost channel valves (I'm English) which begs the question - has Marshall been cheating? it's like putting an OD pedal in an FX loop. Anyone like to comment?

[imaradiostar]

um...isn't that amp looked upon less favorably than classic or modern all-tube distortion designs? I thought it was but I don't know much!

Diode clipping is generally less flexible than an amp with multiple stages distorting and contributing to the overall harmonic structure. Compare the sound of a proco rat to the sound of a plexi on 10. I know it isn't a fair comparison but I think you'd find that there are probably more usable sounds to be found by using both and a certain amount of volume knob twiddling. Any of the diode clipping Marshalls I've played can't do most of the sounds this simple rat into marshall combo can do...but can all those Green Day records really be wrong if you like the way they sound?

I guess what I'm trying to say is to my ears a diode clipper in the middle of a tube amp doesn't sound as good as one in front of a hard-working tube amp like a four input marshall or an old vox or something. Most classic sounds over the years tend to support this theory. There are notable exceptions but I don't like most of the diode clipping amps I've played in my short life. You can't expect a few little diodes to sound exactly like a bunch of tubes but they can still make some neat noises.

jamie

[Chuck H]

Eric Clapton, Jimmy Page and Jeff Beck all used distortion pedals (your English). Most contemporary players that use modern high gain Marshalls seem to like them. Diode clipping is a common part of many players sound.

I used to be an all tube snob. And I still own only all tube amps. Mostly because that's the tone I'm used to and that's what I build. But I like to hear a good diode clipping circuit too. I still have a couple of old pedals that I like though I don't use them much.

A stompbox through a vintage Marshall is the standard by which other rock amps tone is judged. If Marshall wants to incorporate that sound into their designs I'm fine with it. They seem to do it pretty well.

This IS predominantly a tube amp forum. And I like it that way. But diode clipping circuits have been covered here too. Because first and foremost this is an AMP forum. There should be no limitations in the quest for tone. Thats why the focus here IS tube circuits. But sometimes diodes are just the ticket. In fact the diode clipped sound and the sharp character it adds is something that builder who focus on all tube circuits have tried to emulate with tubes. Imagine that. For many years designers tried to make transistors and diodes sound like tubes to no avail. It wasn't until the digital modeling amps that it started to become possible. And IMHO we're not there yet. But to try and emulate diodes with tubes seems counter productive when diodes are eminently more affordable and easier to impliment. And even though I say this I too try to incorporate some of that diode clipped character into my all tube designs. Why? Because there are alot of guys who simply believe "tube=good" "silicone=bad". Still, they want their amps to sound just like SRV (TS-9 Ibenez Tube Screamer) or whoever. So someone has to design it. Doesn't trying to make tubes sound like diodes seem like more of an injustice than including diodes in an overall design.

Chuck

[GraemeSP]

Aha - and there's another Marshall design - the 2550 amp that switches the diodes in using a relay (for footswitch) or Master Vol pot mounted pull switch. Very interesting. BTW I'm rebuilding the front end of my old 2040 Artiste to use the input valves more efficiently. Originally V1 was on the normal channel and V2 on the Reverb channel. I'd heard about a mod using the reverb driver valve as an extra gain stage, but after seeing these designs, realised I can keep the reverb in and rewire the two input valves for two channel - clean and boost. I just wondered about the diodes. Now I know.

[chipaudette]

Yes, the later JCM800s and all the JCM900s had diode clippers in the pre-amp. People loved to rock on those amps. Is that cheating? To me, no. Marshall never claimed they were 100% tube.

To me, diodes just another option for a guitar sound. Because of the prominence of the Marshall 800s and 900s during that time, it's become the sound of 80s and 90s hard rock.

For example, during the classic GnR period, Slash's main go-to amp was his silver-anniversary Marshall (which has diode clippers). I think that he has a great lead tone. Do you think he cared if it had diode clippers in it?

So, hey, if it rocks, why care how it's generated?

Chip

[Gtr_tech]

I don't care for the diode method as its more of a one trick pony. It doesn't really respond well to playing dynamics. I like a front end that will clean up with a soft attack or a rolled down vol control.

[Chuck H]

See...And there you are. Different strokes and all.

As I said before. I am a reformed "all tube" snob. And I do still play through all tube circuits. But I recall constantly in my youth using different distortion pedals to good effect. One of my favorite amps was a Marshall 2100 dual master. What a work horse. Sounded great, always. I didn't have much need for crystal clean tones in that band or I might have needed a different amp. But for what I was doing then nothing could have been better. And yes, That amp had a diode clipping circuit in it. After I started rolling my own I sold it to a kid that was also happy with the tone. To be fair I couldn't play out with it now because it wouldn't cover a wide enough range of sounds. But if I had it here at home I would probably still use it for fun.

Chuck

[Steve Conner]

It's true that the diode clipper doesn't respond well to playing dynamics. But Marshall weren't born yesterday, and this is in fact the very reason why they used it.

High-gain amps can suffer from blocking distortion, which essentially means that they respond too well to playing dynamics. The tone starts to thin out and go all funky on really hard attacks, and sometimes it can disappear altogether and sputter back in after a few seconds. Marshall use the diodes to counteract this and keep the tone together at high gain. They are not in the first stage, and the tubes before them provide some touch sensitivity.

Also, I always wondered why Marshall used a bridge rectifier and 1N4007, not just five 1N4148s as they're cheaper and take up less board space. I recently found out (as a side effect of some experiments at work) that the bridge diodes have a softer knee than 1N4148s or LEDs.

And yes, SRV, Hendrix, and so on, all played through transistors and op-amps. I remember trying to explain to a guitarist friend that the Tube Screamer he paid $200 for didn't actually have tubes in it.

[GraemeSP]

I'm rebuilding the front end of my glass clear sounding 2040 Artiste combo to that of the 4210 JCM800 and that has the diode clippies in, so I'll leave it and see how it sounds. I ran out of new IN4007 so I've put in some unmarked rectifiers lifted from a failed sm power supply - they all check out OK.

I've left in the baxandall tone circuit from the original reverb channel and copied it to the boost channel rather than use the 3 way tone ladder. This is so I don't have to drill another hole for a middle pot and spoil the front panel. I'm going to use the second channel jacks as FX send and return and use simple relays for channel switching rather than the transistors of the original 4210 design. I want to add a pentode/triode switch as well and there's already a spare switch sized hole next to the power switch I can use. I have to sacrifice the presence control for a post PI master volume - again due to lack of real estate for new holes.

Has anyone done a similar mod on an Artiste? The amp as it is was way too clean - I think it was meant for keyboards, guitar AND bass and as such got lost on the way. I've tried to sell it but all potential customers have given up at the last minute. I bought it new in 1972 and it's spent most of it's life under the stairs. I've already changed all the electrolytics for new.

This is a fun task but work keeps interfering, not to mention the missus who wants her dining room table back!

[chipaudette]

I don't have too much to off that's specific to the Artiste...but on your choice of tone stacks...you don't have to go with the Baxandall. You could go with the traditional Treble/Bass/Mid configuration but leave out the pot for the mid. Instead of the pot, replace it with a fixed resistor. Replace it with a 3K, or 6K, or whatever. The result is just the same as if you chose a particular setting for the mid pot and left it there forever. As a result, you can have the sound of the traditional tone stack without cutting another hole in the faceplate.

This is the same trick used by the Blackface Fender Deluxe Reverb. It only has a Treble and Bass knobs, but it still uses the traditional Fender tone stack, which usually has Treble, Bass, and Mid. Instead of using a 10K linear pot for the mid control, Fender replaced it with a 6.8K resistor (equivalent to setting the mid pot to "7"). It works great. If I wanted more mid scoop, I could easily swap out that resistor and replace it with a 2.2K (equivalent to setting the mid pot to "3"). Get the idea?

Since the Marshall tone stack is nearly identical, you could take exactly the same approach if you didn't want to do the Bandaxall.

Oh, and you could do the same trick with the Presence control as well. You said that you didn't want to put in a presence control because you didn't have a hole for it. Well, you could use a fixed resistor to mimic a particular setting of a presence pot. I think that you'll find the Marshall sound sorely lacking if you have no Presence. So, I'd put in the Presence circuit elements (negative feedback elements) and put in a fixed resistor (or pair of resistors depending upon how the pot is wired) to mimic a setting of 6 or something.

Whatever you chose for the Presence or for the tone stack, I hope that you find something that you dig. Good luck!

Chip

[Chuck H]

This is the same trick used by the Blackface Fender Deluxe Reverb. It only has a Treble and Bass knobs, but it still uses the traditional Fender tone stack, which usually has Treble, Bass, and Mid. Instead of using a 10K linear pot for the mid control, Fender replaced it with a 6.8K resistor (equivalent to setting the mid pot to "7"). It works great. If I wanted more mid scoop, I could easily swap out that resistor and replace it with a 2.2K (equivalent to setting the mid pot to "3"). Get the idea?

Just to add to that...I love push/pull pots for mods. You can set a mid "resistor" that is good for general playing, clean to crunch etc. And then use a push/pull pot for the treble control with a mod that bumps up the mid resistor value for fat jazzy stuff and thicker lead playing.

One of my fav's is to have no mid control, but using a push/pull pot on the treble, which usually has a dpdt switch, I can not only kick in a bigger mid resistor, but also a bigger treble resistor. As long as the rest of the amp is voiced accordingly you can go from "pretty" cleans with more bluesy OD, then pull the switch and get a more agressive rock tonality. It's very versitile for a two knob tonestack. Well, two knobs and a switch. It's also cool that you maintain a relatively stock appearance if your doing mods to a classic or a sleeper.

Chuck

[chipaudette]

I'm definitely into switching components into and out of the tone stack. I really like the Mesa Boogie technique of using a switch to increase the Treble cap from 250pf to 1000pf. It gives a very meaty change to the overdrive. Totally killer.

To to this, I tend to use actual toggle switches. I think that I'd like push/pull pots better. Can you recommend a model # or supplier for push/pull pots that might fit within my Fender 65 Deluxe Reverb Reissue?

Thanks,

Chip

[Chuck H]

I really like the Mesa Boogie technique of using a switch to increase the Treble cap from 250pf to 1000pf. It gives a very meaty change to the overdrive. Totally killer.

Thats actually a Randall mod originally done to late SF Fender amps (though I don't doubt that Randy Smith has tried to patent it). The SF Fenders have a bad rep, but that IS a good mod IMHO.

To to this, I tend to use actual toggle switches. I think that I'd like push/pull pots better. Can you recommend a model # or supplier for push/pull pots that might fit within my Fender 65 Deluxe Reverb Reissue?

Dunno. I've never worked in one of those. Are they real guitar amp pots or those cheesy 16mm pc mounted things? Got a picture?

Chuck

[chipaudette]

I'm away from my amp right now so I can't send a picture. They are PC mount, but the shaft seems fairly beefy. The body of the pot seems to be pretty good sized. t takes a 7/16" socket to take off the nut that screws the pot to the faceplate. I've never looked at them and said "Damn, those are pieces of crap".

So, I don't know if they're considered "real" pots or not. If they're not real pots, I'm fine with pulling them off the PCB and using other types.

Basically, I guess I'm asking where you get your push-pull pots. I haven't had great luck finding ones that are sized for amplifiers...the only ones that I've found are sized for guitars.

Thanks,

Chip