Adding distortion: FET Input vs Diode Clipping

[Badside]

So picture a 2204 type build. Adding an extra pre valve is not an option.

The 2204 sound must remain intact, but I need a more "metal" tone available on a footswitch.

I've thought of two options: a FET input (Dumble style), or diode clipping.

FET input (aka built-in booster)
PROS: No "fake" distortion, just pushes the existing valves more. Can adjust the boost level with an extra knob. Lowers input impedance. Simple circuit at the beginning of the amp, taken in and out completely with a simple DPDT relay. Use the Dumble as a reference. Run it straight from the preamp B+ line with a voltage divider.
CONS: Although simple, still more complex than diode clipping. Might pop when switching in and out. More gain therefore more risks of oscillation or other noises.

DIODE clipping
PROS: Super simple, just lift the diodes with a SPDT relay and popping should not be an issue. Has been used by Marshall itself many times. Seems the way to go to make a Marshall sounds more "metal". Requires no power.
CONS: Affects the gain staging and might require a separate volume knob meaning extra switching and possibility of popping, switching in and out will be less subtle. Although a common thing, I can't find a simple schematic of how to add it to a 2204 preamp without changing anything else.

I'm leaning more towards the FET, which is effectively a built-in boost pedal, but seeing that diodes are more common on Marshalls... I'm confused.

Thoughts? Experience?

Thanks

[JazzGuitarGimp]

I have no experience with Marshall amps, so.... grain of salt.

Diode clipping, by its very nature, is going to give you LESS gain. It basically takes your undistorted (or less distorted) signal and chops the tops and bottoms off to make the waveform more square, rendering your signal with less amplitude (= less gain). If you go this route, I suspect you would need to add a make-up gain stage [edit: if you want your guitar to sound as loud or louder than the clean (bypassed) sound].

Of the two choices, I like the FET pre. However, I don't understand the need to build it into the amp. If your intent is to place the FET pre between your guitar and the amp's input, why not just build in into a stomp box? There is so little circuitry in a FET pre that it could easily be built into the same size box that you would wind up using for the footswitch to control it inside the amp, no? Now, it your intent is to place the FET pre between two of the gain stages in the amp, then I get it....

[roberto]

Diode clipping, by its very nature, is going to give you LESS gain. It basically takes your undistorted (or less distorted) signal and chops the tops and bottoms off to make the waveform more square, rendering your signal with less amplitude (= less gain).

This is a typical problem of communication. Technically speaking gain is the ratio between the output and the input, but guitarists use it as "distortion".

So the typical SS part around the tonestack will raise the perceived distortion. There are tons of famous mods about it, and they are very popular, exactly as it is a ts9 in front of a Marshall.

Why don't you re-arrange the premp to obtain a 4 stage OD?

[martin manning]

A lot of people use the term "gain" when what they really mean is distortion. I suppose it comes from the experience of turning up the gain knob and getting more distortion. You also hear statements like "above 10 o'clock it gets gainy." I get overdriven and distorted when I hear someone say that.

Overdriving the input stage will produce tube distortion, which sounds different from diode clipping. I agree with Lou, why not put a booster out front? I get very good results with an MXR Microamp clone into a 2204-ish amp.

[vibratoking]

My experience with the FET preamp is that it won't get you into metal distortion territory as is. You may be able to redesign to get what you want, but the Dumble circuit won't get you there.

I agree, put a pedal in front, but I am sure you have good reasons why you want to build it in the amp.

[Badside]

That is why I used the term "distortion" in the title. I understand diode clipping does not give more gain, rather it limits headroom.

Fun thing, I did a little experiment in the meantime with a small amp that only has 1 preamp stage before a LTPI. I first added a pair of LEDs between signal and ground after the preamp stage... nothing, they wouldn't even light up, signal was too low after just one stage (paralleled 12AX7, 100K anode, 1.5k cathode with 1uF cap, feeding a 1M pot through a 0.0033uF cap).
So I tried regular diodes instead (lower breakdown voltage) and it worked, but interestingly, what little clipping they added, was counteracted by the diminution of gain feeding the following stage (LTPI). In short: it changed the nature of the distortion, but did not increase it (perceptively). Just made things a bit more britte and less warm.

So I guess FET input it is.
Yes, effectively a built-in boost pedal.

I do have boost pedals already, but I want to have an option built in the amp, because when I go for a practice with the band I want just the amp and a footswitch (which is needed for the clean channel and the solo boost anyway). It's just convenience!
On stage I'll have the option of the built-in boost, or a separate pedal.

Plus I was thinking by running the FET at higher than the usual 9V, I can get more headroom out of it and therefore drive the amp harder without getting clipping from the FET itself.

Edit: also I want to be able to go from full-on distortion to clean in an instant. RIght now it means turning off the OD and then switching to the clean channel, no direct switch. I could go around that by having a separate input for each channel though (switching to the clean would mute the crunch channel and therefore the OD too if it's wired to that input only)

[ChrisM]

FET won't affect input impedance. You can set the impedance as you want, just tweak the gate leak resistor.

Also with a pulldown resistor on the FETs input and a solid relay switching scheme you won't have any popping.

Don't look at the ODS FET boost for ideas, it's not that great. I'd look at runoffgroove.com for ideas.


Diode clipping will just be lame. It will cause a large volume reduction when you engage it too btw.

[Badside]

FET won't affect input impedance. You can set the impedance as you want, just tweak the gate leak resistor.

Also with a pulldown resistor on the FETs input and a solid relay switching scheme you won't have any popping.

Don't look at the ODS FET boost for ideas, it's not that great. I'd look at runoffgroove.com for ideas.

Diode clipping will just be lame. It will cause a large volume reduction when you engage it too btw.

I understand what you mean for the input impedance... I just keep reading that everywhere for some reason. My main guitar has a buffer in it because of the piezo anyway, so impedance is of little concern.

Thanks for the info on the ODS FET. I was looking at it mostly for how to power it. At first I thought I'd need a separate low voltage PSU, then I realized I could just run off of the preamp's B+ with a divider after looking at the ODS

[M Fowler]

I build stock Marshall 2204 50w combo amp for some guys with effects loop and they run their distortion pedal at the input which works very effectively.

The Wampler Pinnacle distortion sounds fantastic as does the Zendrive Distortion.

That would be how I would approach the situation if adding a second channel or extra gain tube isn't feasible.

Mark

[kdmay]

I have just added a Jose style clipping circuit to my Metro 50w Plexi (now 2204 / Bogner Blue circuit).

Coming out of the cathode follower and in parallel with the tone stack, add a 10k resistor > 100nf cap (high voltage) > clippers > ground.

For the clippers, I have 2 options:

- Friedman style with the dual MPSA06 for a forward voltage of around 8v. Lots of clipping and gets pretty mean.

- 15v Zeners back to back. I really love this for extra hair and distortion, and the bottom end retains clarity (the transistors can get a little spongy).

I would take this method of clipping any day over a boost pedal / FET input, as your retaining the gain structure integrity of the preamp and clipping the signal AFTER. So, it's like your amp but with more distortion.

A pedal won't sound the same. Not saying it won't sound good, but it will be different.

For 4 components, definitely give it a try!

[Colossal]

+1 to all of kdmays comments; spot on. I love diode clipping as the icing on top of an already-baked 2203. Jose style clipping circuits work well and I like the sound of germanium zener pairs. The sound of diode clipping gives a kind of processed quality to the distortion, a smoothness that sounds you are listening to a recording of great distortion. It is a very fat sound and if you like that completely over-the-top modded Marshall distortion that you know you probably shouldn't play quite so loud but can't bring yourself to turn down, this is in that ballpark with the right voicing. There is also a volume cut with the diodes switched in but this is welcome because you can get the power amp singing a bit more and the whole thing comes together at a manageable volume. The tone might not be for everyone, but it is a killer sound. A Drop D tuned Les Paul into a diode modded Marshall will shock and awe.

[Badside]

I have just added a Jose style clipping circuit to my Metro 50w Plexi (now 2204 / Bogner Blue circuit).

Coming out of the cathode follower and in parallel with the tone stack, add a 10k resistor > 100nf cap (high voltage) > clippers > ground.

For the clippers, I have 2 options:

- Friedman style with the dual MPSA06 for a forward voltage of around 8v. Lots of clipping and gets pretty mean.

- 15v Zeners back to back. I really love this for extra hair and distortion, and the bottom end retains clarity (the transistors can get a little spongy).

Thanks for the info, I was wondering where the best place to put it was. So effectively it's in parallel with the 100k cathode resistor on V2b?

From your description the zeners sound like what I want.

The volume jump is the issue, so I will need to switch between two volume knobs, not a big issue since I'll have another half of a DPDT relay to do that since only one half is required to take the diodes out.

I was almost set on the FET pre, but your comment made me think about it this way: whatever the FET does, I can get it in a pedal. I can't get diode clipping in a pedal... Well I can... but not in the same fashion.

Think I'll order some Zeners and try it on the current 2204 build (this thread is regarding an upcoming build).
When you say "back to back" it's just two of them with their polarities reversed, so basically it clips at +/- 15V?
I could have some fun and add a LED on top of one of them, for some slight assymetry

[Reeltarded]

I sub a pot at the end to control the shunt. The volume loss is huge and the effect is insane without it.

Add a tube. You need makeup gain anyhow.

[kdmay]

[quote="Colossal"]A Drop D tuned Les Paul into a diode modded Marshall will shock and awe.[/quote]

Amen to that Dave!! Sounds killer.

@ Badside: yep, in parallel with the CF 100k resistor.

The volume jump is certainly an issue, particularly if you want to footswitch it in (which sounds like your intention).

What I have done is start with a basic Plexi platform and create 3 unique sounds.

I have a Plexi channel ("clean"), a 2204 / Bogner Blue channel (OD) and a saturation channel (lead). These are footswitchable (well, they will be shortly, I've only completed the saturation side as it stands).

The saturation mode, given it's location, can be switched in for the Plexi mode also, but the reality is that you need a bit of preamp gain in order for the diodes to clip, so 2 gain stages doesn't really get you there IMO (as a big chunk of the tone is coming from the PI and power section in a plexi).

To rectify the volume drop issue, I put a 500k pot in series with the master volume ground reference (wired as a variable resistor). In saturation mode, it kicks in and you set the level to compensate, or in my case, a volume boost for leads. In normal mode (plexi/blue), the relay shorts out the 500k pot.

Works really well and an effective way of getting a lead boost with more hair and distortion, all from your amp. Definitely try it. No need for an extra tube, sorry Reeltarded!!, but good suggestion on the "compliance" resistor.

[Badside]

The volume jump is certainly an issue, particularly if you want to footswitch it in (which sounds like your intention).

...

To rectify the volume drop issue, I put a 500k pot in series with the master volume ground reference (wired as a variable resistor). In saturation mode, it kicks in and you set the level to compensate, or in my case, a volume boost for leads. In normal mode (plexi/blue), the relay shorts out the 500k pot.

Works really well and an effective way of getting a lead boost with more hair and distortion, all from your amp. Definitely try it. No need for an extra tube, sorry Reeltarded!!, but good suggestion on the "compliance" resistor.

Many ways to skin a cat as they say!
I already want to put a "Solo" knob à la Mesa (though I can do it with a pedal in the FX Loop, again the idea is to have all my core tones contained in the amp). I think having a separate master for the "Clipped" mode would be the best way to go. I guess the Solo knob could be inserted under the junction of the ground connection of both Masters so it would work on both channels.

My experience tells me that a 2204 has so much drive going in the power amp (I can't even go past 9 o'clock on the MV) that recovery shouldn't be necessary... unless wants still tries to overdrive the power amp, in which case diode clipping is sort of pointless anyway.

There's also the question of whether or not to have a dedicated gain knob for the Clipped mode... that can become a much more complicated switching scheme. I was thinking of a trim pot between the diodes and ground so I can fine tune the clipping without needing yet another knob on the front.

Also, in the current plans the clean channel is an AB763 type of deal, so it requires both halves of a triode (input and post-TS recovery). But I could just use one half configured as a Plexi Normal channel (the other valve is a bright channel feeding a cold biased stage like a normal 2204) and feed that into the CF with the MV bypassed. That's a cool clean tone as well, but I'm afraid the lack of a dedicate Tone Stack would be an issue (though I could switch a different TS in).

I was trying to keep this as simple as possible, so having a dedicated Blackface clean channel with its own valve, with a Relay switching the Blackface or the 2204 into the FX Loop (serving as a buffer stage before the PI) seemed like the easiest way to do it.
I can even have dedicated inputs so my OD pedals stay out of the clean channel!