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**chipaudette** · 02-11-2008, 11:20 AM

Look up the Marshall JCM900 schematic and you'll notice that Marshall thought that diode clippers were a good idea too. They used a diode bridge instead of just a pair of diodes...but it's the same idea. Of course, unlike your amp, the diodes aren't switchable on the JCM900. I wonder what the JCM900 would sound like with the diodes switched out...

On my own amp (Fender Deluxe Reverb Reissue), I've performed high-gain/distortion experiments by cascading tube gain stages. That has been fun. Similar to you, I've also tried some diode clippers in there to get some enhanced distortion at lower sound levels.

In my experiments, I tried a bunch of different diode configurations to find "that sound" that would really grab me. After each new diode clipper build, I would really enjoy having the new sound. I guess it was mostly the pride of building something yourself that doesn't completely suck. I'd have a lot of fun. But, after a week or so, I always found myself pulling the diodes out of the amp. I guess that after I got over the honeymoon of the new sound, I just didn't find the diodes satisfying. To me, there's just something about reversed diodes that doesn't work...I think its that the sound is too unresponsive to one's playing...it's too static.

That said, it sure was fun trying out different diodes as my clippers. I tried normal silicon diodes and I tried germanium diodes. Then, I tried a pair of germanium on one leg and a single silicon on the other leg (to get more asymmetrical distortion). I've also tried using LEDs, which are also diodes (that's the "D" in LED). I happened to have red LEDs on hand, but you should try other colors too...they operate a different voltage levels so they will sound different. I think that I liked the sound of the LEDs the best...not quite as fizzy as the silicon/germanium diodes. And, if you mount the LEDs externally, you can see them light up when you hit a chord. That's pretty sweet.

Once, I saw a guy mount some red LEDs in the eye sockets of a fake skull. Then, when he'd hit a chord, the eyes would light up. Now that's metal. Old school.

Have fun!

Chip

**rexindigo** · 02-11-2008, 11:34 AM

Tube rectifier as clipping diode

Thats cool, Marshall did it as well!
I will try different diodes as well, because You are right it sounds a little bit fuzzy, but maybe with a capacitor parallel or so this can be optimized....

Well, while reading this thread again, i wondered if it was possible to use a tube rectifier as clipping diode? Just for the guys that hate solid state diodes or for the fun of it...

any suggestions?

**chipaudette** · 02-11-2008, 01:14 PM

So, parallel solid-state diodes work because they have a fixed but non-zero voltage drop across them when they conduct. Flipping that statement around, solid-state diodes require a certain voltage across them before they do anything to your circuit. This conduction voltage is between 0.3-1.7 volts where the specific value is defined by the type of diode you're using. Germanium diodes generally have the lowest conduction voltage and LEDs are the highest.

These diodes work as fuzz/distortion elements precisely because of this non-zero conduction-voltage behavior. If you take a couple of silicon diodes (which require 0.7V to conduct) and arrange them as shunts to ground, your guitar signal will be limited ("clipped") to a max voltage of +0.7V by the diode oriented in the "forward" direction and limited to a max negative voltage of -0.7V by the diode oriented in the "backward" direction. As a result, your guitar signal slams back and forth between +0.7 and -0.7V. In other words, it starts to look like a square wave, which is the classic "fuzz" guitar sound.

With a tube diode, though, the "normal" way of hooking them up results in no requirement for a "conduction voltage". Whenever the signal exceeds 0 V, they conduct. Therefore, putting one forward and one backward results in your signal slamming between +0V and -0V...in other words, you get no signal -- silence. That's not what you want at all.

You could hook only one tube diode up instead of hooking up a pair...this would clip one side of your guitar signal, which would certainly sound distorted. But, it wouldn't sound like the distortion you're used to. Maybe you'd like it maybe not.

Thinking kinda crazy, I suppose that you could hook up the tube diode with a certain type of bias so that it mimics the "conduction voltage" behavior that you get for-free with the solid-state diodes. But, that sounds fairly complicated.

Also, tube diodes are, in general, harder to set up than dropping a couple of diodes into the circuit. Tube diodes require substantial heater current and, I believe, high-voltages.

Good luck!

Chip

**cbarrow7625** · 02-11-2008, 01:31 PM

The Marshall JCM900 diode setup works quite well. It is more than just a bridge. With that 5th diode going through the middlre of the bridge it is more like a frequency doubler. I played with this a bunch a few years back and found that the amp didn't have nearly the same tonality with the diode circuit out.

There are a couple of things you can do to make the diode clipper sound more "tube-like". 1.) make the clipping assymetric. Use 2 diodes facing one direction & one in the other direction. Or 2 & 3, etc. 2.) Use different diodes / LED's , etc. in each half of the clipper for differnet charachteristics. 3.) See the marshall 25th anniversary amp (I forget the model #, it's the one that Slash uses). It has multiple diodes / led's stacked & assymetrical. 4.) lift the diodes with a smallish resistor ( a 1k-15k) to get what Kevin O'Connor calls "variable compliance".. The clipping will be more rounded off than hard edges. 5.) Bias the diodes. You need a + & - supply for this but with it you can set each diode to clip at whatever level you want (Kevin O'connor again). 6.) Another Marshall trick....put a diode in the cathode circuit of the 2nd stage in parallel with the resistor. It creates a heavy assymetrical clipping of the stage.

There's lots that can be done with diodes. Not my favorite sound but it si worth playing around with in your situation.

Chris

**pdf64** · 02-11-2008, 09:43 PM

Fender use a 12ax7 as a tube diode clipper in their performer / roc-pro series, see
http://www.fender.com/support/amp_sc..._Schematic.pdf
Hope that helps - Peter

**cbarrow7625** · 02-11-2008, 09:47 PM

That's pretty cool. I always wanted to try a tube diode clipper. I wonder how it sounds?

That's a good example of how to bias solid state diodes as well. The basic approach is identical. Fender set theiirs to clip at around 2 Volts but you could set it to whatever level is appropriate for the drive level in the circuit at that point using that approach.

Thanks for the schematic!

Chris

**d95err** · 02-17-2008, 07:30 AM

Originally posted by rexindigo View Post

Thats cool, Marshall did it as well!
I will try different diodes as well, because You are right it sounds a little bit fuzzy, but maybe with a capacitor parallel or so this can be optimized....

Well, while reading this thread again, i wondered if it was possible to use a tube rectifier as clipping diode? Just for the guys that hate solid state diodes or for the fun of it...

any suggestions?

Put a resistor in series with the diodes. That will smooth the clipping out a bit, making it less harsh and more "tube-like". Check out this page for some ideas:
http://www.muzique.com/lab/sat.htm

Rectifier tubes are probably not a good idea as diodes because of the huge heater current demand. Regular 12AX7 can be used as diodes though. Can't remember how (I think you just tie the cathode to the grid or something like that).

Kevin O'Connor writes a lot about these things in "The Ultimate Tone 1".

**rexindigo** · 02-18-2008, 06:35 PM

hi d95err,
what value would you recommend for the serial resistor?

**TubeDude** · 02-28-2008, 12:58 AM

Use LEDs Instead

Use LEDs instead of diodes. Wire them in the same as you have the diodes. I like this setup a LOT better than diodes. Play around with different colors too, they will sound different. The diodes give more of a broader and softer clipping but plenty of AC/DC type distortion. Won't give you Metallica sound...

Later
Brad

**rexindigo** · 03-01-2008, 06:30 AM

Hi Brad, thank You, this is a very good trick, i changed to LEDs and the sound is incredible, it almost sounds like real saturated tubes...in any case much more than diodes. almost no cutback in volume as well,
Now I have a switchable distortion sound and can just use my passive AB Box no additional cables etc. real nice sound, I will try to post a soundfile as soon as i get my new mixer.
This Forum is incredible, Thank You all,
Special Thanks to Chip for his excursion about diode theory.

**TubeDude** · 03-01-2008, 11:13 PM

GREAT! I'm glad you liked that little trick. Diodes are just too harsh sounding to me; even germanium ones that people rave about. You have to use input/output filters more using them. Now try using 3 or better yet, 4 LEDs and see how you like that. You can don't have to use 2 normal polarity and 2 reversed. Stagger them some to get different overdrive/distortion sounds. Like 3 normal polarity and 1 reversed, etc. Some people like the sound better that way. I particularly like the sound of 4 (2 normal and 2 reversed); green and red ones. Orange and yellow ones will give you a slightly different sound also. Been wanting to try blue ones but haven't. Experimenting is the key; don't disregard anything until you try it and hear it yourself. I've been using that trick for a while now. I've also been designing and making tube preamp/overdrive/distortion pedals and am getting ready to start selling them on the internet. I've got some slick tricks I've done on that circuit too. I normally run a 12AX7 tube in it for all out distortion but for mild overdrive, I use a 12AU7. This pedal will give you the saturated tube sound with any amp. It's not an over-the-top distortion but plenty enough. I've got a upgraded version that uses an op-amp plus the tube and it's all the distortion and signal boost you can handle! But especially sounds good with tube amps; drives them really hard if you need it too. I also build handmade tube guitar/bass amps and microphone preamps. Is your amp tube or solid state? Anyways, I'm always available it you want to discuss/ask any other questions about clipping circuits or anything else. Private Message me and I'll give you my email address if you want. I can show/tell you some cool stuff. I've been an Engineer for 20 years. Music is practically what I live for.

**rexindigo** · 03-02-2008, 06:06 PM

Hi Brad, i just drew it, so if You could please let me know if I'm right in this, but what would You recommend: Red Green Cascaded and reverse or Cascades of the same coloures. (see attached Pdf)
Its a Tube amp i did this to, (a Marshall Plexy homebrew clone, Model 1987), and i added the LEDs right between the first and the second tube stage, because I could just place it at the front panel in front of the Volume control of the bright channel very easily.

Attached Files

LED1.pdf (3.5 KB, 1229 views)

**rexindigo** · 03-03-2008, 06:18 PM

In the mean time i did a little research on the internet and found a nice example of LED clipping in a tube amp. So i just copied that to my design and it sounds really good:
http://www.tube-town.net/diy/tt-sam/tt-sam-sch01.JPG

Anyway usually I can not judge a modification before i heard it in rehearsal, which will be on friday, so than i will see if i like it at all. Ill try to make a recording of it then.

**Mark_48** · 03-09-2008, 01:59 PM

Originally posted by cbarrow7625 View Post

That's pretty cool. I always wanted to try a tube diode clipper. I wonder how it sounds?

That's a good example of how to bias solid state diodes as well. The basic approach is identical. Fender set theiirs to clip at around 2 Volts but you could set it to whatever level is appropriate for the drive level in the circuit at that point using that approach.

Thanks for the schematic!

Chris

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Diode clipper in a tube amp, what do you think of it?

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