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

[d95err]

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

That's just a cheap and dirty marketing trick. Using a tube clipper at low voltage in a solid state amp, just to be able to market it as having "tube sound". I'm sure a common silicon diode clipper would sound just as good in that context.

[Mark_48]

Tube clipping is presumably smoother than SS diodes. Attached is a section of the RocPro's manual describing the tube in the amp. The wording used is a little deceptive on what exactly is being used for a preamp.

[tattybob]

This is a damn good thread.
im new to this so forgive anything crazy i come out with.

I have a JCM900 dual reverb.

I read a while ago about the diode clipping stage and had some previous experience with building one or two FX pedals.

I believe the clean channel on the JCM900 4100 uses Red LED clipping and the lead channel uses the bridge recifier. Id be into swapping out the bridge recifier for LED's, or even bypassing it with some sort of compensation if the tone suffers. making that switchable.

In addition, Ive been reading the beavis audio website for a while now, and I wondered about putting in a rotary switch set up. (making something a little more complex) to dial between Bypass, BR, LED, Germanium, or even MOSFET clipping circuits. Any thoughts?

At the moment i have that notepad file of gain & tone shaping mods which are all resistor and cap replacements, but i want to do some similar componant switching with these (Orginal values to modified values or something)

Its all a bit of fun.

Id have no idea about how to wire up a 12AX7 to clip the signal

[rexindigo]

sound clip of the diode clipper in my plexy clone

Hi there,
at last I have changed it again to diodes with parallel cap and resistor, and here is a clip of it... i hope you like it! (the hum in the background is my pc)
for me its great now and i will add this to all of my next amp projects

[kg]

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.

not entirely true... the tube still has it's plate resistance which will prevent it from perfectly clamping your signal. what will actually happen is a rather pleasing compression effect.

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.

asymmetric clipping can work, but generally it falls into same trap as grid blocking during grid conduction. essentially if you attempt to pull your g1 voltage above vk, the grid will collect electrons--in other words, become more negative. you have literally created a diode in the grid circuit.

the only way to bleed off this stored charge in an RC coupled amp is through the grid leak resistor, which, along with the coupling cap, has some finite time constant associated with it. as multiple cycles propagate through the amp, the effect worsens, until the grid has stored so many electrons that it pinches off the tube. the result is a flatulent, unpleasant distortion.

one way to FIX this issue is to make sure that the grid will also conduct in the opposite direction.. ie use the tube grid diode as one half of your clipper circuit. this can be made to sound very good.

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.

i would not use a tube diode intended for power supply use... but ANY tube can be made into a diode by connecting its electrodes in various configurations. connecting g1 to k will give you a relatively higher rp, whereas connecting g1 to plate will give you a very low effective rp tube (but may burn up your grid if dissipation becomes too high).

[TubeDude]

KG is correct on everything that he said. But I still prefer using LEDs to get that good overdrive sound (other than tubes!). Let me say this before saying anything else: if you want tube type distortion then buy a low wattage tube amp and CRANK it or a (real, higher voltage) tube pedal. There's simply no substitute for a saturated tube; especially the output tubes! You can do it with preamp tubes and get good sounds but not as good as saturating the output tubes. Now for the sound, I'm not talking about ridiculous, over-the-top distortion. Just a good broad and raw overdrive. The key to using LEDs for good overdrive is to put them in a latter part of the preamp stage, so the signal will be high enough to make them conduct... If you are slick and plan it out enough, then you can use your preamp or gain volume to adjust the amount. If you still have to crank your amp to mind blowing levels to get the LEDs to overdrive, then you need to redo your setup. They are not in the right place in the circuit. You don't have to put a lot of signal to them to get them to overdrive. The right amount of LEDs seem to be 4 for what sound I like.

[kg]

KG is correct on everything that he said. But I still prefer using LEDs to get that good overdrive sound (other than tubes!). Let me say this before saying anything else: if you want tube type distortion then buy a low wattage tube amp and CRANK it or a (real, higher voltage) tube pedal. There's simply no substitute for a saturated tube; especially the output tubes! You can do it with preamp tubes and get good sounds but not as good as saturating the output tubes. Now for the sound, I'm not talking about ridiculous, over-the-top distortion. Just a good broad and raw overdrive. The key to using LEDs for good overdrive is to put them in a latter part of the preamp stage, so the signal will be high enough to make them conduct... If you are slick and plan it out enough, then you can use your preamp or gain volume to adjust the amount. If you still have to crank your amp to mind blowing levels to get the LEDs to overdrive, then you need to redo your setup. They are not in the right place in the circuit. You don't have to put a lot of signal to them to get them to overdrive. The right amount of LEDs seem to be 4 for what sound I like.

i agree. when i use ss diodes, i prefer LEDs for clipping duty.

i also prefer them for cathode biasing, but that's another thread.

[boyer]

In addition to having a (foot) switch to turn the clippers on/off, could you also have a control to vary the level of distortion? Perhaps a 1m pot between the signal source and the clipper pair?

[TubeDude]

Now, that's what I was talking about! Yes you could have a "drive" control for the amount of distortion. That's what I meant about putting the LEDs in the correct place in the circuit. It depends on the amp setup you have already; of course. If you don't have a preamp or pre volume; it's easy enough to put one in so you will have control of the amount of overdrive...

[TubeGuy]

On the subject of Marshalls and diode clipping:

The gain channel pre-amp on an 800 series 2210/2205/4210/4211/4212 (all the same pre) has two things going on:

1) V1b has a diode in parallel with the cathode resistor

2) Between side 2 of the dual gain pot and the volume, there is a bridge rectifier/diode setup.

I understand how diode clipping can be done between ground and the signal path, but I'm not clear on how 1) and 2) create clipping. Can someone enlighten me as to the mechanics of that?

(yes I realize this thread is old but I searched and its right on topic for what I wanted to know!)

[mooreamps]

Hi there,
at last I have changed it again to diodes with parallel cap and resistor, and here is a clip of it... i hope you like it! (the hum in the background is my pc)
for me its great now and i will add this to all of my next amp projects

YA, baby !!! Very nice....

Good job....
-g

[TubeGuy]

Bump! anyone?

[TubeDude]

TubGuy, I'm not sure I understand the question. Do you want to know how diodes clip the signal or what??? By the way rexindigo, that sounds pretty good considering what you did to it...

[cbarrow7625]

OK Tubeguy, here is my best explanation:

1.) With high drive signals (in excess of the grid-to-cathode voltage), the bottom half of the AC waveform gets soft-clipped by the tube. You run out of cathode voltage swing & it flat-bottoms the waveform. The diode (cathode of it facing the tube cathode, anode to ground) clips the positive-going half of the waveform atthe diode voltage (about 0.6V in this case). THis also ends up being a relatively soft-clip (compared with just shunting diodes to ground inbetween stages. But it ends up being an assymetrical clipping with one half of the waveform getting dynamically clipped (as the cathode voltage changes) and the other half getting clipped at 0.6V all the time.

2.) The bridge rectifer is wired so that the signal & ground are going into the bridge rectifier just like you would wire in a transformer in a powr supply. The extra diode has its anode connected to the + terminal on the bridge & its cathode to the - terminal on the bridge. This diode creates a path for both the negative going & positive going AC waveforms to pass from the top of the bridge (signal) to the bottom (ground). Draw it out & you'll see waht I mean. The path for each half of the waveform is 3 diodes (about 1.8V). The cool thing about this setup is that both the positive-going & negative going signals pass through the "extra dioide" in the middle going in opposite directions. This set-up is very similar to a frequency double circuit...and that is essentially what it sounds like as a clipper, it sounds like a frequency doubler. Everything that gets clipped basically has its harmonics pushed up by an octave (double the frequency) and it ends up sounding very smooth and sweet. Try this setup vs. 3 series diodes in the normal configuration (just side by side strings) & you'll see what I mean. The harmonic content is at a higher frequency.

I played with both of these set-ups a bunch of times but I finally learned that diode clipping is not where its at for me so i abandoned them. However, these 2 particular versions of diode clipping are my absolute favorites in a tube amp. They first one, in the cathode, actually works with the tube - what's not to like there? The bridge rectifier setup just sounds more natural & tube-like to me than shunting diodes straight to ground.

Hopefully I expalined what I know well enough to make sense. LEt me know.

[rexindigo]

Thank you Tubedude, to be honest in the mean time I changed the whole setup again, took out the diode clipper and did the one-wire mod which cascades the two input channels and added a pre-PI master volume, which gives me a lot of nice preamp overdrive.