Diode Clippers in Tube Amps

By "clip the signal" he means distort. Basically, in guitar amps and effects, you amplify the signal and route it through at least two diodes, which allow electricity to go only one way, mounted in different directions. The cuts the tip of the sound off and gives a distorted sound. A "bridge" is simply 2 sets of diods mounted opposite directions.

Diode clippers limit the level of a signal at some point, often right before the grid of a tube. Any signal above a certain voltage level goes to ground and is "clipped" off. It's a bit different from the way the tube naturally "clips" signals of higher levels than the tube can handle. The result is similar on the far extremes of hard clipping distortion, but natural tube "clipping" occurs in stages, with initially a soft clipping on the positive peak as grid current begins to flow, and because grid current charges up the coupling capacitor from the previous stage with the electrons drawn from the grid on the positive peaks, it produces a negative bias voltage on the grid that makes the negative peaks go lower while the positive peaks stay clamped at about 0V. So the initial harder clipping is on the negative peak only of an overdriven stage, making an asymmetrical waveform full of even harmonics. When it's pushed so hard that it's clipping symmetrically, the harmonics are all odd ones (which, contrary to widespread Internet mythology, still sound quite "musical," but they're a different sound.)

Clipping diodes are usually arranged to produce symmetrical clipping, and it comes on more sharply than grid overdrive clipping.

That charging of the coupling capacitor with grid current electrons, which have to slowly leak down to ground through the grid resistor, can overbias a tube for an instant after a very strong signal spike so the tube stays "cut off" and won't reproduce any sound until the charge leaks down. That's called "blocking distortion." If the time constant of the coupling capacitor and grid resistor are very short, it can produce a "buzz saw" sound that's very much liked in rock and high-energy blues. But if the coupling cap and/or grid resistor are too big, the tube can stay "blocked" long enough to produce silent "drop outs" in the music, like a bad cell phone connection. Clipping diodes don't produce blocking distortion, and that's one reason they've been used. They also offer more consistent results, unaffected by changes in the individual tubes used.

It's possible to use Zener diodes set at a voltage just above the tube's negative cutoff voltage, to let the tube distort naturally, but limit blocking distortion by only letting it charge up barely above the cutoff voltage, so it'll leak down more quickly. That can let you hear a nice "buzz saw" sound after hitting a big power chord, instead of hearing a click, silence for a half second, then hear the ringing chord swell and fade away. It's also possible to use one set just below the cutoff voltage, so you never get hard clipping on the negative peak. It's impossible to generalize about what diodes do in an amp without analyzing the particular circuit. And nothing beats putting an oscilloscope on it and checking it out in action. (Except, maybe, plugging it in and playing through it.)
8)

geeze richochet, you must build effects pedals too.

Haven't tried that, actually. Here's a great site if you're interested in building them: http://geofex.com/

(It's got some good tube amp info, too.)

Yeeah, i've been messing around the insides of amps and pedals for about a year. Its alot of fun, and you notice funny stuff, like how the only between the DOD preamp 250 and MXR distorion+ is the price.

Yeah, you find a lot of little coincidences like that if you look for them.

I've built a few circuts to plug my guitar into experimenting to see if I could come up with something different. Its good to expand your knowledge learning all you can about the electronics used for your guitar, amp and effects. Plus you'll get a better understanding on how to adjust them to get the best tone that makes you sound original.

joe

So, i guess in other words, the clean sounds i get from the amp are tube, and the distortion sounds are transistor sounds. Is that correct?

Not necessarily. That would be true if you played a tube amp set clean with solid state effect pedals plugged into it. It's debatable how you'd classify "solid state distortion" vs. "tube distortion" with diode clippers on the grid of a tube. See above about how they can be used differently. I think it's best to think of it as tube sound with some solid state assistance. It's really pretty dang silly to worry about whether sound is coming from tubes, solid state devices or both. What matters is if it sounds good to you.

Say, if you take out the diode bridge, and do nesessary adjustments, could you make such an amp fully valve? As in when you turn it up, the valves distort without diode assistance. And would it be possible to make a gain control for pure tube overdrive? Or would it be nessesary to turn up and annoy your neighbours to get a good tube overdrive?

Sure you could. The devil's in the details, that being "the necessary adjustments." People modify amps all the time. But, like building a hotrod car, it can get complicated when you find that changing one part makes others not work right, and they have to be changed. I'd recommend reading up on tube amp design and modding and basic electronic theory till you feel really comfortable with how they work before attempting something like that.

(And from a mechanical standpoint, it's a pain in the rear to modify amps built on printed circuit boards.)

... And you want to make sure the diode bridge you are removing isn't the power supply rectifier!! That would suck.

The diode definately isnt the power supply rectifyer. I've checed the schematic. Would it be a good idea to put a master volume after the valve grids, so i can get a great tube drive at low volumes?

I was interested in getting the Marshall JCM900 4501 combo, which is apparently all tube. I did A LOT of recearch on it, i studied the schematics and everything. I also sent Marshall an Email asking for details bout the amp, like is it all tube, or has solidstate parts in it. So the guy said that the preamp is hybrid(yet the amp does have 3 ecc83 valves) and the poweramp is all valve. So, are the clipper diodes in the preamp stage?
If it so, how would it change the sound if i make the preamp all tube by removing the diodes?

PS
What famous marshalls also feature clipping diodes for a special "edge" to the sound/to create the overdrive? Ive heard that Slash's signature 2555 has them.