Marshall JCM-800 "clipping diodes"

[trem]

I am thinking it is approximately 1986. Is there a procedure to remove/bypass the solid state components that make up the drive/over drive effect? My buddy has one of these heads, I believe it is a 2205. He is not happy with the sound and asked me for help. I only took a quick look at the schematic, seems to be a few diodes and a handful of transistors, via an IC chip perhaps.
Anyway... If anybody can point me in the right direction, I would appreciate it. I do not mind reading at all. I have always had poor success with the search function on INTERNET forums. But I am always happy to give them another try.
Thank You

[Robert M. Martinelli]

Hi,
clipping diodes are there to alter the waveshape by squaring it, before bypassing that section, be aware that you can achieve different degrees of clipping by changing the diodes' type. The clipping level depends on the diodes' forward ( threshold ) voltage, 0.2/0.3 for Germanium diodes, 0.5/0.6 for silicon diodes, 1.6 V for red LEDs, 1.8 V for green LEDs etc.

The lower the diodes' forward voltage, the harder the clipping effect they will have, and the more the waveshape will resemble a square wave, so more odd harmonics will be present.

You could also try "asymmetric" clipping ( e.g. one diode for the positive half of the waveshape and two diodes for the negative one ) which tends to introduce even harmonics, making for a "more gentle" distortion.

The clipping stage is usually paralleled to the signal, so cutting it away from the signal path is usually pretty straightforward. The unclipped signal will of course have a greater amplitude, so you might want ( or not ) to add a voltage divider to keep the signal's amplitude within certain limits.

Hope this helps

Best regards

Bob

[Guitarist]

There are a ton of mods to do with this amp. Are you still working on it? Also, your friend could sell the amp. I know this is blunt but like an going with the wrong girlfriend, a painful but correct move. Yet, if his idol is Slash then you might want to persevere.

[booj]

cut them out

[trem]

Thanks everybody.
I took the diodes out. I did not know quite what to think. Sounded better for sure, but not really "right". Installed new diodes. Found a guy with an early 80's vertical inputs, he wanted to do a trade. It was a little beat, but with new e-caps, in jacks, some DeOxit and a bias check, the amp was much more to my friends liking.
I just do not have enough electronics knowledge to have messed around with his other amp. I am fine working from a schematic, with proven mods. But any type of "engineering" on my part turns into a guessing game. It was just smarter to get a different amp. Didn't want to get in over my head.
Thanks Again

[radtekvp]

nonlinear components

One of the "secrets" to achieving a particular sound is that even very slightly different parts will produce some subtle (and sometimes desirably subtle) effects. All of the distortion effects are achieved by some sort of non-linear element in the circuit. But all diodes are not alike. Much more complex than the simplistic (?) diode equation or a few forward voltage characteristics, the tiny details of the behavior of a diode in a specific circuit may have surprisingly large effects in the sound.

If diodes were all so simple as the first order equations suggest there would be only a few type numbers - but there are *thousands* of different diode type numbers. In addition, designers often spend a great deal of effort in selecting a specific component, and then, to achieve repeatability, manufacturers will purchase an entire manufacturing run from a supplier. This means that even if you purchase a replacement with the same type number you might not get the same sound.

An excellent writeup was posted years ago on the web that described the why's and wherefores of a particular inductor in one of the classic fuzz boxes that was known for a great sound. Analysis, as I recall, found that the inductor was actually *badly* designed and had a particular nonlinear response that GAVE that unique sound. Attempts to replicate the sound later even with the same type number inductor failed - BECAUSE it was originally manufactured with a quirky problem and the new ones were made "correctly".