Modding jcm800

[fossilshark]

Time for my yearly post here 

The jcm800 2204 I built sounds really nice and i am happy with it, but I am looking for a slightly different sound.

No matter what pedal I use the distortion is just slightly "chunky" exactly like Slash's rig on appetite for destruction.

I want the distortion more "fizzy" if that makes any sense. Like a live tape of tony iommi in the late 70s. His LA100BLs didnt have a master volume or high gain channel they are basically plexis with different frequency responses I think.

Any suggestions on where to start with making my high gain sound more "fizzy"?

[JPK]

How's about posting an as-built schematic?   

[d95err]

A treble booster is a lage part of Iommi’s sound. Treble boosters are very simple single transistor circuits, easy to DIY.

As a start, I’d try a treble booster and then getting rid of the bright cap across the gain pot on the JCM800.

[fossilshark]

Its built to exact specs of this schematic minus the bright cap (who actually installs that anyways).

Yes i use a rangemaster treble booster i spent about a year playing with every different mod you can do to it. My treble booster is exactly what iommi uses and i am confident in that. For the past 5 years i have basically exclusively listened to black sabbath live tapes up until this year.

I have the 1969 schematics for the LA100BL and LA50BL from laney themselves and i have built an LA50BL but not having $800 to drop on the original partridge output transformer makes it sound way too muddy for my liking. Its basically a bass amp.

I just want to mod my 800 to have the distortion sound different if thats possible.

[Williamblake]

Did you try anything else but the 820R cathode resistor in the second gain stage? What about a 1.5k. This is what defines the headroom in that preamp or so i think. 

[JPK]

I started reading the below link and found something maybe you could experiment with easily: The Cold clipper gain stage. Experiment with lower (or higher) than 10k resistor values to see if you like that sound better. It says the purpose of this circuit was to smooth out the overdrive tone. Maybe you don't like it that smooth?


https://robrobinette.com/How_the_Marshall_JCM800_Works.htm

[AlNewman]

You could try experimenting with the value of the cathode bypass cap on the input stage which should make it a bit harrier.  You could experiment with 1 or 5 uf, once you find the right value you could also parallel it with the .68 and make it switchable if it gives you the effect you're looking for.

[fossilshark]

these are all excellent suggestions. I am almost finished a "lab unit" wich is an amp i built in high school with a swiss cheese chassis im going to convert to an 800 2204 to perform all these test on.

I will run some experiments with these suggestions and report back. thank you all for the suggestions.

[nandrewjackson]

Change interstage coupling caps from 22n to 10n or 6.8n


+1 for experimenting with the cold clipping cathode R.

[d95err]

You could try the Friedmann style second gain stage, with a switch to select between three different cathode options. I think his amps effectively switch between 10k, 5k and 2k7/.68u.

[Williamblake]

Did you try anything else but the 820R cathode resistor in the second gain stage? What about a 1.5k. This is what defines the headroom in that preamp or so i think.

Just came across i do think 820R is my happy value here, too and increasing the resistance will ruin your headroom.
On the other hand i now remember adding a small bybass like 20nf to this cathode will give you a knack, resulting in an amp that sounds broken. Maybe that could be the fizz you want?