Marshall JMP Mod in Progress

[tonmazz]

Guys,

Hey. I am new to this forum and just found it while doing a little research for my JMP 100 mod. This is my first and I'm really learning a lot. I wanted someone to bounce some stuff off of and it looks like I came to the right place. I have several questions as it relates to diode clipping. I have a diode clipping circuit I'm considering and I like high gain, but not overly buzzy, gain. What value diodes should I be using. I have planned to try 20v but am curious if others have tried other values and what the results have been. Also looking at 2.7 ohm resistors with .68U caps for cathodes of V1. Any thoghts on that value if Im shooting for high gain, think 80s metal tones, maybe some heavy Ozzy sounds. Waiting for the parts to arrive so I can begin. Any advice appreciated. Thanks Tony

[antiunderscores]

2.7 ohm, or 2.7 k? im assuming its 2.7 k which is going to give you fairly low gain.
As you lower that you're going to get more gain, the lowest is usually 820, as on a marshall. But its all relevant, because as you increase the value of the plate you are going to get more distortion, if your value of the plate is something like 220k than 2.7k on the cathode should be ok, if you have a value of 150k or 100k or lower you are either going to want to drop the size of the cathode resistor or raise the size of the plate.

That cap is acting to brighten the amp but with that value i imagine its letting all audible frequencies past so it is boosting the sound levels but keeping the distortion levels at that 2.7k levels. Though again, i could be wrong.

With the whole diode thing, from my limited experience, jumping over the diode distortion stages in my amp was the way to go. It just sounds so much more organic, if you turned the gain down it used to be the same hard clipping fuzzy sound, just quieter. But at high gains it was ok so you could always have a dual pot for your gain or add an extra pot to adjust and mix in the diode sound. Though its my first serious amp project and still a work in progress, so im much like you.

to talk a bit more on the plate/cathode...
some classic combinations have been:
Marshall: 100k plate, 820ohm cathode
Fender: 100k plate, 1.5k cathode on first stage and 820 cathode on second.
Vox: 220k plate, 1.5k
I think at the moment my amp is running 120k and ~820 but i have a switch so it can be 120k and 1.5k and it sound pretty good for that just starting to break up, but still jangly clean IMO.

just remember any gain on the very first stage is going to be passed on to the next stages, making them more likely to break up, so do it one mod at a time, and if you actually distort the first stage then its going to be distorted no matter what setting you have your amp on but i dont think that happens often because the input signal is pretty low usually

[tonmazz]

Cap Issue

Thanks for the input. I have tried a few of those combinations and found a few that I like. I do have one issue that is bugging me. I have put a .68 cap over a 2.7 resistor on the cathode, coming from pin 3 on V1. It seems to create a microphonic type squeal when I add the cap. I can use a different value and it's fine. I know this is a common combination so I am a bit at a loss as to why it's doing it. This was one of the first changes so I know it isn't related to any other change. Any advice is welcome. Oh by the way I did try different caps of the same value and they act the same way. Thanks.

[tonmazz]

Thanks for the input. I have tried a few of those combinations and found a few that I like. I do have one issue that is bugging me. I have put a .68 cap over a 2.7 resistor on the cathode, coming from pin 3 on V1. It seems to create a microphonic type squeal when I add the cap. I can use a different value and it's fine. I know this is a common combination so I am a bit at a loss as to why it's doing it. This was one of the first changes so I know it isn't related to any other change. Any advice is welcome. Oh by the way I did try different caps of the same value and they act the same way. Thanks.

I figured it out, unshielded wire on the input jack. Fixed it up and all the issues with the cap went away. I'm messing with diode clipping and want to put a multi position switch of some type to go between the various values of diodes. Does anyone know who makes such a switch? I know it's overkill but I might as well make it the way I want it, right? Thanks.