Marshall MG15ZW Gain

[JoeBaker]

Hi can anybody help. I have a MG15ZW, the normal channel is fine but it really has a lot of gain in the lead channel to much in my opinion, even with weak pickups, and with the gain control set below 1. I have asked Marshall about reducing it they weren’t that helpful. I have tried changing the gain (G1M) on the diagram as it was crackly but it has made little difference. In my opinion changing resistor (R8) from 560 ohms to a greater value would reduce the input signal, before it arrives at IC, but would that be enough to reduce the gain? I would value an opinion on this, any help appreciated, thanks in advance, joe

Thanks to the person who posted the circuit diagram on http://music-electronics-forum.com/t11641/

[cbarrow7625]

Rather than removing & replacing R8, it would be easier to just put a resistor in parallel with R6 to get the same result (reduced gain). Try putting a 100K resistor in parallel with R6 to start. That should reduce the gain significantly. If that isn't enough reduction, try a smaller value. If it is too much, try a larger value.

Use some clip leads to try a few different values quickly. you should find a resistor value that gets your desired results pretty quickly.

[JoeBaker]

I’ll give that a go, it’s a much better idea than mine, I’ll try what you suggest with some clipped leads to start with, thank you very much.

[cbarrow7625]

Cool. Let us know how it works.

Just so you know, I suggested the easiest thing to do, not neccessarily the best. It seemed to be in the spirit of your original question though.

It sounds like you would probably also like a softer onset of distortion as well as less of it. Does that sound right? To do that, you may have to do a little more complicated surgery (still simple though). you may want to consider putting some resistance in series with the two LED's in the op-amp feedback loop. That will help soften the onset of distortion. To do this you will need to unsolder the two LED's on one end (either end, doesn't matter as long as the ends both go to the same op-amp pin) and solder those two ends together. Then add a series resistor from the ends of the LED's back to where you unsoldered them from.

In that circuit a good value to start with would be about 220k. Add more resistance for a softer distortion onset and less for a harder onset. You may find that a combo of this mod and the one suggested earlier may get the best results. Getting the values "correct" on both mods at the same time will be a balancing act. You may have to go back and forth a few times to get the right values in both places. If you plan ahead you can use clip leads for both resistor substitutions and get it right pretty quickly. Good luck.

[JoeBaker]

Thank you, I would probably like a softer onset of distortion as the main problem with the amp at the moment is that it has too much distortion to soon, and it’s too harsh as it drives the tone to much. I like old style amps where the distortion is more controllable where you can clean up the sound using the volume pot on the guitar. This isn’t available on this channel at the moment; I hope modifying this section of the preamp will help. The clean channel sound is extremely good for a solid state amp, it has a really warm tone from its power amp similar to a Fender valve amp and is ideal for blues or rock rhythm and clean lead when cranked to full volume, and is ideal for gigging it’s extremely loud for a 15watt. I wish the lead channel had a similar sound but with more crunch that would make it perfect in my opinion, hopefully the mod will achieve that, I see no reason why it shouldn’t. Many thanks Rob