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Solid State Amplifiers => Amplifier Discussion => Topic started by: buffalocomputer on May 19, 2021, 06:40:11 AM

Title: Marshall lead mosfet 100 lead channel very quiet
Post by: buffalocomputer on May 19, 2021, 06:40:11 AM
Just got one of these and a 60's 4x12 cabinet for $125, but the lead channel is extremely quiet. I can hear that it has the crunch, so I know that part of the circuit is good.


The amp does not respond to the volume knob on the lead channel, so I'm expecting it's a problem with the pot.

If turns out not to be the volume pot, what other components could cause that pot not to respond on the lead channel?

Also, what would be some good mods that I could add that I can make switchable with installing switches on the front plate?

Title: Re: Marshall lead mosfet 100 lead channel very quiet
Post by: Enzo on May 19, 2021, 10:30:23 AM
I suggest making the amp work before even thinking about mods.

Do nit assume it is a bad component.  It could be, but could also be a broken connection, broken wire, cracked circuit trace, etc.   Apply a signal and trace it through the channel looking for where it disappears.

My first reaction is a bad semiconductor somewhere.

Title: Re: Marshall lead mosfet 100 lead channel very quiet
Post by: Tassieviking on May 20, 2021, 02:43:01 AM
First thing to do is to find out which model MosFet you got, they made more then one model as far as I know.
Then i would find the correct circuit for it.
Try DrTubes website for the circuit to start with.
https://drtube.com/en/library/schematics/69-marshall-schemas
Once we know what you have we can make better guesses
Title: Re: Marshall lead mosfet 100 lead channel very quiet
Post by: Bjorn218 on June 10, 2021, 02:39:08 AM
You have already pulled the grain pot out?  It’s a push/pull. You also have to activate the gain channel with a TRS plug or was it TS, I can’t remember just a plug where the foot switch plugs will do. That’s what I had to do with mine.  It’s a TS now that I think of it as I used a TRS plug and pulled it back out about a 1/4” to make the connection.