Marshall 3203 Artist Bias

[jgcable]

I have a question. I want to install I bias pot on a Marshall 3203.
#1. Does anybody know where the bias resistor is on this amp?
#2. I am told it has a resistor that should be inline or somewhere near the phase inverter tube. Below is a picture of the 22K trim pot I need to install. If you notice, the trim pot has 3 legs on it but the resistor I am replacing it with only has 2. How to I solder it to the board? Do I cut off a leg? Sorry for the bush league questions.

[Rupe]

Re: Marshall 3203 Artist Bias

I would to Dave Sarge at Engineered Sound in State College, PA. He has done all of the work on my amps, including a nice tone mod on my 3203 to get it to more "plexi-like" specs. He is a consultant to many of today's amp builders and used to work with Randal Smith. He is currenttly an EE professor at PSU. You can reach him at 814-238-6942, or [email protected]
Tell him Bill Rupert referred you.

[DonP]

Re: Marshall 3203 Artist Bias

1. Start from the grids of the EL34's and work your way back. You should go through a pair of high value resistors (like 220K). There will be some small filter caps by the resistor.

2a. You connect two legs together (the wiper and one of the ends).

2b. Consider remote mounting with wires going to the board. Sort of like how the DSL's are, so that you don't have to tear it apart to bias it. Make sure you don't let these wires ground out.

P.S. Are you sure this is a fixed bias amp? The wattage is so low it could be a cathode bias. Very slim chance of that on a Marshall.

[charvelguy]

Re: Marshall 3203 Artist Bias

I like the remote bias idea. Handy when you don't have to take the chassis out for access.

[Twisteramps]

Re: Marshall 3203 Artist Bias

Sorry I didn't notice this sooner...

http://www.drtube.com/schematics/marshall/3203pwr.gif

Look to the left of center of the schematic - R2. it's 56k, you'll want to make it smaller and tie it to the pot so that at full rotation you have more than 56k, and at full cut you have less - I'd say with a 22k pot, go for around 44k or so. That'll give you a range from 44k to 66k. Look at this schematic and note the bias adjust on the far left. That should help. If you're not sure, have someone else do it. You can screw up an amp in a hurry with a bad bias mod.

http://www.drtube.com/schematics/marshall/jcm800pw.gif

Pete

[DonP]

Re: Marshall 3203 Artist Bias

Pete,

You have a schem for the preamp? I'm guessing it's a 3005/5005 preamp, but just want to make sure.

[charvelguy]

Re: Marshall 3203 Artist Bias

It's on the same site..
I think I may eventually do this to mine as well.

http://www.drtube.com/schematics/marshall/3203pre.gif

[jgcable]

Re: Marshall 3203 Artist Bias

Ok Pete, so I replace the 56K resistor with a 44K. I get that. It will give me a range from 44K to 66K.
Now, where exactly do I mount the 22K trim pot? I was going to remote mount it so I could get to it without removing the chassis. I am also going to snip one leg of the C13 bright cap and wire in a switch to the back of the chassis to act as a bright switch for the clean channel.
Also, the trim pot has 3 legs. 2 on one side and 1 on the other. Do I tie both of the legs on one side together to act as one contact?