Author Topic: Marshall lead 12 preamp help  (Read 10853 times)

gregdeerberg

Marshall lead 12 preamp help
« on: June 19, 2011, 11:02:15 PM »
I built this and it is acting very strange. Not sure what I did wrong? It only works when my guitar is from about 0-3 on the volume pot. If I go louder it craps out. I used a 100k resistor for the gain pot as well because I didn't have a 22k. That has to be set in a certain range as well.....somewhere in the middle of the travel. I did substitute a few parts because I didn't have some on hand. R1=470K...C1=2.2uf....R10=150k. I also did away with the tone stack and just sent pin 7 out to a 2.2uf cap. I also have pin 4 going to ground. I am using a single 9v to pin 8. I tried 18 volts and got the same effect but just louder. Do I need to feed B+ anywhere else? Any ideas? I have a link to the schematic below. Thanks



http://www.drtube.com/schematics/marshall/3005.gif

Mephistocles

Re: Marshall lead 12 preamp help
« Reply #1 on: June 26, 2011, 02:27:50 PM »
I built this and it is acting very strange. Not sure what I did wrong? It only works when my guitar is from about 0-3 on the volume pot. If I go louder it craps out. I used a 100k resistor for the gain pot as well because I didn't have a 22k. That has to be set in a certain range as well.....somewhere in the middle of the travel. I did substitute a few parts because I didn't have some on hand. R1=470K...C1=2.2uf....R10=150k. I also did away with the tone stack and just sent pin 7 out to a 2.2uf cap. I also have pin 4 going to ground. I am using a single 9v to pin 8. I tried 18 volts and got the same effect but just louder. Do I need to feed B+ anywhere else? Any ideas? I have a link to the schematic below. Thanks



http://www.drtube.com/schematics/marshall/3005.gif

I was interested, yet annoyed by this schematic. Not everything is straightforward to me. Nevertheless, I'll help you out.

- Solder a 27k resistor between the 100k pot 1st and 3rd leg.
- For R1, anything between 100k and 1.5M, really not very critical, as long as you don't get too low.
- What? 2.2u for C1? Try something more like 10n to 1u non-electrolyte.
- R10 some lower? Why not two 470k resistors parallel, for example.
- Use symmetric power supply. Connect two 9V batteries is series, with the - connected to the + with a battery clip. Solder those two wires together. Ground is connected to this point. Now, the batteries + and the - are +9V and -9V.

I hope you only wanted to build the preamp section and not the poweramp section... In that case there is still hope.