tunghaichuan
September 7th, 2008, 09:02 PM
...a Marshall Lead 12 conversion.
I had this old 80s solid state Marshall Lead 12. It never worked right, and I originally bought it to mod. There was something weird about the input jacks. I tried replacing them and it got a little better, but the amp was never 100%
A stock Lead 12 actually sounds very good. It does a pretty credible impersonation of the 80s Marshall sound. It does a very good squeaky clean sound as well.
As you can see from the pix, the amp looks totally stock until you pull off the top back panel which hides the tubes. The only indicator that it might be different is the Edcor transformer sticker I put on the back of the chassis.
I built an AX84 P1, Rev. 10 circuit into it but with a twist. Instead of an EL84, I used a 7868 which is like a larger version of the EL84. The EL84 is a 12W tube and puts out about 5W of actual audio power. The 7868 is a 19W tube and puts out about 8W-10W of actual audio power. The 7868 uses a special 9-pin novar/magnoval socket and looks like a large 12AX7 or EL84. The 7868 is electrically identical to its octal brother, the 7591. Some Ampeg amps of the 60s used 7868s and 7591 tubes.
About 10 years ago, I built the stock version of the AX84 P1, Rev. 10 circuit. It sounds very good. It only gets some crunch when the volume knob is dimed. But it sounds very good with my Boss Blues Driver pedal. Instant Marshall tone.
I basically just gutted the chassis pulling all the SS circuitry. I then built in the P1 circuit after punching/drilling all the holes. I used a surplus Stancor PT and a special-order Edcor single ended OT. The Edcor has an unused ultra linear tap which I may hook up to see how it sounds. One thing I don't like about the Edcor is that it has fast on connectors which slip onto exposed solder tabs: bad idea! There is about 300v on the primary side of the OT (blue and red wires).
It powered right up after I finished it, but didn't seem as loud as it should. However, 8w isn't that much louder than 5W. As usual the three band Marshall-style EQ is kind of wonky. It needs to have the bugs worked out of it.
Over all I'm pretty satisfied with this project, but if I were to do it again, I'd use two EL84s in parallel single ended rather than the 7868. They are kind of pricey, even the currently manufactured tubes.
I'll try to get some sound clips up when I get the bugs worked out.
Some obligatory pix:
http://www.diycustomamps.com/images/lead12/Lead12_1.JPG
http://www.diycustomamps.com/images/lead12/Lead12_2.JPG
http://www.diycustomamps.com/images/lead12/Lead12_3.JPG
http://www.diycustomamps.com/images/lead12/Lead12_4.JPG
http://www.diycustomamps.com/images/lead12/Lead12_5.JPG
http://www.diycustomamps.com/images/lead12/Lead12_6.JPG
http://www.diycustomamps.com/images/lead12/Lead12_7.JPG
tung
I had this old 80s solid state Marshall Lead 12. It never worked right, and I originally bought it to mod. There was something weird about the input jacks. I tried replacing them and it got a little better, but the amp was never 100%
A stock Lead 12 actually sounds very good. It does a pretty credible impersonation of the 80s Marshall sound. It does a very good squeaky clean sound as well.
As you can see from the pix, the amp looks totally stock until you pull off the top back panel which hides the tubes. The only indicator that it might be different is the Edcor transformer sticker I put on the back of the chassis.
I built an AX84 P1, Rev. 10 circuit into it but with a twist. Instead of an EL84, I used a 7868 which is like a larger version of the EL84. The EL84 is a 12W tube and puts out about 5W of actual audio power. The 7868 is a 19W tube and puts out about 8W-10W of actual audio power. The 7868 uses a special 9-pin novar/magnoval socket and looks like a large 12AX7 or EL84. The 7868 is electrically identical to its octal brother, the 7591. Some Ampeg amps of the 60s used 7868s and 7591 tubes.
About 10 years ago, I built the stock version of the AX84 P1, Rev. 10 circuit. It sounds very good. It only gets some crunch when the volume knob is dimed. But it sounds very good with my Boss Blues Driver pedal. Instant Marshall tone.
I basically just gutted the chassis pulling all the SS circuitry. I then built in the P1 circuit after punching/drilling all the holes. I used a surplus Stancor PT and a special-order Edcor single ended OT. The Edcor has an unused ultra linear tap which I may hook up to see how it sounds. One thing I don't like about the Edcor is that it has fast on connectors which slip onto exposed solder tabs: bad idea! There is about 300v on the primary side of the OT (blue and red wires).
It powered right up after I finished it, but didn't seem as loud as it should. However, 8w isn't that much louder than 5W. As usual the three band Marshall-style EQ is kind of wonky. It needs to have the bugs worked out of it.
Over all I'm pretty satisfied with this project, but if I were to do it again, I'd use two EL84s in parallel single ended rather than the 7868. They are kind of pricey, even the currently manufactured tubes.
I'll try to get some sound clips up when I get the bugs worked out.
Some obligatory pix:
http://www.diycustomamps.com/images/lead12/Lead12_1.JPG
http://www.diycustomamps.com/images/lead12/Lead12_2.JPG
http://www.diycustomamps.com/images/lead12/Lead12_3.JPG
http://www.diycustomamps.com/images/lead12/Lead12_4.JPG
http://www.diycustomamps.com/images/lead12/Lead12_5.JPG
http://www.diycustomamps.com/images/lead12/Lead12_6.JPG
http://www.diycustomamps.com/images/lead12/Lead12_7.JPG
tung