12 series build troubleshooting

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12 series build troubleshooting

Post by nperros » Sat Apr 18, 2015 1:46 pm

Hi folks,

I'm new to the forum and was hoping I could rely on your collective wisdom in finishing up my 12 series build. I'm experiencing two problems with the amp.

First, when the amp is dimed, I'm getting an ugly, out of tune bass note along with the notes I'm actually playing. I believe that this is usually caused by an under-filtered power supply. It was also my understanding that the power supplies on the 12 series Marshalls were lower than for other plexis and that this is where some of their charm comes from. So do I bump up the capacitor size in the filtration section, or is this something intrinsic in 12 series amps?

Second, I'm getting interference in the input that sounds like cell phone noise but is present even when there's no cell phones around. Is the solution to use shielded cable to the 68 k resistors? Should I take the resistors off the board and put them directly on the tube socket?


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Re: 12 series build troubleshooting

Post by rgorke » Thu Apr 30, 2015 3:12 pm

First, welcome aboard.

Second, what exact values are you using in your filtering?

Third, a few photos always help.

Fourth, some clips of the "noise" helps as well.
"If you make a mistake, do it twice and smile and let people think you meant it." Jan Van Halen.

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