Oscillation in '67 Marshall 50w

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JTM100
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Location: Florida

Oscillation in '67 Marshall 50w

Post by JTM100 » Fri Apr 07, 2017 2:37 pm

I have a '67 Marshall 50w. It is the earlier shared cathode JTM circuit with a solid state rectifier and EL34's. I keep getting this weird oscillation and static out of it intermittently. If I let a note sustain, you can hear almost a subtle vibe/trem kind of sound in the note. As the not starts to trail off there is a static sound at the end of it. The amp is not a trem model. The problem occurs anywhere the amp is played. I have swapped the tubes out, but that didnt fix it. I also tried installing 1.5k surpression resistors on each power tube socket. The 47pf fizz cap is present on the phase inverter. Does anyone have any ideas what might be causing this? The amp has all of it's original components in it.

Tazin
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Re: Oscillation in '67 Marshall 50w

Post by Tazin » Fri Apr 07, 2017 4:30 pm

Could be a leaky signal cap...One of the 0.022uF.

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neikeel
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Location: Suffolk, England

Re: Oscillation in '67 Marshall 50w

Post by neikeel » Fri Apr 07, 2017 5:32 pm

Does it vary with the choice of input/volume? Is it dependent on settings of the EQ?

Have you cleaned all the jack contacts and check the power lead and fuse holder contacts?

Pics might be useful.
Neil

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