My amp is an oscillator !

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My amp is an oscillator !

Postby nigelwright7557 » Mon Jun 24, 2013 10:22 am

I have just completed a 80 watt valve amplifier.
I found the amp oscillates at half volume. Either side it is fine.
So I took out the phase splitter valve and the oscillation is still there in the first 2 gain stages.

I have tried extra decoupling but that made no difference.
Removing the bypass capacitors stops the oscillation but the gain is then too small.
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Re: My amp is an oscillator !

Postby Shannon Parks » Wed Jun 26, 2013 5:11 am

What's going on at CONN1 and how about the rest of the schematic?

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Re: My amp is an oscillator !

Postby nigelwright7557 » Thu Jun 27, 2013 3:11 pm

Conn 1 selects between one gain stage and two.

Its only the shown part that has the problem.

I had a look at the pcb and it is very poor in regards keeping input wires short.

In the end I took out the cathode capacitors and it stopped oscillating.
It still has plenty of gain to reach full power.
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