• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

Why a sudden distortion in treble of this SET amplifier?

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I have had this very sophisticated SET amplifier with 520B valves on the output for a few years now; recently I noticed a unusual distortion between 5k and 12k that was not present before. Neither is the distortion in three other amps I have so I don't think it's related to the AC line. Nothing else change in the amp's sound that I could tell.

If it is a bad part in the amp, what are the first suspects? cap, resistor, tube?
 
I have tried different tubes and the problem persists.

The amp was made in Denmark by J. Nielsen. But the designer has no idea what could be the cause.

I also thought it could be the PS caps, especially considering the number of 150uF caps he uses in series for the B+ filtering. I think there are 24 of those caps in series... Checking that will be a major work.
 
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I have tried different tubes and the problem persists.

The amp was made in Denmark by J. Nielsen. But the designer has no idea what could be the cause.

I also thought it could be the PS caps, especially considering the number of 150uF caps he uses in series for the B+ filtering. I think there are 24 of those caps in series... Checking that will be a major work.

Just a thought,

Have you checked the bias of the O/P tubes? also bias voltage and B+ supply voltage + ripple? Can you direct us to a schematic it might be easier to give some more information..

Regards
M. Gregg
 
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