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Thuper

Tube amp for Cornwall III

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Sorry for the delayed response lost track of the thread. Marty (Thebes) emailed me with a nudge ;)

So I take this noise is more or less about the same with no sources attached and not effected by the amps volume control? Also this noise is identical out of both speakers?

How far away from the output tubes is this Triode/Pentode switch?

One thing that happens when switching between these two modes is that the input sensitivty of the amp is lower in pentode mode as compared to Triode mode which makes pentode mode more susecptable to noise but usually this would be noise from outside sources connected to the amplifier.

If the switch is half way across the amp noise can be the result but usually the mode of operation will not effect it much. I wonder if the contacts for pentode mode have become oxidized from a lack of use. Try working the switch back and forth vigorously (with the amp off).

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The switch is on the back of the amp, between the speaker posts. So it's ca 4 inches from the output tubes.

I have tried working the switch, but it didn't help.

I've noticed that the amp is sensitive to radiation, so I thought I should make me a shielded powercord. I had the powercord from my DVD player running in front of the amp, with hum as a result... The powercord runs a inch or so from the Triode/Pentode switch.

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In my experience, this same described noise was caused by noisy driver tubes in my Foreplay III; these are/were 12AU7A, not branded, that came with the kit. Was kinda weid though that they both started doing it at about the same time. Replaced them both (several months ago) and have not noticed that noise since.

Rick

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Well... I've changed the driver tubes. I have a pair og Sylvania and GE tubes here. And the hiss sound is there also when I use these.

Frode

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