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Noise from output tubes

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9/2/2004 8:35 PM

Noise from output tubes

I was using the Groove Tubes Speaker Emulator II as a load for my Express-type bassman (2x6L6). The output tubes make sound when I play. It's a weak sound, but still audible and gets louder if as the volume on the amp is turned up. IIRC, I have the bias set at 36ma at 480vdc which should be roughly 70% dissipation. Is this sound normal? The only reason I can hear the sound is because the head is loaded. While looking at the tubes I can see mild blue flashes while I play. Are the tubes supposed to flash mildly as you play? I've never looked at them before. I did not measure my screens? Could they be melting?

Jeff

9/2/2004 8:53 PM

The blue comming from the tube's is normal. There is an article online somewhere describing the normal blue glow, and the "bad" blue glow from a gassy tube that's about to pop. I have never heard of output tubes making noises, and I doubt they'd be able to. But the output transformer definitely will make noise if it's played hard (or not so hard). This is because of the magnetic feilds in the transformer are enticing the windings and laminations to move. If the transformer is not perfectly potted up, the parts of it can move ever so slightly, resulting in "singing" OT's. every time I test an amp into a dead resistor load, I can hear it though the OT.

9/2/2004 8:56 PM
Jeff	Wow, quick and extremely helpful reply. You're probably right about it being the OT. I cannot really tell. Since this was my first time loading and having the room quiet enough to hear the weird noise I was a little nervous. Thanks Jeff

9/3/2004 2:43 AM
Dai Hirokawa	FWIW，when I use my Palmer spk emulator w/my Marshall and crank it, I can definitely hear noise coming from the OT.

9/3/2004 3:37 AM
Jeff	Glad I'm not the only one. It's weird to have the amp cranked and be able to hear it. Jeff

9/3/2004 4:56 AM
Enzo	Output transformers "sing" all the time, and the harder you push them the more you can hear it. Usually you don't hear it because of the sound coming from the speaker masking it. Play any amp into a dummy load and you will probably hear some of it when you crank.

9/3/2004 9:05 AM
MatthiasM	google "magnetostriction" (n/t) ..

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