tube noise

[Gaffa]

I recently finished my K-12G stereo amplifier kit that I purchased a few months back. The kit was a pleasure to build and sounds absolutely fantastic with the exception of one little fault: if I tap on my work bench, the amp chassis (equipped with anti-vibration feet), or the circuit board, I hear vibration noise through my speakers (it’s really bad if I touch the top of any of the four valves/tubes). The noise is even more disturbing if I’ve turned the volume up above roughly halfway, to the point where I’m worried about blowing a speaker driver. Could someone please offer some advice on what is causing the problem and perhaps a way to eliminate or at least reduce the problem?

P.S. This is my first audio/tube amp venture.

[gni]

Microphonics most likely. . .

http://www.ehow.com/how_2043383_-loc...amplifier.html

http://thetubestore.com/12ax7review.html

http://studentweb.eku.edu/justin_holton/tubes.html#a7

http://www.guitarrepair.com/abouttubes.html

or Grounding. . . .make sure the unit is grounded well. . .recheck
those solder joints.

[oshifis]

The term "noise" may be incorrect here. The tubes convert mechanical vibration to electrical signal when the distance of the electodes are changed due to shock. There are techniques to isolate the tubes so that the vibration of the chassis is not conducted to them.
Some call 60/120 Hz hum as noise. That is also incorrect, noise is random variation of signal.

[TubeHead Johnny]

What type of preamp tubes are you using? Microphonic tubes can be tamed by using the short plate types. I've had good luck with JJ ECC83s.

[Gaffa]

Thanks to "qni" for the useful links & help.

I pulled the 4 tubes from their sockets, tightened all 36 pin receptacles, aligned the tubes carefully while re-inserting, and then fired up the amp. I tapped on the bench, the chassis, the tube tops (with the eraser end of a pencil), and even jiggled the tubes a bit... the "noise" problem was gone! Nothing but clean sound now. Wonderful.

Oh yeah, I e-mailed the S-5 Electronics guys and they had a few helpful suggestions too (mostly about properly grounding the volume pot--which I did).

The help was most appreciated.

[richwalters]

Worth mentioning, on the subject of acoustical tube noise I was so put off by putting those so-called ring dampers on tubes that makes everything about the look of a tube amp completely wrong. I found this little trick with rubber grommets and sprung brass strip just provided the right damping on 12BY7 tubes to stop ringing. These tubes have so much gain, even in triode config that the "ping noise" can sing.

rich

[SY]

Nice!

[ray_moth]

It's always worth taking precautions against tube vibration but I've also found that microphonics can vary widely with different manufacturers. My GE 6AU6s, for instance, are much worse than my RCA ones.

[richwalters]

Ray, that falls right in with my findings: On my NOS "ping" scale, RCA come on top for the best overall consistent quality (noise and performance symmetry) followed by the rare Marconi, Sylvania,TungSol, GE comes lowest.
Those Marconi tubes (the pinnacle of British manufacturing) provided by far the lowest thd and are mighty hard to get hold of.
Daresay, current production 6550 TungSol give lower thd than NOS KT88's.

richj