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Odd tube rattle problem.

3/31/2006 8:51 PM
Odd tube rattle problem.
I have a GT Soul-O-75 here that sounds like it has a microphonic 6L6. When grabbing the tube, the rattle stops (doesn't help that the tube is so close to the speaker).  
I replace the 6L6, play thru the amp forever, and decide that was the culprit. Owner calls me up, and is estatic that I found the problem. Tells me he's played thru it for two days without hearing the rattle (also doen't help that he's obsessing on the rattle that you can only here at lower volumes on 1 or 2 notes. But it is there). After patting myself on the back, I get the call...  
Anyway, this wouldn't be that big a deal, but it has happend two times. When it came back the first time, I just figured it was another bad tube. But after replacing it again, there was a repeat of what I wrote in paragraph 2.  
Is it farfetched to assume that there is something else exciting the tube other than the speaker?  
It is so intermittent, that I can't trust the results of plugging into another speaker cab. BTW, it happens on the same socket.  
Any ideas?  
Thanks in advance.  
4/1/2006 2:15 PM

Clean and re-tension the tube can cause what appears to be a microphonic tube, but is actually bad socket connection.
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4/2/2006 4:45 PM
Thanks for the reply, but...
Initially, I tried retensioning because it was doing the common crackling thing when excited by the speaker.The socket was shot, so it got replaced. Problem solved. But then it came back about a month later with this new problem. The tube is very tight in the socket. It definitely stops when I grab the tube though. The amp is currently not here, but I guess I should inspect the socket when it comes back. This is a high quality socket, so I don't expect that to be the culprit. Interesting about how a dirty socket can cause a tube to appear microphonic. Haven't come across that one before. That's one for the archives.  
Any more idea?  
4/2/2006 9:34 PM
Carl@Zwengel Amps

I get the same power tube rattle with about 80% of the new tubes I use. The tubes do seem to work ok and dont fail but I can understand that it's a bit of an annoyance. As an aside, I've noticed that most of the current EL-34's and 5881wxt's DO NOT have this litle rattle. However they're not 6L6's either so there may be some strange manufacturing thing going on.  
Without being antagonistic, you could tell the customer to either pony up the long green for a set of NOS RCA's or Sylvania 6L6's or he could just tolerate the little rattles.  
4/4/2006 2:46 PM

Thanks for the reply, Carl!  
Yeah, I suppose I should try a few more tubes if you've been experiencing a high rate of rattling tubes. We did play thru it for about 20 minutes, and decided they were okay. In my case, they wait until the customer takes the amp home until they start doing their thing.  
Yeah, I did tell him that he's not going to even notice it playing at louder volumes with a band. Which of course is what it's designed to do. Nice guy, but he's dwelling on that "one" note that excites the tube while playing fairly quiet in a small room.  
picky, picky picky :)  
Thanks again.  
4/5/2006 12:59 AM

I do this with EL84s destined for PV Classic 30 and other small amps that leave the power tubes dangling. Hold the tube up in front of your face and rap on the side of it hard with a knuckle. If you can hear a rattle or a bit of looseness inside then, it will be a rattler in the amp soon enough. I select tighter tubes for that, and the looser ones go in amps less prone to rattle.
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