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In Reply to: New tubes for an Anthem. posted by RAYDOG on July 25, 2002 at 05:03:21:
Contact SFI (there was a phone number posted here a few days ago, maybe a search would uncover it again!). I have had experience with my own gear with the same type of "whistle/squeal" problem, but it wasn't SFI stuff, so I can't really say what you've got there (mine tuned out to be a voltage regulator that was under spec'd for the amount of tubes it was supposed to supply to- the protection circuitry kept cutting in and causing oscillation!).
Nick at SFI should be able to help you track the tube or part problem down!
BTW, they do sound good, don't they?!
Cheers
Dman
Follow Ups:
Damm, I was thinking it was just a microphonic preamp tube. I hope it's nothing worse. I'm not the type to go digging around in the guts of my amp and the last thing I want to do is send it off somewhere for weeks for repair. I once had to send a pair of Infinity Composition speakers in for warrenty repair, both amps blew during a thunder storm. It took over 4 months to get them back. I was not amused.
"Honey, that's the same amp we have always had, I swear!!"
Don't get freaked out - it probably is the preamp tubes. The "underspeced regulator causing oscillation" problem is certainly not very common and was probably a flaw in that particular product and certainly does *not* represent a common problem in a wide range of amps from different vendors. LOTS of tube gear doesn't even have regulation. -jd
...You might want to give the tube sockets a little cleaning as well (when doing what "ozzy" and I have suggested)! I just thought of that too!
Cheers
Dman
Of course the easiest way to tell if you have a microphinic tube is to turn the unit on, let it warm up for about 1/2 an hour, then lightly tap on each tube. If there are microphonic ones in there, you should clearly hear them through the speakers! If you don't, then the problem is a little deeper!
But I think you already knew that!
Not trying to scare you, but thats exactly what I thought was wrong with my preamp before getting it to my "tube guru" friend.
Cheers
Dman
It does sound like a microphonic preamp tube. Sometimes all you have to do is pull them out and reseat them. While you are doing this, you can swap left/right channel tubes and further isolate the culprit. But just reseating them might be all that is neeeded.Oz
RD,Rotate the 12AU7 into different positions. Do the same for the 12AT7's. My bet is if the microphonic tube is in the first gain stage---getting it out of that stage and substituting a good tube will resolve much of the problem.
If, after all the maneuvering of tubes DOESN'T resolve or at least lessen the problem, you'll need to look for cold/broken solder points. Dried out caps (especially ceramic disc babies). And possible instability or oscillations in the gain stage/PI.
I'd try to first rotate a KNOWN good 12AT7 and 12AU7 into the unit. MOve it around to the vaious positions and see if that helps. It would be a lot less hassle if the culprit was a loosey-goosey tube.
Certain 12AT7 and 12AU7 are kinda noted for microphony. Probably one of the reason why so many "ruggedized" versions of each tube exists.
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