Microphony and Feedback Loop

[ALRubinger]

Hi all:

So, good news: BeePre on first build is working! And sounds great at the low volumes I can test this late at night.

Hiccup: there's lots of microphony coming out of one channel when touching the controls or any part of the chassis plate.  Even walking in front of the unit can trigger some, and this can create a feedback loop when the amp is also on.  Volume and balance on the BeePre has no effect on the microphony hum. Switching the 300B tubes makes the microphony hum come out the other channel.

If you could be so kind:

1) Confirm tube is bad?
2) I so, am I eligible for a replacement?

Thanks in advance!

S,
ALR

 

[fullheadofnothing]

All tubes are microphonic to some degree. Direct-heated tubes are microphonic to a greater degree than others. There are lots of ways of dealing with the issue, mostly involving adding mass. Making the tube envelope heavier with the flask weights Dan has written about is a good idea. Also, some people like to leave out the silicone O-rings packed in the kit. If you are one of those people, please put them in, they help a lot. While a little noise when touching a chassis may seem like a huge deal, remember that the point of this gear is to put on music and listen to it. Relax and enjoy rather than poke and worry!

If it is going off when you walk by, then I'm going to make an educated guess that it is on a floor standing piece of furniture, and you aren't on a concrete slab floor. If you have the ability to use a wall mounted shelf instead, that will help tremendously. If not, try adding some mass to your stand. Also if possible, move your stand, as it could be on a particularly bouncy part of your floor.

Finally, tubes need to break in. Give it at least 50 hours of run time before deciding it isn't working. Additionally as a tube warms up (i.e. in the first half hour or so after turning on) it will make more noise than after it has reached its operating temp.

 

[ALRubinger]

Thank you, Joshua!

I'd written because one of the tubes is drastically more sensitive than the other; walking by can trigger a feedback loop as I mention.  The other channel just gets a little jitter from time to time, which is totally acceptable.  But I'm having issues controlling the one problematic channel.

I'll give it some more break-in, but something about this operation is saying to me it's a bit wonky as it stands.   In your experience, this is normal behavior at first?

S,
ALR