Echoing and strange feedback issues on a new build crack.

[loz_the_guru]

Hi all,

I finished my crack build yesterday, and have sat down to listen today. I've run into an odd issue on the left channel.

I can hear an strange echoing sound. It's not a buzz, and it seems like the amplifier is picking up sounds/vibrations from it's surroundings. For example if I tap on the desk the amp sits on I can here it almost as if I was tapping on a pain of glass. The effect is worse if I tap on the metal top plate.

Any ideas on what the issue might be? Apologies for the rather vague description of the sound - it's almost as if there's some sort of vibrating/oscillating echo? Does this make sense to anyone?

EDIT: I've attached a photo of my wiring in case it helps. Please note that I bridged A3 and A8, as I pulled one of the LED legs off during soldering . I've ordered another in the mean time.

[Paul Birkeland]

You may just have a slightly microphonic tube, which isn't a cause for concern.

[loz_the_guru]

You may just have a slightly microphonic tube, which isn't a cause for concern.

Thanks for the reply Paul. Is there anything that can be done about it? It's really annoying, even at low volumes, and completely ruins the experience at low volumes.

Is it likely to be the power tube or the output tube, and is there anyway of determining? The power tube is quite corroded on the outside - could this be the cause? I've attached another photo.

Thanks!

[Paul Birkeland]

It's a physical attribute of some tubes.  If you tap on each one while the amp is operating, you might notice that one is far more sensitive.  In my experience, it's usually the 12AU7.  If you put the Crack on a heavier table or put a bit of weight on top of the power transformer on the Crack, this may reduce the microphonics to an acceptable level.  A wall shelf will also generally solve this issue.

12AU7 tubes aren't super expensive, so you could also buy a few spares if that ends up being the source of the microphonics.

[loz_the_guru]

It's a physical attribute of some tubes.  If you tap on each one while the amp is operating, you might notice that one is far more sensitive.  In my experience, it's usually the 12AU7.  If you put the Crack on a heavier table or put a bit of weight on top of the power transformer on the Crack, this may reduce the microphonics to an acceptable level.  A wall shelf will also generally solve this issue.

12AU7 tubes aren't super expensive, so you could also buy a few spares if that ends up being the source of the microphonics.

Thanks. Tapping on the larger of the two tubes (this is the 6080, right?) seems to make significantly more noise. Presumably swapping this out will alleviate the problem?

[Paul Birkeland]

It's certainly worth a shot, but the tube will otherwise operate and work just fine.