two minor problems, maybe

[moviedave]

When I turn my crack+speedball up, with no source plugged in, I start to get a buzz around 12 O'clock, which gets louder the higher up I go.

Second, and this only seems to really happen when I have my 5998 plugged in, every tap on the case, or movement of the volume pot gets transferred though my headphones and I can hear every detail.  That doesn't happen ALL the time, but with some frequency.  Are either of these anything to worry about.

[Paul Birkeland]

With no source plugged in, yes, you can get some buzz if you crank the volume up.

The other noise is microphony, which is also normal with some tube types in particular, though very rare in a 5998 or 6080.

[Grainger49]

To test it you need to short the inputs.  Taking a pair of jumpers, one for each channel, and jumping from the outer conductor to the inner conductor will do it.  With an open input all sorts of noises can creep in. 

If it still buzzes, Dan has a post in Technical Topics folder about this.  You should read all the stickys there.

[moviedave]

Thanks.  Shorting the inputs did the trick.  The microphony however, remains.  I wonder if this was a bad bad of 5998's.  I sent the first tube I received back for this problem, and it's replacement has it too.

[Grainger49]

Some tubes are microphonic almost by design; that is the whole run of tubes are microphonic.  Some manufacturer's tubes tend toward being microphonic.  Others are less so.  So changing the tubes can make a dramatic effect.

There is a tube rolling thread in the Crack folder.  It will have lots, and lots of suggestions.