Crackling in left channel on recently built Crack

[nebulous]

Hello All,

I recently built a Crack amp and everything seemed fine, voltages appeared OK, and music played fine. However, in the first or second day of use I noticed a slight crackling in the left channel.  It's a soft, static-y rustling that persists for about a half a second and comes at random intervals (i.e. amp has been on for a half hour, no static and then it will starting coming at 30 sec to 5 minute intervals). The noise will come and go even if I do not have an input plugged into the amp. One final note, I followed the 5/6/10 revision on page 19 of the manual that says to solder power transformer terminal 4 to terminal 14U (instead of terminal 22L).  Any advice on how to remedy this problem would be great, thanks!

Here's my setup:
Computer-->DAC-->Crack Amp-->Senn hd 598

The Senn's are pretty low impedance, I am planning on upgrading.

Here is a list of the voltages I felt were marginal (not exactly on):

Terminal	Desired Voltage	Read Voltage	Percentage
1	90	76	84%
5	90	76	84%
A1	90	78	86.7%
A6	90	76.8	85.3%
B1	90	76	84%
B4	90	78	86.7%

[vetmed]

I would try replacing the 12au7. The noise and the low voltage suggest to me that the tube you are using is on its way out. Hope you find this useful.
           Regards
               Robert Lees

[nebulous]

Thank you for the advice. Could you or anyone else recommend a replacement tube and where to acquire it on the internet?

[Paul Joppa]

I agree that it might well be a tube issue. However, I should note that a low 12AU7 plate voltage in this circuit suggests the tube is conducting particularly well, with emission and transconductance on the high side. That doesn't prove it's quiet, of course! I'd say it could be either tube thoough.

It's possible this will go away in a while - 10-100 hours maybe? - that does happen sometimes with tubes. And of course it could be a marginal connection - just re-seating the tubes in their sockets might even cure it if it's a bad socket connection.