Crack Static

[twm]

I'm getting a random, intermittent, static buzz in the left channel of my Crack. 

When I turn on the power, I'll get a brief period of similar noise in the left channel but it quietens down after a couple of seconds.  At first, the left channel would be quiet for about an hour, then the static/buzz would begin again intermittently. I didn't know if I might have a questionable solder joint, a failing tube or something totally different, so I decided to wait a few days to see if anything changed. Its been a week and now the static starts almost immediately, but is still intermittent and only in the left channel.

Any suggestions on how to debug this? If I try swapping out a tube, should I start with the 12au7?

Thanks.

 ---todd

[Laudanum]

I'd start with the tubes.  I had the same thing with a bad  6AS7, left channel.  You could actually see a white arc between the plates during the static/buzz.  But a tube can be bad without seeing anything wrong.  It could be either tube.  Always a good idea to have a spare or two of both the 12AU7 and the 6080 (or 6AS7).  Even if they are not your tubes of choice.  You can pick up new production 12AU7's for 10 or 15 bucks.  Some of the less popular NOS 12AU7's can be found on ebay for just a few dollars.  6080's and 6AS7's arent expensive either.  In the meantime, you can try the chopstick test which has been mentioned here by the experts.   Turn the amp upside down and use a wooden (non-conductive) chopstick or a piece of small diameter wooden dowel rod and lightly push/tap on the solder joints while listening to the amp (keep the volume on the low side). Be careful not to bash in the resistors or led's or other delicate components.   I have a feeling it's one of the tubes but thats not a very scientific nor expert opinion on my part plus, the chopstick test cant hurt.  If you want quick access to some spare tubes, try the larger, local musical intrument retailers.  12AU7 and 6AS7 (6080) are pretty common in tubes guitart amplifiers.

[twm]

It looks like I had a bad tube.  I put in a JJ 12au7 and so far so good. No more noise and it markedly improved the sound immediately.

Thanks for the help!

 ---todd

[Laudanum]

Easy fix, happens frequently enough with tubes so you gotta have some spares.  Good to hear you got it sorted out.

[Grainger49]

twm,

Now try some tube rolling.  I went wild buying all kinds of 12AU7s and found some that were really cheap.  I think I had a pair of Toshibas that sounded nice and cost next to nothing.  Don't be afraid to try odd tubes.  The good ones are well documented here on the board.  A favorite to me is the Sylvania JAN 6189WA.  I posted about tube equivalents for FP 1 & 2 but it is buried since there has been no responses.  All the tube numbers should work in Crack as well.  

http://www.bottlehead.com/smf/index.php/topic,1027.0.html

[twm]

Thanks for the suggestion.  Now that I've debugged and resolved the issue with the static, I'm ready to start tube rolling. 

I'm amazed how much of a difference swapping out the tube made. When I finished my Crack, I really wasn't very impressed with the sound, but with a better 12au7 and higher impedance cans, I'm really liking what I hear now. Good stuff.

 ---todd