2. amps-preamps
  3. 1301704400

Conrad Johnson Classic noisy channel.

I have owned this from new and upgraded it to se.Before and after the mod,the left channel tube rush becomes noisy after 3 months use.I am a moderate user,approx 3-4 hours use per day and do not leave the pre on when not in use.I have used 3 sets of mullard m8080/cv4058 tubes with the same result.Swapping tubes from left to right,noise swaps to right channel.Have had pre checked and all tube voltages are within spec.Are there any other classic users with this problem.Dave.
dre
dre OP
6 posts
 
Thank's for the reply.I have contacted cj and pre has been looked at by a qualified tube tech.Noise is tube related but cause of problem cannot be found.Tube plate voltage 150 volt + range recommended,4 x measured tubes 163/159/160/156 volts.Cathodes are 5.43 volts.I would send it to cj but as I live in New Zealand this would be a problem.
detredwings
162 posts
 
Hi Dre,

I would take a scope and look at the voltages again and see if the left side has some garbage "AC" on it? It could be a bad cap not filtering out the garbage. Your voltages seem okay. If there was excessive current draw on the left side the voltage reading would reflect that. ( E = I*R ) If everything is clean on both sides maybe you could try a U.S. 6C4 military tube. It should be a little more rugged. Ask CJ to make sure it's okay to use them first. Three months is took fast for any preamp tube to go bad. Being that's it's always the left side something fishy.
detredwings
162 posts
 
Sorry "to fast", typing in a hurry.
Post removed 
mlsstl
1,409 posts
 
An aside: odd how hard it is to pin down a minor issue in a new product.
I don't think small signal tube failure at 3 or 4 months is a "minor" issue. Doing some math based on the info given, the tubes in the one channel are going bad after 400 or 500 hours. The normal expectation for such tubes would easily be 10,000 hours and generally much more. His preamp tubes aren't even lasting a quarter of the expected life for a power output tube.

However, I suspect that in the hands of a good tech with proper equipment, the underlying problem would quickly reveal itself. However, it is not a problem easily fixed by the unaided consumer.
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