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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
elizabeth
6,726 posts
 
An aside: odd how hard it is to pin down a minor issue in a new product. And how even tiny internal parts variations can have a profound effect.
I had a Phono box, and got a second one. They were supposed to be identical, in fact they had sequential serial numbers! yet one (the 2nd one) sounded better than the first. So much so i returned the first one for a restocking fee!! As i would always reguard it as inferior. (even thought nothing was really wrong with it.)
mlsstl
694 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.
aspens
10 posts
 
Ask a tech to evaluate the MOSFETs. There are two, strapped onto heat sinks, in the middle of the circuit board.

I had my PV15 into CJ twice (in the ’Ed’ years) and they could not find the issue. Said it tested perfectly. Yet the unit emitted constant low rustle like noises from the left channel as clear as a bell.

Finally had a local guy figure it out. He knew just by the noise behavior what it was before cracking open the top panel. Problem, current punches through the MOSFETs. He put in one that are rated higher. The PV15 sounds awesome now. Can't believe I waited so long to sort it out.
erik_squires
5,268 posts
 
The PV10 has a single power supply though, not sure about the classic. The odd part is that it's only happening on one channel.

Later PV models had split power supplies per channel.
erik_squires
5,268 posts
 
Wait, swapping tubes works? Why don't you just swap across sets?? :)

Keep the good one and swap out the noisy one. Am I missing something?
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