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
I had a CJ PV10 preamp that developed noise after a number of years of use. It turned out to be a bad capacitor in the power supply.

It your case it sounds tube related, but it is possible that a circuit voltage is off and may be stressing tubes faster than normal. If that is the case, only a trip to a qualified shop is going to be able to find and fix the problem.
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.
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.
Sorry "to fast", typing in a hurry.
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.)
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.
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.
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.
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?
I have had a similar noise emanating from either the left or right channel when a tube begins to act up. But this noise is more of a rush, less of a rustling cracking noise with occasional ticks and pops. although both are about the same low volume. Of course, this noise is only heard when no music is playing. If you swap tubes, however, and the noise moves to the other channel, the problem is in the tube. Then you put in NOS tubes.

Yet, if you swap tubes and the noise remains in the same one channel, the problem is in a component(s) on that channel in the circuit board. If your tech pulls the tube, and the noise is still there on his oscilloscope, look elsewhere, down stream of the amplifying tube. If, as in my case, the C1 upgrade was performed, a capacitor or two on the offending channel can be pulled to pin point the offending component part.

In my case, an issue that I asked CJ twice to sort out and they did/could not was found by a local tech with considerable experience, particularly with studio and music equipment. The problem, one I endured for 10 years, on this PV15 was in one MOSFET, where there are two, one for each channel. I don’t know the technical stuff, but he said that MOSFETs can be damaged by current and this almost always will compromise the part.

In my case, he showed me exactly what was going amiss. Once compromised, the channel will always be noisy, even if the unit specs out to CJ unit values. Typically, when a capacitor is bad, it has noises such as this, but often after the component warms up, it will pop, then play fine for the listening session. If that capacitor is in the power supply, maybe that is not the case.

The tech I used replaced the two MOSFETs with matched pairs (and he ordered 8 to sort them), and the PV15 works brilliantly. But that was my problem, not a capacitor issue in the power supply. The MOSFETs he installed are 400v MOSFETs. One thing to keep in mind, I’ve been led to believe that MOSFETs are notorious for being noisy at high frequencies, about 25% of stock. So some care is required to ensure two good ones are employed.

I must of spent four hours on several separate occasions on the internet searching "Conrad Johnson noisy channel" and spoke to a number of techs, including CJ (several times) about this. Before the local tech fixed the issue, I wrote to CJ for the third time ( including having them physically look at the machine twice). Their current service manager told me he could fix the machine, and he rightly knew it was not a capacitor issue. Then he dropped a bomb: the minimum charge was three hours, $396. To me the message was clear enough, "We really don’t want to help you and we are not going to stand behind the earlier CJ work or promises." I hate to be so harsh but that was one of the most disappointing emails CJ could have sent me, particularly since I own other CJ components.

I got the PV15 fixed for $70, parts included. The unit works brilliantly now.

This write up is provided to detail my case for one reason only. To help other folks fix their machines and not waste the time I did. So, if other folks have a noisy channel on a CJ preamp/linestage of this vintage, and it is not a tube problem, look to the MOSFETs as one possible issue.
So as the PV series progressed they added regulator sections, for reliability and channel isolation.

The 10, having a single regulation pathway, would get noisy on both channels at the same time, but I believe by the 12 it was staggered, with one shared regulator, followed by two individual sections.

In either event, I strongly recommend heat sink fins on those little regulators to improve the lifespan. About $0.20 or something, super cheap. Use a tiny bit of grease and pop them on. 

Best,

E
Thanks! for sharing-  aspen & erik_squires