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My Tubed Amps Entered "Twilight Zone"

Last week, My Conrad-Johnson tubed PV5 preamp and MV50 power amp started to emit a loud hissing sound through both speakers about 10 seconds after turning on. So loud, that I had to shut them down. I checked all the tubes and they all were lighting up. MV50 was re-tubed 2 months ago by CJ. I then hooked up the PV5 to my Bryston 3B-ST power amp. The sound was perfect, with no hiss at all. I then tried my Bryston B-60P integrated as a preamp with the CJ MV50 power amp. No hiss and the sound was great. I re-connected the PV5 to the MV50 and used different interconnects and the loud hiss returned. Very frustrating experience. I called CJ service department. Knut suggested that one or both power supplies of the PV5 or MV50 needs replaced. If the power supplies are bad, why do they play well with the Bryston SS amps? The loud hissing occours only when the CJ equipment is used together? Any suggestions of what is going on here? Thanks, Pete.
prlscs
6 responses Add your response
weiserb's avatar
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weiserb
476 posts
 
Send in the amp first.
basement
245 posts
 
My guitar amp did this before and it actually turned out to be a bad tube. While I can't explain it, I would think it would be worth it to try some different tubes before you go through the hassle of sending it in.
albertporter's avatar
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albertporter
4,949 posts
 
The sound you describe is something I have come to associate with Tesla E 34L power output tubes.

Although I like Tesla's, if your CJ uses EL34's and the factory choose that tube, I would look to that as a possible source of noise. One amplifier that had this identical problem "repaired itself" after additional hours of burn in and re-biassing the circuit.
zaikesman
3,017 posts
 
Very strange indeed. I would go back and hook up each C-J component to each Bryston component again, and run them that way for a few days to see if the problem eventually crops up when the PV-5 and MV-50 are isolated from one another or not. (About AP's suggestion, I am pretty sure C-J is using Sovtek/E-H EL-34's; I take it that your output tubes are biasing properly?) If the problem never shows up when using these in combination with the SS components, but comes back as soon as you reconnect the C-J's, then you might want to try subbing out some spare tubes (if you have them) in the preamp's line stage sockets (the fact that the problem apppears in both channels makes it unlikely that this a faulty tube problem in the amp).
prlscs OP
40 posts
 
I want to thank everybody for the suggestions to help me with the loud hissing problem. Today, I began to play with tubes in both amps. I readjusted the bias on power amp, removed and reinstalled all of the tubes. Then I began substituting a NOS RCA 5751 in the line stage of PV5 preamp. One of the 5751's was very "noisy", causing the hissing sound. I bought this preamp in February and never replaced any tubes, until today. I still wonder why the hiss disappeared when connected to Bryston 3B-ST power amp? Thanks again, Pete.
zaikesman
3,017 posts
 
It could have just been an intermitent problem that didn't show up the first time you connected the C-J pre to the Bryston, or the different input load seen by the cathode follower tube when swapping amps could, I suppose, have caused it to behave a little differently. Welcome to the wonderful world of preamp tubes, separating the merely committed from the truly insane!
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