2. amps-preamps
  3. 1431834468

Weird problem. Bad tube or something else?

I just bought a pair of tested new Amperex 7308 tubes for my Audio Research LS-7 preamp and put them in two days ago. These replaced Electro Harmonix tubes in the V1 and V2 positions. The sound was slightly crackly/breaking up, so I reseated the tubes. The crackling went away, but the sound was extremely cold, hard, bright, and the soundstage lacked depth. I was not happy. I figured it might be from the tubes being shipped and not used in who knows how long, so I let the system play for 12 hours straight. No change. About 26 hours later, things started improving. That night, I turned the system off and went to bed. The next day, I turned the system on (Accuphase T100 tuner, Audio Research LS-7 preamp, Classe CA 200 amplifier, Magnepan 3.6R) and I heard two to three loud cracks in the left channel. Immediately, the amplifier went into protection mode and shut down. I put a cheap pair of speakers in place of the Magnepans to rule them out and fired it up again. Same problem. A few loud cracks from the left channel and the amp went into protection mode.

Suspecting that the newly purchased Amperex 7308 tubes in the V1 and V2 positions might be the culprit, I replaced them with the pair of Electro Harmonix 6922s that I had been using before buying the 7308s. The system played fine. I put the 7308s back in and same problem. Cracking followed by amp shutting down. I then switched the 7308s' positions to see if the cracking would follow the tubes. Of course I could not duplicate the problem and the system played fine. Now not knowing what to do, I took the preamp out of the system and did a very thorough cleaning of the tube sockets and, while I was at it, the RCA jacks, with Caig Deoxit D100, isopropyl alcohol, canned air, the whole deal. I did see some oxidation on the toothpicks that I did the cleaning with. I re-cleaned until I saw no oxidation (no oxidation was seen during the third cleaning). This thing is probably cleaner now than new. I put the pre back into the system with the 7308 tubes and all was fine. I switched the 7308 tube positions left/right and it still played fine. I have not put the Magnepans back in yet, so I don't know if the cold, hard, bright sound went away or if the soundstage depth is back to normal.

Could this be a tube problem or could it have been a tube socked oxidation issue? What else could cause something like this (cracking followed by amp shutting down). I was expecting to be blown away by the sound with the vintage 7308s, but I was seriously let down. Hopefully it was an oxidation problem and I will be blown away when I put the Magnepans back into the system.
ketchup

Showing 6 responses by ketchup

ketchup OP
459 posts
 
Yes, I cleaned the 7308 pins before putting them in for the first time. I should have mentioned that.

I just measured the pins on the 7308s and the 6922s. All were about 0.038" - 0.039" in diameter. The pins on the 7308s were more consistent in diameter and the pins on the 6299s were maybe 0.0005" smaller in diameter than the 7308 pins. I say maybe because the digital calipers I used are only accurate to 0.0005" and it depends where on the pin you measure (middle, top, bottom). So, for all intents and purposes, you could say that the 7308 and 6922 pins are the same diameter.

Hopefully it was a poor connection problem. Could that cause an amplifier to go into protection mode?
ketchup OP
459 posts
 
Viridian, I thought about retensioning the sockets, but didn't. Maybe I will get back in there and do it. I'm not really sure if the problem is solved yet. Some more usage to make sure the amplifier does not shut down and finally hooking the Maggies back up to see how it sounds will be the true test. I can not tell how anything sounds through these cheap JBL bookshelfs placed on the floor!

By the way, my 7308s are the gold pin versions, too.
ketchup OP
459 posts
 
Lol. Nice one Viridian.
ketchup OP
459 posts
 
It sure did go south quickly, huh?
ketchup OP
459 posts
 
Update- I hooked up the Maggies last night and played the system for about 3 hours. It sounded great. The cold, hard, brightness was gone and the soundstage was deep and wide. The system was turned off last night. When I came home from work today, I turned the system on. As soon as the preamp's mute circuit turned off, I heard less than one second of music followed by a snap and the amplifier going into protection mode. This was with the 7308 tubes back in the pre. I put the EH 6922s back in and guess what- no cracking, no amp shutting down. Everything seems fine. To recap, I have never experienced the problem with the EH 6922 tubes, just with the 7308s.

I guess it's time to retension the tube sockets as I have no other idea what to do. It seems strange. If the sockets are the problem, why doesn't it occur with the EH 6922s in the pre? Perhaps it's a slight variation in pin position between the tubes. Totally guessing here.

If anyone has any other ideas I would obviously love to hear them.
ketchup OP
459 posts
 
I just wanted to say thanks to everyone who helped out with this problem. I retensioned the tube sockets which were definitely in need of help. The tubes are now very hard to push in and pull out. I would say that it takes about 2-3 times more force to insert or remove the tubes now, which is good. I'm sure all pins are making good contact with the sockets. Unfortunately, the problem with the 7308 tubes did not go away. I was able to trace he problem to one of the tubes as the snapping sound would move from the left to right speaker depending on which one of the 7308s were in the V1 and V2 positions. I have been playing the system regularly for many hours a day for the past few weeks with the EH tubes in the preamp and it works flawlessly. The seller guarantees his tubes for whatever reason, so luckily I was able to send them back for a refund. Thanks again for all the help, especially the experts here (you know who you are!).
More to discover