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
mechans
2,999 posts
 
Did you clean the 7308s' pins as well. It sounds like your problem was poor connection between the tubes and your pre. Now that you've cleaned the sockets, do the same with the pins. The sockets were obviously oxidized. If the pins on the EH 6922s were fatter, it would help explain why they worked.
It is unlikely that the tubes were arcing or shorting since burning them in "worked."
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?
viridian's avatar
viridian
4,184 posts
 
I usually retension the sockets as well using a dental tool to gently pull out the metal receivers that make contact with the tube pins. Sounds like you have solved the problem.

It's all system dependent but IMHO the Amperex tubes should be a large improvement over the EH tubes. I am using the gold pin versions with great success.
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.
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