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.
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I'm thinking that running the 7308s in the V3 and V4 positions for some amount of time, including powering the preamp up a few times when it is not warmed up, might provide useful information.

Also, I see in the schematic for the LS-7 (at arcdb.ws) that V2 corresponds to the left channel, and V1 to the right channel. You had mentioned that symptoms originally appeared in the left channel, so it would seem that you should focus on the V2 socket and on the 7308 that you initially installed there.

It looks like one of the two triode sections in V3 and one of the two sections in V4 are used in one channel, BTW, and the other section of each of those tubes is used in the other channel.

Also, although I don't know how it might be related to the problem I'll mention that the design appears to use some amount of feedback from the output (from a point just before the muting relay and the 200 ohm series resistor that is at the output) to V1 or V2 for the corresponding channel. Perhaps that will be suggestive of something to someone else.

Good luck. Regards,
-- Al
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I think I would find someone locally to test the 7308 tubes.

It could be possible the LS7 left channel is driving the 7308 tube too hard. The EH tube is not affected because it is more rugged and can be driven harder.

LS7 schematic
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An intermittent problem like this will not turn up on a tube tester. It does appear though that one or more of the Mullards are at fault and should be returned if warranty is still available.