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.
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."
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?
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.
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.
One thing that comes to mind is that sometimes when we clean or use contact enhancers on tube pins the material can migrate from one pin to another via the tube socket surface causing an intermittent or dead short. So better safe than sorry, I always clean the surface of the tube socket to prevent such an occurrence.of course, your problem might be something else entirely.
where did you get the tubes from... there is a lot of tube label swapping going on.. I brought a pair of mallard from a guy off the internet, and was told he was a reputable dealer, my cj amp did the same thing and stopped working. talked to my friend andy at (didn't have them in at the time) and another tube guy and they explained this to me. had to have mind fixed....
The guy you bought your Mallards from was probably a quack.
Lol. Nice one Viridian.
Viridian gets the "chuckle of the day" award :-)
If your Mallards blow up ... be sure to duck.
But did he duck his responsibility?
Geoff must you persist in forwarding that tired old canard?
wow, what responces......what was the joke.............
This has turned into a fowl thread.
Geez, somebody's feathers are getting ruffled.
It sure did go south quickly, huh?
This has turned into a fowl thread.
Nah. IMO the thread is just ducky.

-- Al :-)
Sounds like there might be bill if anything needs repairing (sorry, had to go there).
Retension the tube sockets...that is it based on your comments. I had the very same thing happen to me and it was this exact problem and fix.
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.
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 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
I would put one EH tube in one channel and a 7308 in the other. If you have problems first switch sides and then sub the other 7308 for the one in the amp, then again switch the tubes side for side. This will tell you if the issue lies with one channel, one of the 7308s or one of the 7308s when used in a specific channel. Get going; right now you're just playing Ketchup.
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
They were for the most part used in Tektronix oscilloscopes and they will be noisy and often have grid leakage.

In addition you mentioned that they sounded cold, hard and bright. That's exactly what happens if they are worn out.

That tube is wonderful, but the LS-7 doesn't have a lot of air on top. A properly tested and operating Amperex 7308 is glorious, but the top end is relaxed just a teeny bit. So you have to be careful about system matching.

Another tube that would work well would be a Holland made 6922. Incredible tube but more at the tippy-top compared to an Amperex 7308. These may be branded Philips or even Mullard. They are kind of expensive. If you want the same sound for less bucks get the ones made in India branded BEL E88CC in the red box. Same guts. I mean exact same. Sound about the same in my listening. But half the price.
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.
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!).
sounds like a bad tube with electrical arcing, had the same happen with an AT7 and a power tube. I could see the arcing
when watching at the base of the tube