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
upscaleaudio's avatar
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upscaleaudio
213 posts
 
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.
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!).
davide256's avatar
davide256
362 posts
 
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
jackweston
4 posts
 
Buy yourself a set of  4  RCA brand 1960's vintage  7308 Siemens Halske triodes. They sound way better in this linestage that  7308 amperex tubes do.  I have tested them all including  NOS burned in  1960's  Amperex  6922's.  Make sure they are first tested for transconductance and shorts before you install them.  I have used a set of these in my  ARC LS 7 for 12 years and the sound is awesome. My LS 7 linestage has been modified  heavily with high grade caps and many other improvements.  This preamp has a stepped attenuator, TRT Steath coupling caps  ( 4 uf 425V) with Steath bypasses, zero recovery diodes, gold plated tube sockets,  replaced the two ( 2)  stock power supply caps with Panasonic low ESR electroylytics with 2.0uf  V Cap teflon bypass on the larger value cap, installed  IEC Furuttech socket, Sonicap teflon .001 uf bypass on power regulator, and chassis damping  ( Isodamp) . But even if all the mods were not performed the S & H  1960's  7308 triodes would still make a difference. They need some burn in time if they are NOS.   Would be good for you to purchase a tube tester so when you buy the tubes you know they are good right out the gate!   Regards,  Jack 
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