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Second failure of new amplifier

Two weeks ago I burned my new amplifier equipped with eight 6L6 tubes. I had been using the amp for one week before that and everything was OK. The failure occurred when I was switching the amp to the standby position and then to the "off" position (two small tubes and two central power tubes flared, so I switched it off and disconnected from the mains immediately). Switched on - no sound. I found out then that the failure was caused by one damaged 6L6 tube and one burnt fuse.

I replaced the fuse and one tube and everything was OK, until I decided to listen to the music today. I listened to it for 15 minutes, then the same problem occurred - when I was switching it to the standby position and then to the "off" position, there was a clapping sound, one 12AU7 tube ignited (flared strongly) and the tubes do not glow after switching on. I will explore this problem further when I come back from work.

When I was purchasing this amplifier (I would prefer not to disclose the manufacturer), my local dealer claimed that the amp had a "stabilized power supply design" or something like that. One of my audiophile friends dissuaded me from bying this amp, saying that 4 output tubes per channel may cause a problem. The dealer said "don't listen to him - he does not know what a "stabilized design" means."

I think I know now what is "stabilized design" - it's burning fuses and tubes every second day. My dealer is reluctant to take it back even for resale (I suggested that he took it for repair and sold it and only after that give me money), but he claims that the guarantee is void because I replaced the stock tubes (12AU7 and 6922) with my own (vintage)tubes and that I should have waited for a couple of months (while it was burning-in) and should have used it with stock tubes without replacing them for other tubes. He added that according to the European ISO standards, changing the tubes would void the warranty.

My questions are:

1. Does tube-rolling in a new amplifier always void the warranty?
2. Is it true that the auto-bias function (especially when the amp is powered with so many (8!) output tubes is unreliable as compared to manual bias adjustment (like my other amp - Cary SLI-80)?
3. Is it normal when a brand new amplifier behaves like this?

I will appreciate all comments, opinions and suggestions.
transl
cyclonicman
693 posts
 
Timrhu,

I don't think that a defective tube should destroy an amplifier. Even a brand new functioning tube will one day become lifeless and defective. At that point, will it destroy the circuit? I guess it is possible, but it never has happened in my experience. I don't think there are too many threads on this topic either, which would seem like this is a rather unique and unfortunate incident for Transl.
I still don't think he has done anything wrong, except that he purchased this amp from some loser!
transl OP
156 posts
 
I want to thank all guys for compassion and understanding. I haven't decided yet what to do - the dealer keeps (and will obviously continue to keep) silent, while the amp is working OK with the stock tubes, except that I installed the Mullard CV4110 tubes in the 6922 position (tomorrow I expect NOS TungSol 5881 tubes to arrive and I am not sure whether I will install them or not under the circumstances). I will be watching for the amp's behavior.
davidsss
43 posts
 
I have a new amp and have had tube failure. I am new to tubes an did not replace the tubes. These things can happen. But the service I expect is the service I'm getting - the importer came to my house, replaced the tube (and the other one in the pair for a match) and the fuse. Unfortunately it seems to have taken something else with it so the amp is in for repair. I understand that tubes which test fine can become defective, it's just bad luck. But you should expect the sort of service I'm getting - no questions asked, it's being fixed.

Just for information - the tube in my amp arced inside the tube and although I turned the amp off in a hurry it was dead after this (fuse blown). Now if a replacement tube is put in there is a big orange glow in the tube in the position the offending tube came from. Hope it's back soon though.

DS
swampwalker's avatar
swampwalker
5,138 posts
 
If it works fine with the stock tubes, then I do think that have to look elsewhere for the problem (i.e., something wrong with the NOS tubes (a short, etc). I can't imagine that the amp would work fine with new tubes and not NOS. I also agree with those who said that rolling in identical NOS tubes should not/cannot void the warranty. Tubes wear out and they also fail occassionall. They are a normal replacement item. This would be like a car manufacturer telling you your warranty on the entire car is void because you replaced the brake pads at Sears instead of the dealer.
transl OP
156 posts
 
Amp failure update:

Yesterday I received an octet of brand new NOS/NIB TungSol 5881 tubes (60s vintage). Trembling with fear, I inserted them in proper positions and listened for a couple of hours. Even in half an hour, the sound was fantastic. I switched the amp off without a problem.

Today in the morning I switched the amp again and very soon heard some noise (cracking sound) from the left speaker. Switched the amp off, let the tubes cool down and swapped the output tubes from left to right and vise versa (exchanged the channels). The problem did not appear again. Went to work. Will resume the listening in the evening. What could have caused the cracking sound? Gas burning inside the tube(s)?
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