2. amps-preamps
  3. 1204287063

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
rcprince
3,062 posts
 
Undertow: While your analysis would normally be correct on the above facts, the poster had an earlier thread where he said that he had used the amp for a week with no problems with the stock tubes, then decided to tube-roll and started having the problems after he did that--the amp failed only after he put in the NOS tubes. Hence my advice, which was based on knowing about that prior thread. And for those of you who are interested, you can get the name of the manufacturer from that thread too.
undertow
1,033 posts
 
Rcprince
, No I think I read the STOCK tubes welded themselves in place he was able to actually remove them and replace the fuses etc... After that with the NOS... I might be wrong
transl OP
156 posts
 
OK. To resolve the second failure, I replaced the fuse and installed the stock pre-amp tubes. The amp sounds terrific, by the way (which makes me think that some modern amplifiers are optimized for modern tubes - if you replace them with NOS tubes, you sometimes hear "mushy" sound and a loss of focus)... I will see how it behaves with the stock pre-amp tubes.
mlsstl
1,424 posts
 
Transl, sounds like nothing more complicated than one of your NOS tubes was defective. That certainly can happen.

As such, I'd just run it stock for a while to make sure nothing else is going on. If it continues to work well, then you can continue with your tube rolling experiments.
tonyptony
615 posts
 
I suppose it's also possible that even like-type NOS tubes may have caused the problem even if they were good. But that would presume the amp had no autobias, or that it was not rebiased when the NOS tubes were put in, AND that a scenario like that would cause just such a problem. Hard to imagine for anything with a reasonable design, but if it's even likely that the amp would become so unstable with anything other than the manufacturer's specified tubes then I might reconsider owning that piece of equipment.
More to discover