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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
gs5556
1,234 posts
 
Is it always the same tube socket? If it is, check the socket under a bright light. Look for burn marks, carbon deposits or mechanical damage in the pin holders and solder points. Use a magnifying glass as even a carbon coating could cause an arc short - and it does seem like there's a short somewhere. Could have been initially caused by a bad tube and replacements may suffer the same fate.

Rolling tubes may have an effect in causing tube failure if the amplifier has a fixed bias setting. Even if the tubes are compatible substitutes, there could be a slight change. But not enought to result in a short IMO.

Curious... why use the standby switch to power down? Is it recommended? Using the standby does not allow the tube B+ voltage and power supply caps to power down gradually. Also, the bias settings may be affected when you roll tubes which may cause the plate current to increase. These may be reasons why the first tube and fuse blew in the first place or just pushed it to premature failure. I'm just going by my experience with guitar amps - power on with standby but power down with just the power switch - but whatever your manual says overrides.
cyclonicman
693 posts
 
I think your dealer is full of it. I have never heard rolling tubes can void a warranty and if this particular amp will not allow for tube rolling than it should be noted in the warranty or this unscrupulous dealer should have stated it to you. Usually unauthorized modifications or abuse will void the warranty. If possible speak to another amp manufacturer to get an idea of their thoughts on tube rolling in their amps. I know my amp calls for 6550 tubes and I wanted to use KT88's. I called Conrad Johnson and although it wouldn't void my warranty, I was told that I would be unable to bias the KT88's, but that is a far cry from voiding a warranty. I also contacted Cary at one point because I wanted to replace the 6SN7 tubes in my SLP98 preamp with 5692 tubes. The Cary tech told me that it would be no problem and that I would probably like the 5692 tubes very much. This has to work out for you, as you have done nothing wrong. If you cannot get satisfaction from this dealer, then you should contact the better business bureau. Good Luck to you!!
transl OP
156 posts
 
Gs5556:
'Curious... why use the standby switch to power down?' - how can I avoid the standby switch? Shall I make two quick clicks - standby and then immediately to the "off" position?

By the way, the amp is working now. I replaced the fuse and installed the stock EH 6922 and 12AU7 preamplifier tubes. Sounds very nice! I will watch how it behaves with stock tubes. Afraid to roll tubes forewer (but not with the second amp - Cary SLI-80).
timrhu
2,779 posts
 
I gather you don't have much technical experience with tubes. You can get a tube data manual and find that any given tube
That made me chuckle as I began my life as an electronic technician back in the days of tubes. Used to have a tube tester in my garage. I fully understand the principle of suitable substitution.
That being said, I'll admit I don't know much about warranties for tube electronics. It still would surprise me if a manufacturer is obligated to replace a defective unit when the defect was caused by a faulty tube (not supplied by the manufacturer) being placed in the equipment by the new owner. Just an assumption as I haven't read the warranty agreement, I could be wrong.
If the product had a defective tube socket that wasn't apparent until the original tube was removed that's a different issue and Transl was fortunate to discover it. This needs to be discussed with the dealer.
mlsstl
1,436 posts
 
Timrhu, what would you say if Ford or GM voided your car warranty because you put in Bosch spark plugs instead of the Champions (or whatever) that it originally came with? Or limited your brands of gasoline to Exxon/Mobil only. ("I'm sorry, your warranty claim is denied since you used BP/Amoco gas.")

Another problem with that logic is the brand of tube used is often meaningless since many tube manufacturers are no longer in business even though the brand may still exist. Those tubes are being made elsewhere.

I have to say that in 40 years of involvement with the technical end of audio this is the first time I've ever heard someone trained in audio electronics claim a warranty was likely to be voided based on the brand of vacuum tube used. Interesting.
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