Opinions Wanted: Tube Power Amplifier Reliability

WLGMuzza

Well-Known Member
Oct 21, 2019
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Hi all

Seeking some advice please. Please note, I’m not looking for any technical diagnosis / troubleshooting. Before I take any next steps I’m canvassing opinion on expected ‘reliability’ of one element of my system.

I have a very well known, high end power amplifier. It is a stereo 12x tube unit using 6550s or KT88s. The amplifier is self-biasing with an expected tube life of 3,000 hours.

The amplifier is coming to the end of its warranty period (this has been ongoing for the past 4x years) and I’m trying to understand whether the behavior described below is typical.
  • 7 of 12 x 6550 factory supplied tubes were replaced before 1900 hours, sometimes in service and sometimes on power up.
  • Replaced with a full set of aftermarket KT88s from The Tube Store. Continued to have problems but nothing worse than the above.
  • Went back to the factory who said they would not look at any potential issue with the amplifier until the aftermarket tubes were replaced with ones they had supplied.
  • Replaced the aftermarket tubes and purchased 4x extras at 220% premium over matched, tested versions from The Tube Store.
  • 3x new factory tubes have failed before 900 hours
The local retailer has all the details of my system and I have provided dumps of the error logs as requested. The local retailer and the factory are not suggesting any issues with my ancillaries, power etc. so I'm not looking for diagnostic advice.

The bottom line is the factory has said the amplifier is not at fault and the failures are ’tube variability’. They will not look at the issue any further.

Should suck it up, or should I take this further?

Thanks for your valuable input.
 

Ron Resnick

Site Co-Owner, Administrator
Jan 25, 2015
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This amp could only be VTL Siegfried II.

Did any of the three tubes that failed before 900 hours do any collateral damage to the amplifier (like taking out a related resistor or capacitor or melting a fuse socket)?
 
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facten

Well-Known Member
Feb 13, 2022
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The amplifier is coming to the end of its warranty period (this has been ongoing for the past 4x years) and I’m trying to understand whether the behavior described below is typical

I have 3 tube integrated amps in use in 2 systems. I've never had the issues that you have described, so at least in my case it is not typical. And, I have luckily only had 1 power tube fail , a Psvane KT88, and it was after 100 hours. If it were me I'd try to force the issue with the manufacturer; somewhat hard for me to believe that so many of the manufacturer supplied tubes as well as those you purchased from TheTubeStore failed in and off themselves
 

Sablon Audio

Industry Expert, VIP Donor
May 22, 2015
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Your experience appears to go beyond tube variability, not least of all given that they are current production rather than a crap shoot of ‘nos’ examples. Have you checked / monitored your wall voltage to rule out the possibility of it running ‘hot’?
 

WLGMuzza

Well-Known Member
Oct 21, 2019
21
12
68
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This amp could only be VTL Siegfried II.

Did any of the three tubes that failed before 900 hours do any collateral damage to the amplifier (like taking out a related resistor or capacitor or melting a fuse socket)?
Hi Ron, no, the amplifier shut down, as expected, before any damage. That function is working well .
 

WLGMuzza

Well-Known Member
Oct 21, 2019
21
12
68
59
Your experience appears to go beyond tube variability, not least of all given that they are current production rather than a crap shoot of ‘nos’ examples. Have you checked / monitored your wall voltage to rule out the possibility of it running ‘hot’?
Hi Sablon, input AC was monitored over the period of a month at different tes of the day and is 230V +/- 7V. The factory do not believe there is an issue with that.
 
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Ron Resnick

Site Co-Owner, Administrator
Jan 25, 2015
11,751
7,373
1,465
Beverly Hills, CA
Hi Ron, no, the amplifier shut down, as expected, before any damage. That function is working well .

Thank you. I do not know if there is any sonic penalty for the VTL protection circuit in the Siegfried II, but VTL truly has solved the traditional high-power tube amp problem of a dying tube frying related circuitry or melting a fuse holder. I literally have never heard of a Siegfried II needing to be sent back to the factory because a tube failed.
 

WLGMuzza

Well-Known Member
Oct 21, 2019
21
12
68
59
I won't confirm or deny the manufacturer at this stage, but a fourth tube has just gone at 1067 hours, on power up.

This consumption rate means, as with the first set of 12, all 12x new tubes I purchased late December 21 will likely be replaced prior to the 3,000 expected life span.

Another question; the local distributor recommends swapping out tubes in pairs when one fails as the new tube is far stronger than the old one. Apparently this causes an imbalance across the pair, therefore is likely to cause the pre-existing tube in the original pair to also fail prematurely.

Is this an issue with autobias amplifiers or is it BS?

I appreciate all input as I assess my next steps to get an acceptable outcome.

Thanks
 

mtemur

Well-Known Member
Mar 26, 2019
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Another question; the local distributor recommends swapping out tubes in pairs when one fails as the new tube is far stronger than the old one.
I think local dealer is right. Even if the amplifier has auto bias you should replace tubes in pairs.

Normally tube amplifiers are very reliable. For example I served on board of an old sub for 6 years which was full of equipment with tube electronics. For general announcement and paging (1MC and 7MC) a two channel tube amplifier was used. Two 807 output tubes were used if I’m not mistaken and it was always on, 24/7 for 365 days. I don’t remember ever those tubes failed.

IMHO it is better to consider changing the amplifier with another, less powerful one. I believe a tube amplifier performs better when designed as;
- SET
- push pull with only two tubes per channel
- without using 6550, KT88, KT90, KT120 etc kind of tubes
- without going a couple of hundred watts level.

Any amplifier meets those doesn’t mean it’s a good amplifier. For example wavac 833 is a SET amplifier but even the four chassis version doesn’t sounds good IME.
 
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