Power Tube Specs, performance and longevity

[retired farmer]

Did you like them at least? I bought a quad of sohpia electric kt88s a month ago I can tell you nothing about life span
But they are voiced nicely for my melody amp. They call them long life what ever that is supposed to mean.

A set of EH KT88's, a set of EH 6550's, and a set of Preferred Series KT88's.

 

[Ron Resnick]

A set of EH KT88's, a set of EH 6550's, and a set of Preferred Series KT88's.

What are your sonic impressions of the EH KT88s versus the Tube Store Preferred KT88s?

 

[Ron Resnick]

Is anyone familiar with the JJ KT88s?

 

[AMR / iFi audio]

I guess that's the rub, I don't have those details, instead relying on the tubes tested and certified by the manufacturer to reach their claimed average life.

I paid over double the price for those tubes compared to a set from an independent retailer. The average life is less than 1900 hours, not close to 3,000 hours, compared to the 'non-authorised' version at 1400 hours.

Do EH publish tube life figures? I suspect not given the variable operating parameters across different applications.

The EH datasheet I have found doesn't provide info on longevity.

 

[WLGMuzza]

Did you like them at least? I bought a quad of sohpia electric kt88s a month ago I can tell you nothing about life span
But they are voiced nicely for my melody amp. They call them long life what ever that is supposed to mean.

I prefer the KT88s to the 6550, punchier, better bass, not quite as holographic though.

 

[WLGMuzza]

What are your sonic impressions of the EH KT88s versus the Tube Store Preferred KT88s?

It was some months between last listening to the Preferred Series , due to the high failure rate eating my spares, and the manufacturer supplied EHs going in. The best I can answer is that I found a greater difference between the 6550s and the Preferred Series KT88'S than those to the EH version.

 

[MarcelloCreato]

Very interesting discussion. I have been in dispute with the local agent for a very well known manufacturer of high end tube amplifiers. The estimated average tube life for my power amplifier is 3,000 hours on KT88 tubes due to some fancy power on sequencing and autobias.

I've had 3x sets of tubes, two supplied by the ampliifer manufacturer, and one set supplied and tested by a reputable tube dealer.

None have gotten substantively close to the advertised, and statement in the manual, 3,000 hours, with the first set that came with the amplifier only making it to 1900 hours before total replacement.

The reason given by the manufacturer is 'tube variability despite selling me 2x sets of factory tested and burnt-in items. It was also suggested to me that the latest tubes (those manufacturered in the past 2-3 years) are less reliable than those produced in the prior 5-10 years.

Let the buyer beware......

YMMV

Cheers

Following this discussion with interest. My question to WLGMuzza would be, looking at his phrase "the first set that came with the amplifier only making it to 1900 hours before total replacement." : what is your trigger to say that your tubes need replacement? Do they stop working? Or do you test them periodically and when are they out of spec you say "OK end of life reached", or do they just sound bad, or don't they reach their bias point anymore...or...?
As far as tube life goes, there are several mechanisms that determine tube life, one of which is the already mentioned plate dissipation/ heat production in Watts, calculated as the product of Plate-Cathode voltage and Plate Current. But theres more, like are the screen grids glowing? Very often a cause of failure in the long run. Also cathode flaking, and also cathode treatment in the factory: like how good are the oxides bonded to the cathode base material? And some more, like vacuum quality, plate material, quality of manufacture especially the spot welding of grid wires to the stands.... Each weak point responding to another form of startup- or operating conditions.

 

[WLGMuzza]

Following this discussion with interest. My question to WLGMuzza would be, looking at his phrase "the first set that came with the amplifier only making it to 1900 hours before total replacement." : what is your trigger to say that your tubes need replacement? Do they stop working? Or do you test them periodically and when are they out of spec you say "OK end of life reached", or do they just sound bad, or don't they reach their bias point anymore...or...?
As far as tube life goes, there are several mechanisms that determine tube life, one of which is the already mentioned plate dissipation/ heat production in Watts, calculated as the product of Plate-Cathode voltage and Plate Current. But theres more, like are the screen grids glowing? Very often a cause of failure in the long run. Also cathode flaking, and also cathode treatment in the factory: like how good are the oxides bonded to the cathode base material? And some more, like vacuum quality, plate material, quality of manufacture especially the spot welding of grid wires to the stands.... Each weak point responding to another form of startup- or operating conditions.

The amplifier has built-in diagnostics. If a tube experiences an issue the amplifier shuts down and indicates which tube should be replaced.

After 9x failures at 1900 hours I bit the bullet and replaced all 12 hoping to get better reliability. Wasn't the case.

 

[MarcelloCreato]

WLGMUzza Can I send you a PM? To exchange info on brand/model of the amp. Maybe I can help.
For sending PM I suppose our email addresses should be known by the system. Dunno how that works on this forum.
Are you sure that the diagnostics system is all OK? Maybe it is a bit more 'triggerhappy' than the manufacturer intended for some reason.
Mostly such diagnostics systems are plate or cathode current monitors that trigger when this current exceeds a predetermined limit.
Does not have to mean immediately that all tubes have worn out, on the contrary, most of the time in my experience it's a sudden issue in one tube for whatever reason, and the rest of the tubes can be still perfectly OK.