Cautionary tale about KT 88 tubes

[PeterK]

As many of you know I have an Airtight ATM 2 stereo amplifier bought second hand ( thanks Dave F). For a number of years I have been running it successfully on the tubes supplied by Airtight originally but eventually they failed. I replaced with Electro Harmonix KT 88 tubes and just this summer one went taking out one side of the amp doing a lot of damage. I got the amp repaired 2 months ago and put in matched pairs of Gold Lion KT 88. Last week one of these failed doing more damage I suspect. Another inspection and repair job lined up next week.

However, I noticed on the official Airtight website that they indicated that from September 2017 they will no longer be using KT 88 tubes due to their inability to source reliable KT 88 tubes and will be using 6550 tubes instead. Seem like they are on something there from my experience. Any thoughts on this issue?

It has been incredibly frustrating and quite costly! Any recommendations re 6550 tubes?

On the plus side Noel and Ivan in Cloney have been extremely helpful in helping me diagnose what had gone initially, the amp or the Quad speakers.These guys show why we need to support bricks and mortar retailers. Firstly they are gentlemen, and secondly they help customers way beyond what one could expect. Kudos to them and long may they thrive.

[Fran]

Theres a fair bit of stuff out there about bad octal tubes, but it seems hard to differentiate if its batch/factory/country/tube based. All I could suggest is to use a reputable supplier and the guy I always recommend is a fella called Jim McShane in the US. He knows hifi applications, specialises in tubes new and old and won't set you wrong.

http://www.mcshanedesign.net/

Last dealings with him - meself and Ivor went halvers on 2 sets of EL34s from him but have dealt with him a few times over the years.

Fran

[Diapason]

I know Steve always* seemed to be killing KT88s in his Jadis amps, but that all changed when he moved to KT120s. The sound also moved up several notches, so perhaps you could see if that's an option in your amp?

*Not really, but I know he went through a few over several years.

[Maciej]

Personally I had no issues with Sovtek KT88 so far. I’ve been using them for over 2 years.
As Fran said reputable seller such as Upscaleaudio is the key to get decent tubes.
I found KT88 sound more dynamic with better extension on both ends vs 6550 for example Svetlana 6550 winged c

[Ivor]

I too have had Gold LIon KT88s do damage to an Airtight ATM1 but then again I've had EL34s give a fireworks display as well. I recently got a set of 6CA7 valves - essentially EL34s from www.watfordvalves.com and I went for Electro Harmonix as they were the original brand supplied by Airtight - that said the original valves were superduper Electro Harmonix/Airtight yokes and the recent w=ones were a very reasonable £85 delivered for a matched quad set. They sound lovely too.

Thankfully Airtight is a relatively simple design and the damage could be repaired - not by me I hasten to add, it's beyond my competence and confidence.

[tony]

Touch wood I used this company in the states for cryoed Gold Lion KT88's and no probs so far.
Wonder with the large Jadis amps is it the sheer number of valves that can lead to problems? Doesn't explain the issues wit the Airtight Amps.

What exactly did it take out in the amps? I was lucky it seems with my Prima Luna amp. Something happened but Fran identified it was just a resistor that had gone. Can't understand how they do not have protection built in to prevent big damage being done if you lose a valve.

[Ivor]

Doesn't explain the issues wit the Airtight Amps.

What exactly did it take out in the amps?

As I said above Airtight are pretty straightforward... no PCBs or fancy wiring - all point to point.
My first disaster happened when I left the amp to warm up and noticed that feared burning smell as I passed the cave door. I rushed to turn off the amp but a rogue valve had started burning into "what lies beneath" and the smoke signals weren't good. Thankfully it was just some wires and maybe a resistor - it could have been far worse.
Second time it happened I can blame exterior forces! A picture had fallen from the wall above the amp and while I though it had done minimal damage, just a small nick on the chassis, it could well have hit a valve. Thankfully I was sitting there when the valve started to glow like no valve should glow. It had been on for about an hour so I'm guessing it had just reached full temp. I took it out and replaced it - just the dodgy one. While the amp continued to play fine the bias reading for that valve socket was zero so I didn't risk playing anymore. Again it was just a few wires with burnt insulation and a blown resistor.

[tony]

https://www.cryoset.com/ Sorry forgot to link the site.
Yes Ivor your travails sound similar to mine with the PrimaLuna in the first instance, your second issue was just part of house ownership! Things just fall off the wall for no unearthly reason except to cause consternation among us innocents. Both cases the just relatively small intervention required to get the amps up and running again.

Peter's seem to be more serious. Just wondering what it took out in his case.

[Rob]

Although the 6550 and the KT-88 are not quite the same tube (35 watts plate dissipation versus normatively 40 watts, and a 15kohm plate resistance versus 12 kohms), the tubes are pretty close characteristically, which may lead some manufacturers to issue one design under different labels, so would be useful to check feedback about a given model online.

Not too familiar with Airtight amps but word is that higher power tube amps, especially of the 80s/90s vintage, generally tend to push the tubes (typically KT88s/6550s) fairly hard, and sometimes the screen grids would collapse, especially in ultra-linear mode (think the Airtights tend to use that mode) because it typically places the same strength of signal at the screen as at the anode. It can be the same with pseudo-triode mode where the anode and screen are tied together. The EL34 got a rep for unreliability for similar reasons - some manufacturers would use premium NOS valves for testing, and would drive them hard safely but the late period equivalents weren't nearly as good, which was understandable since NOS tubes cost a high amount of money in their day and tube manufacturers had the pick of premium materials. Maybe the KT-150 would be a better choice since it is rated for somewhat tougher operation?

[sdiesel77]

I've tried few KT88 brands (Electro Harmonix, Gold Lion, Shuguang GEKT88, Tesla, Psvane Treasure).
From my own personal experience, I find the Gold Lion and Tesla to be unreliable. One Gold Lion burnt suddenly one day which also burnt an amp resistor.
The EH reliability is a bit better than the last two brands and sounds ok. The Shuguang GEKT88 reliability is ok and sounds quite good.
The Psvane Treasure are reliable and sounds really good.

I'm also thinking of now trying the KT120