[Syrah]

Hi All,

I bought two quartets of Gold Lion KT77s for my amps back in June 2010.  I had an issue with one amp a few months ago, where two of the tubes started to glow bright red, and seemed to be getting hotter and hotter.  So I turned off the amp, replaced the tubes with some extras, rebiased, and then it worked fine again.  Now this has happened to my other amp.

The amps are Dodd 120w monoblocks.

Is this a typical sign of tube wear?

Given that they are 3 1/2 years old, with normal listening (maybe 10-20 hrs/week) I wouldn't be surprised if it is.  I thought that typically tubes start to crackle, and then pop, when they are near death.  I didn't think they would just suddenly decide to overheat themselves.

Thanks.

[thorman]

I do not claim to be an expert but have been using tubes for 40 years and from my experience you are very lucky by being warned you need new tubes....Many of us have had bad luck by getting no warning and a tube blows and takes Amp/preamp parts with it. So in my mind now is the time to buy new tubes and move on...Toss the questionable tubes out so you don't try to re-use them . Usually when I buy a Quad of power tubes , I buy 5 .......This allows me to have a spare .......You never know !

I would say what you are experiencing is very typical with a little good luck thrown in...............Don't wait.....Change em..........

[FullRangeMan]

Multiple tubes, multiple problems, the solution is SET.

[JakeJ]

To answer the OPs question, yes, the glow is due to wear.  The tubes are not able to maintain bias causing the voltage to go out of spec which in turn allows too much current to pass through the plates so they glow red hot as a result.  Yup, it's very bad and I agree you are lucky.  It would be a good idea to figure out roughly when you reach a certain time limit, say 1500 hours, you can track the hours and replace before there is a catastrophic failure.  Know anyone with a tube tester?



Keep beatin' that drum, fullrangeman!

[Atlplasma]

Couldn't this be a capacitor issue (failure or cold solder joint)?

[Syrah]

Thanks Guys.  I don't have a tube tester or a friend with one.  It sounds like I should consider buying one.  Any thoughts on this one?

I'd like to be able to test my 6SN7s too, which does not seem possible with this.  But the price seems good for what it is and it will test my KT77s and my 12au7s.

http://www.tnt-audio.com/accessories/orange_valve_tester_e.html

[avahifi]

When a power tube starts glowing cherry red, you really do not need a tube tester.  That tube is done for.  Most likely an internal short caused by one of the tiny little metal elements in the tube failing from years of thermal cycles.

Tube glows red, bad tube, replace it.

Frank VA

[Syrah]

Thanks.  I was thinking more whether or not it would be useful to detect that it is degrading before they go nuclear on me.  One thing is for sure, I'm not warming up my tube amps any more while I'm out of the room, etc.

[avahifi]

A very good reason not to leave your equipment on all the time.  Bad things can go wrong when you are not looking and if you are not there to notice a small fault, it can cook itself into a major calamity given enough time.

Frank VA