I am still glad I carry extra tubes !

[godofthunder]

At a out door gig out side of Buffalo NY last night, my Hiwatt 100 was acting up at sound check. Low output and a noisy decay. Pulled the back panel and the cause was obvious, a failed EL 34, the top had gone white and upon closer inspection the tube had cracked halfway around the diameter. I pulled the bad tube and put in a spare EL34 and was up and running in less than 15 min.

[Bionic-Joe]

Y'know...An amp causing problems like that...I would dump it. Say...I have an Idea...I would be willing to trade my .67 Project Non-Revers Thunderbird with ALL of the original parts, including 2 chrome pickups, Pots, Brass shielding plate bridge tailpiece, tuners, etc. for that Hiwatt.

[Lightyear]

If you're taking trades I have a deed to bridge we can talk about

[Highlander]

Scott... there are a few old DR103's knocking around over here... I can always do some searching for you if you go for that trade...

[Dave W]

Well, it's good that you had the tube, but chances are it was already gone, deteriorated over time. The chance that an EL34 was good and then just suddenly failed is very slim.

[Freuds_Cat]

  

[Pilgrim]

Very nice work - and extraordinary good luck that the tube failed so visibly.  They're usually not that obliging.

[godofthunder]

I don't believe it suddenly failed, certainley not at sound check. I run at least 2 heads each night and frankly by the end of the night I can't hear much of anything, for me problems always arise at the next gig during sound check. Joe if you would like to trade that amp for the bird you have a deal.

[Bionic-Joe]

You mean the Hiwatt? If it is a 100 watt DR103, you have a deal.

[Psycho Bass Guy]

The EL34, did in fact, suddenly fail. I say that with condfidence due to one detail:

the top had gone white.

The 'white' was caused by the chemical reaction of the air with the tube's getter and occurs immediately upon contact. Leaky tubes will glow flourescent in operation as the leaking air becomes ionized by the high voltage and will glow purple or orange.  It would have been pretty hard to miss seeing. The ionization process also keeps leaking air from reacting with the getter, so it will only turn white if the leak is severe and occurs while the tube is not in use.

The tube had a flaw in its envelope or took a whack, probably the former, as EL34's are one of the most popular power tube types and I've seen more than few that tested fine but had obvious physical flaws and it stands to reason that there would be others with not-so visible flaws. If the glass composition itself was not chemically correct, the simple process of heating and cooling during operation could have broken it.

I still don't reccomend swapping the dead tube with a replacement that has not been tested in the amp. It is quite possible to damage the amp with a tube that need more bias than the "old" ones. My advice would be to make sure that you have quality tubes rather than rely on the unknown factor of field replacements. It's always possible to have a tube fail, same for transistors. (As a matter of fact, I've had far more transistors fail in use than tubes.) Both should be serviced in the shop, not on a gig

[godofthunder]

Believe me I understand that this type of work should be carried out by someone qualified. After the gig the amp went straight into the shop. But being miles from any sort of tech help with a show to do I'll take the chance and make a diy fix.  The show must go on !

[Psycho Bass Guy]

My point being that to have a tube just shatter is extremely rare and more indicative of a badly built tube or mistreatment (I've seen a$$hole drunks in bars try to bust tubes in amps on and offstage just for kicks more than once.)  than something you're going to have to worry about in general. Carrying "backups" for a good set of tubes properly installed is far more likely to result in breaking an unused set of tubes in transit than it ever is to 'save the day.'

You experienced a type of tube failure that doesn't damage the amp. In that, you were extremely lucky. Had the glass broken like that under operation, you could have burned out your power transformer, fried your speakers and had a nice fireworks display. You were, again, extremely lucky when the tube you substituted was happy to run at the emission your amp's output was biased for. It is more probable than not that the average similar swap will not go so well.

The envelope vacuum seal was the first element to undergo QC testing back in the day, but nowadays, depending on the manufacturer, may not even be done at all. What brand was it?