Broken tube (12ax7) ?

equalsql · October 2014

Turned on my 5E7 clone last night and nothing happened, some hum but no signal getting through, when I looked in the back of the amp I noticed V2 was not qlowing quite enough, so I turned off the amp and had look at it, and as I touched it I noticed it was cracked and the the top of the tube glass came off with the slightest of touch. The amp had not been moved or knocked since my previous night's practise session, so I'm assuming that it's down to just weakness in the glass due to heat cycling.

Personally, I've never seen this before. The tube was a Sovtek. I replaced it with a JJ and the amp works fine now.
For you long-standing tube-amp users, is this a common way for tubes to go?

ecc83 · October 2014

No, not common but 't;appen.

The very first few HT-20.40,60 combos that came over had an odd instance of a cracked pre amp valve (gone white).

Not sure they ever got to the bottom of the problem, just stopped happening!

BTW always wear gardening gloves or use a rag when handling cracked valves. Not only do you not want to get cut but you don't want any of the chemical ***t in your bloodstream either.

Dave.

Cirrus · October 2014

I think I've lost a couple that way. On one, the very top little pointy bit cracked off apparently of its own accord leaving a pin-sized hole. On another, there was a hairline crack on the base between two pins.

Both were old - I think it was an RFT and a Brimar.

I also had broken glass the time a preamp valve wasn't coming out, so I pulled it too hard and as it came out it flew from my hand, bounced off the ceiling and shattered on the floor behind me.

Not best practice, that!

ICBM · October 2014

Fairly common, yes - or at least, it's one of the more common valve failures rather than it happening to a lot of valves, possibly! Bearing in mind that I mostly see amps when they're not working.

Common causes seem to be...

Cold. Although I wouldn't have expected this because the UK doesn't actually get that cold, it does seem to be noticeable that amps stored out in garages etc over the winter have a much higher proportion.

Crap valve sockets, particularly a plastic type commonly used in the 90s. Marshall (JTM30/60 series, Trace Elliot Velocette for example) seem to get it a lot - the plastic actually shrinks over time as it gets hot, bending the valve pins inwards and cracking the glass.

Being a Sovtek or JJ. Why these two brands do it more than others, I'm not sure. It may just be a sample-size oddity.

ecc83 · October 2014

Cold IC?

I was servicing VALVED car radios in my early teens and I do not recall ever having a cracked valve?

I always thought that the "synchronous vibrator" was bloody clever!

Dave.

ICBM · October 2014

ecc83 said:

Cold IC?
I was servicing VALVED car radios in my early teens and I do not recall ever having a cracked valve?

Yes, it surprised me too when I first started noticing it, but I've seen enough now that I don't think it's coincidence. It's not just the modern valves either, quite a few old Mullards and Brimars as well.

The sample size on these is quite large too, I work for someone who has a vast collection of old amps stored in an unheated building.

Cirrus · October 2014

ecc83 said:

"synchronous vibrator"

I typed this into google looking for more information and I have to say most of the search results weren't quite on topic . What it one of those?

)

ecc83 · October 2014

ICBM said:

ecc83 said:

Cold IC?
I was servicing VALVED car radios in my early teens and I do not recall ever having a cracked valve?

Yes, it surprised me too when I first started noticing it, but I've seen enough now that I don't think it's coincidence. It's not just the modern valves either, quite a few old Mullards and Brimars as well.

The sample size on these is quite large too, I work for someone who has a vast collection of old amps stored in an unheated building.

Ok, well you can't beat the numbers!

Good job they don't use them in planes anymore!

BTW bit of trivia re cold. The early transistor radios would stop working in the winter but when brought into the workshop we could find nothing wrong with them. Turned out that some local oscillator transistors (OC45?) had marginal gains and at low temps there was not enough leakage collector current to get them started.

Stuff it up your jumper for 5mins and it would burst into life!

Dave.

equalsql · October 2014

ecc83 said:

No, not common but 't;appen.
The very first few HT-20.40,60 combos that came over had an odd instance of a cracked pre amp valve (gone white).
Not sure they ever got to the bottom of the problem, just stopped happening!
BTW always wear gardening gloves or use a rag when handling cracked valves. Not only do you not want to get cut but you don't want any of the chemical ***t in your bloodstream either.
Dave.

I had to lever it out from the base using a small screwdriver, I didn't fancy slicing my fingers on it ( I checked that the caps were drained before doing this) . I'm going to have a proper look at the internals tonight under my magnify glass, I've always been interested in how these tubes are constructed
B-)

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Broken tube (12ax7) ?

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