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Tube cracking problem on MG-Head.

post #1 of 9
Thread Starter 
My expensive Mullard CV4004 (driver tube) just developed a crack and bit the dust today. Pulling out "Getting the most out of vacuum tubes" by Robert Tomer, I found out that the most probable cause for the crack are the tight tube sockets. The contacts on the tube pins are supposed to float within the socket so that they will not put too much stress on the glass envelope. The contacts in the sockets on my MG Head did not seem to float and the tube is quite tight in the socket. Think thermal expansion and glass stress everytime the tube heats up and cools down.

Has anyone else experienced this?

Wing Keong.
post #2 of 9
The tube sits tightly in the sockets of my MG Head. No give or wiggle room.

I’ve change hundreds of tubes in my MG Head and I’ve never had a problem.

Try bending a pin with your fingernail. It’s made of very soft steel and should give before the glass would fracture.

Inserting or removing the tube improperly can do more damage

post #3 of 9
post #4 of 9
i'm not sure what they mean by "float." these sockets just have some pins inside that clamp onto the pins of the tube right?--kinda like any other socket--RCA, 1/4"... etc. not sure what kind of "float" they would be referring to. maybe they mean that the pins shouldn't be too tight?

i dunno.

but of course, what do you expect when you subject a 40 year old piece of glass to high-temperature thermal expansion repetedly for a long period of time? it's gonna break eventually. though i would have thought the inner-components would have broken down first. perhaps the glass was formed unevenly, and over time, stress broke along that original weak point.

who knows.

but i think this adds to the argument to leave on your amp for maximum tube life.
post #5 of 9
Yamamoto tube sockets are nothing but audiophile jewellery according to those on Audioasylum. Teflon may be a great insulator, even greater than ceramics but the pins are meant to be a joke. They're so lose apparently that people get intermittent connections and if they don't, the removable nature of the pins for cleaning means then when a tube is removed the pins come with it too. Some people have gotten sick of them and even replaced them with bog standard ceramic sockets. For a $50USD tube socket this isn't really good enough...
post #6 of 9
What’s the expansion coefficient of glass? Not a whole lot I’d assume and the tube doesn’t get real hot so it probably doesn’t expand much. If glass expanded and contracted a lot light bulbs would break since they are fixed into the base.

By float he means the female socket pins will hold the tube pins tightly but still have some space inside the ceramic/teflon socket to move a little. If you take a new tube socket and shake it the female pins will rattle around, that is they float inside the socket. This lets them slide over the tube pins but still hold it tightly.
post #7 of 9
yeah, true... but even with very small amount of expansion, it's still enough to stress and crack things that are weak. just look around outside.... cracks in walls, sidewalks, wood, etc....... never saw glass break, but still........ but glass is amorphic, so perhaps it has some self-healing properties too. dunno.....

but all my tube gear, on sockets of all sizes, i can't move the tube around. they're all held down pretty good. so i don't seem to find any float on any piece i own.
post #8 of 9

Probably a faulty tube

If the MG Head sockets were so tight as to cause tubes to crack, there would be a lot more posts about cracked tubes in MG Heads on this forum.

I'm thinking more likely that the Mullard in question was defective, or maybe weakened due to stress, shock or damage. If you've had multiple tubes crack in your MG Head, then I'd say the socket is defective.

The tubes are pretty tight in my Head too. AFAIK they're supposed to be; if they were loose they would make poor contact, and the tubes would pop out of the sockets during transit.

BTW the driver tube doesn't get very hot, so heat isn't likely the (direct) cause of the cracking.

post #9 of 9
Thread Starter 
The Mullard has been sitting in the Head for almost a year and a half now. It's been removed few times for rolling and such so who knows. In any case, it's good that I have a spare and that I plan to rebuild the Head with another even cheaper and good sounding tube set ( 6N1P, SV83 ) so these tubes won't cost me a fortune to replace.
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