amp explodes a power tube

[JC@]

Hello.
What may cause a tube to start consuming huge current?
It happens randomly. Sometimes it does, sometimes it doesn't. But when it does, the power tube literally explodes. Well, not really (implodes?), but it does break the glass and all.

[loudthud]

Usually it's loss of grid bias. Check for tight connections, a cracked grid resistor or stopper and a leaky coupling cap. Also check the tube socket.

[JC@]

Usually it's loss of grid bias. Check for tight connections, a cracked grid resistor or stopper and a leaky coupling cap. Also check the tube socket.

loudthud, transformers can't do this, or may be?
It's a fender clone, about 50 years old, "50 watter". I visually inspected all that you mention, but I will replace grid resistors and stoppers, and unsolder and double check both coupling caps again.

About tube sockets, I will try to inspect them closely as I wouldn't like to just replace them blindly.

The owner says he believes it is always the same tube that blows, and my checkings confirm it is just one of the sides (tube) that goes wildly runaway.

Thank you

[JC@]

One more question: what do you use to clean really filthy socket contacts? It would be nice something like a tiny brass brush like the ones used to clean a gun's bore (I can't remember how it is called in english, sorry)

[loudthud]

The contact cleaner I use isn't available anymore due to environmental restrictions. You'll have to search it see what's available in your area. I've never used one of those gun brushes. Use it lightly so as not to remove plating on the contacts, just enough to remove any corrosion. Use some type of long slender tool to carefully bend the tube socket contacts for increased tention.

If the output transformer goes open, the screen grid will try to draw massive current but I don't think the tube would get so hot as to crack the glass. Does the tube Red-Plate?

[JC@]

>Does the tube Red-Plate?

No it doesn't, even that the milliammeter was showing a rapid encrease of current on that tube before it reached the top of the scale (200mA). The owner came with two cracked tubes, and he just told me that he could see "blue sparks" inside the tube before it went off. Twice.

[loudthud]

>Does the tube Red-Plate?

No it doesn't, even that the milliammeter was showing a rapid encrease of current on that tube before it reached the top of the scale (200mA). The owner came with two cracked tubes, and he just told me that he could see "blue sparks" inside the tube before it went off. Twice.

I guess that means it could be the transformer. I wouldn't replace it until we find further evidence that it is the cause. I've seen a couple of tubes melt the glass before when they red-plated, but the only thing I've heard that will crack it is beer.

[JC@]

I guess that means it could be the transformer. I wouldn't replace it until we find further evidence that it is the cause. I've seen a couple of tubes melt the glass before when they red-plated, but the only thing I've heard that will crack it is beer.

When I finish the refurbishing (caps, some cables, bias circuit, cleaning and retensioning of the power tube sockets..) which is almost done, I will isolate the output transformer and check it out.

No problem. In a way, I would like it to be the transformer because that will end with the chase.

[Enzo]

Nowhere so far did I see where you tried different tubes in it.

In my experience, leaky couplign caps stay that way, they don't come and go as a rule. When I see current going skyward and then plunging back to earth, I suspect the tube is failing before anything else. Swap places with another tube, does the symptom move with the tube or stay with the socket? I'd just sub another set of tubes in there first and see what happens.

[JC@]

Nowhere so far did I see where you tried different tubes in it.

In my experience, leaky couplign caps stay that way, they don't come and go as a rule. When I see current going skyward and then plunging back to earth, I suspect the tube is failing before anything else. Swap places with another tube, does the symptom move with the tube or stay with the socket? I'd just sub another set of tubes in there first and see what happens.

Hi Enzo, thanks for coming.
THIS was my next thought and message.
Of course I tried different tubes, and the problem appears at least in my testings, on one of the sockets. But the funny thing is that it is just as described: skyward, remove in the socket, skyward, change of tube, normal, off/on power, skyward, of/off power, steady again... Changed tubes, and again the same thing. So many times that I'm not sure it is the socket the cause of the problem.
I would love to have it fixed but I can't isolate or reproduce the problem.

I was thinking if one half of the winding in the output transformer could be intermitently open...

[Enzo]

If it goes open, there will be no plate voltage on that tube. SO next time the current takes a dive, check for missing plate voltage, that will tell you if the OT is opening. For that matter, a loss of screen will turn a tube off, so look for loss of B+ on pins 4.