Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => AmpTools/Tech Tips => Topic started by: antieatingactivist on November 22, 2009, 02:09:56 am
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Inside the power tube, mainly the EL34, the small spring like electrode that wraps around the cathode (I'm pretty sure it is the control grid) glows red when you crank it and play hard. With a property working and adjusted amp, should this happen. Is it bad for the tube or is it completely normal? I'm always afraid of killing new tubes.
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... the small spring like electrode that wraps around the cathode ...
There are 3 spring-like electrodes wrapped around the cathode. Any of them glowing red is a problem. They should not glow red at idle or while playing; only the filament, at the center of the cathode, should glow in operation. You might want to double-check in a dark room to make sure what is glowing.
If one of the rids really is glowing, your tube is in danger of imminent failure. Depending on which grid is glowing, other parts of your amp may be damaged as well.
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It might be the cathode glowing. It looks like a little spiral glowing, not a grid wrapped around support rods.
I understand how a tube works, i just havent dissected one yet. Maybe I should do that.
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> havent dissected one yet. Maybe I should do that.
I grew up smashing tubes, and look what happened to me.
These days, people won't even let their children play with asbestos or snort leaded gasoline.
Be careful when smashing tubes. You are not supposed to sniff or lick the insides. There's some exotic elements in there, and stuff like asbestos and lead which we used to play with are now blamed for long-term serious health problems.
It's your body. I am not a doctor nor a lawyer. But if you were my kid, I'd make you do it outside in a slight breeze, give you tweezers so you didn't have to touch much, and make you wash-up good when done.
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I'd add a plexi face shield and an old rag to wrap around the tube when your'e whacking on it- try to crack the glass near the junction with the base, maybe a medium sized screwdriver will be enough. you don't want to crunch the innards... yet. ...and go watch that Mullard video someone posted a link to a while back. It'll give you some respect for 'old school' automation- and the nice young Brit girls who put those tubes together. and the bloke with the asbestos gloves who has to catch the glass tubing as it comes out of the foundry through a hole in the floor.
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A glass cutter tool may work well too, to help the glass break cleanly from the base, that is if you're interested in keeping that part relatively intact.
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Some differences between your EL34 and the video - mainly beam forming plates instead of a suppressor grid (g3) for the 6L6. Everything else is ~same. If you've got a grid glowing in an EL34, it's very probably a screen grid (g2) instead of a suppressor anyway and the video shows the other stuff that's common to what you have. The red grid isn't likely the control grid (g1), either. It can be, but I'd guess not. Control and suppressor current will nominally be as close to nothing as I expect imperfect devices to get. The screen, on the other hand, deals with much more current.
If the problem just reared it's head, I'd expect a tube fault first, but only as long as I had healthy screen resistors in circuit - like, 1k/5V or fatter.