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One of my 6550 single top getter tubes that have VERY little hours on them I noticed started glowing a little red on the inside crease near the top of the tube and extends down about 50% of the length of the blackplate. I called the tube dealer I got them from and he said that is not really a problem, that since I can't individually bias each tube separately that tube is probably a little more used than the others and is sucking up more current. He will refund my money towards another tube, but all the ones he has there now are NOS of course and a more money.He also said that since I bias my tubes at 50 mA that may be part of it. My plate voltage is 495 volts and bias is 50 mA per tube. He said in an amp that was 600 volts and biased at 30 mA the tube probably would not glow like this.
What is this glow thing all about in that area? Is it going to eventually, or even pretty soon burn a whole in the plate? I know black plates are pretty damn reliable workhorses. I don't use the single top getters that much, but still, like all my tubes to be top notch.
Any input appreciated.
Follow Ups:
I have a 6L6 that does the same thing. Been like that for a while and sounds fine. I'd have to agree with GP53 and thought about a misalignment. I feel the heater element has become too close to the plate in this location.
If you can swap the position of the tube in the amp and have the glow follow the tube to another socket, then I'd say it points to the tube and not the bias. The bias needs to be correct too of course.
Yes, I was able to swap the tube into different sockets & the glow follows the tube.Have not tried lowering the bias below the recommended 50 mA that Cary states the bias should be set at to see if it stops glowing then, my thinking is to just return the offensive glass bas**** Still though, these 6550 I thought to be one tough tube, so it sort of sticks in my crawl to return it and then pay MORE for another after this experience.
I'd back off the bias just a tad if I were you.
I found if anything tubes sound a litle better if run slightly hotter than factory settings, but get a little lean if run much below. I try to stay w/in 5 or 10% of factory settings. What is the suggested bias setting for a blackplate 6550 then at 495 volts and how much lower than 50 mA can you go and still get top shelf performance? I did back it down to 45 mA and still glowed. Sound seems unaffected though by this glow, just NERVOUS!
Howdy,I've got a GEC KT66 that has the same thing on the crease. I think mine is a misalignment of the internal structure itself. Sounds great
in the W5M although I watch it carefully........GP53
Trust me, I'm watching this one VERY close too. Actually, I made my mind up to send it back. Think that it means the crease is weak and about to burn through? What happens then?
The crease in the tube's plate is glowing because that is where the current is greatest in this type of tube (a beam tetrode.) It doesn't mean the crease is weak. And no, it won't burn a hole there, but that doesn't mean it is a good situation. Maybe you should back off the bias current a little on both channels.If you replace just the one tube, it will need to be matched to the other one in that channel, or you may have the same problem again (possibly in the other tube.)
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