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KT120 One new tube glows brighter. Within normal?

Old Guy8

Old Guy8

Cliff & Cleo
Subscriber
Rogue Cronus Magnum V1 with a NEW quad of KT120s. Less than 10 hours on them.
All tubes manually bias at 35 mv as Rogue recommends.

Three of the tubes look to glow at nearly same brightness. While tube at right is brighter with a slight arc up. As I hope you can see.
Amp sounds fine to me.
Now I can't attest that previous set of tubes were all of same brightness. At a later time after amp and tubes cool I will rotate tube positions.

My question is. Does this brighter tube look like it's within normal operation? Is it a red flag of possible trouble? What does red plating look like?

I can contact Rogue this next week, with questions?
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It probably nothing if they ate biased correctly. The heater filament is just sticking out further in the one. I have a 12BH7 tube that is really bright at the bottom. The heater filament t is just hanging lower is all. If the tube isn't red plating it should be fine.
 
Lavane said:
It probably nothing if they ate biased correctly. The heater filament is just sticking out further in the one. I have a 12BH7 tube that is really bright at the bottom. The heater filament t is just hanging lower is all. If the tube isn't red plating it should be fine.
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Thanks Lavane, that's a load off mind.
So tell me what does a red plating event looks like? I assume I would know things aren't normal? Arcing within tube?
 
And here's another, the plate actually glows cherry red when too much current is pushed through the tube. This can become SO severe that the plate glows bright orange, the brighter the glow the faster the destruction of the tube....

Tube redplating.jpg
 
I am glad to read this -- thread answers a question of mine .. running a Jolida amp, and one of the EL34's has always been a bit brighter than the other 3 .. never swapped tubes or anything - amp sounds fine, so not going to worry about this ..and definitely not a redplate condition ..
 
Old Guy8 said:
Three of the tubes look to glow at nearly same brightness. While tube at right is brighter with a slight arc up.
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As others have said, the brightness is not an issue. But what do you mean when you say, "a slight arc up"?

Does the tube flash or visibly arc when the amp first powers up or while it's running?
 
Slight arc up refers to the filament wire that's protruding from the cathode sleeve.
It's the part makin the glow and is got a bit of an arch to it where they bent it on order to stuff it all in the cathode.
Nothing to worry about.
If you have a dead tube crack he glass and take it apart to familiarize yourself with their construction.
 
FlaCharlie said:
As others have said, the brightness is not an issue. But what do you mean when you say, "a slight arc up"?

Does the tube flash or visibly arc when the amp first powers up or while it's running?
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Charlie,
I used a confusing word in arc. It can mean different things. There's no flashing at all. I ment that the filliment has an upward curve shape to it. So i guess that's why its brighter. The filliment is more visible than on the other tubes, because it protrudes up higher in one spot. My picture could have been a little closer to it, to see it better.

After i wrote this. Saw Train's explanation which is better.

Knock on wood. Yet to have a red plate tube, so far.
 
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A friend of mine, was an engineering apprentice working in a UK valve factory. Back in his youth. Should have him tell me more stories of it.
One story. Was when he was eating a sandwich while working on the valve line. And his foreman asked him, if he was enjoying his sandwich? Then chewed him out royally.
 
Lavane said:
It probably nothing if they ate biased correctly. The heater filament is just sticking out further in the one. I have a 12BH7 tube that is really bright at the bottom. The heater filament t is just hanging lower is all. If the tube isn't red plating it should be fine.
Click to expand...
I have the same(heater filament sticking up a-bit) with one KT88 but tube is running fine.
 
Old Guy8, I have the Atlas power amp version of your Cronus and posted about hearing power supply noise, with my ear right to the speakers. I'm curious what your experience is with the Cronus.
 
oldflame said:
Old Guy8, I have the Atlas power amp version of your Cronus and posted about hearing power supply noise, with my ear right to the speakers. I'm curious what your experience is with the Cronus.
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I recently read your thread about that. A mystery to me, too.
I don't think I've experienced that. I have had a low hiss with ear at speaker, with another tube amp. From a noisey input tube. But no problems with this amp. Have you called Rogue about it? They may be preoccupied with KT120 shortage issue. Unfortunately.
 
When I called Rogue, their first thought, like others have posted, was maybe a cap(s). The noise reminds me of my early Dyna 400, where snubber caps had to be added across the bridge rectifier. The Dyna, however, only had a buzz with no 60/120Hz hum. Glad to know your Crony is quiet.
 
oldflame said:
When I called Rogue, their first thought, like others have posted, was maybe a cap(s). The noise reminds me of my early Dyna 400, where snubber caps had to be added across the bridge rectifier. The Dyna, however, only had a buzz with no 60/120Hz hum. Glad to know your Crony is quiet.
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If down the road you chose to have Rogue repair/replace caps. They had options not listed on thier site.
Other Rogue Cronus Magnum owners have had them upgrade the caps. While there for repair.

I've considered having them upgrade mine to a V3 for triode and add a phono pre. Which mine doesn't have. It's pricey and shipping, gives me pause.
 
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