I just got a quad of EH KT88s. One of them has a spot on one of the plates that glows yellow, even when run at a lower bias setting (Simple SE). It's a small spot that is well-defined, not a dull glowing area which is what I normally see on tubes with alignment issues. When the tube is off, that spot seems to coincide with one of the welds on the extra radiator fins that these tubes have. The cage on these looks just like the TungSol reissue 6550, but those lack this extra fin.
I was a bit alarmed, so I put the whole quad into the Dynaco ST70 which is fixed bias. I biased them all to about 60mA, which is something like 25-30W. Three of the four seem have one or more of these little spots, though the one is much brighter than the others. I ran them for a couple of hours. Bias was stable and everything seemed fine. As a test, I ran that pair harder and started getting a little plate glow. It was the usual dull red and was even across the length of the plate.
So I am not sure what to think. Anyone else ever see this? I'll snap a picture tonight, as I need to swap the tubes around to get a better view.
I was a bit alarmed, so I put the whole quad into the Dynaco ST70 which is fixed bias. I biased them all to about 60mA, which is something like 25-30W. Three of the four seem have one or more of these little spots, though the one is much brighter than the others. I ran them for a couple of hours. Bias was stable and everything seemed fine. As a test, I ran that pair harder and started getting a little plate glow. It was the usual dull red and was even across the length of the plate.
So I am not sure what to think. Anyone else ever see this? I'll snap a picture tonight, as I need to swap the tubes around to get a better view.
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Waiting to see the pictures, but that sounds very strange and rather concerning.. I'd probably be planning on sending them back to the seller given the description you've provided.
If the spots coincide with weld points, they may be reflective, and be reflecting light from some other source. The obvious source of light inside the valve is the cathode, although it may require multiple reflections to reach the outer surface of the plate. Is the spot still there if all voltages other than the heater supply are removed?
Excellent question. I will try this. I don't have a direct line of sight because of the way the tube sockets are arranged. I feel like I ruled out a reflection off the glass, but I am not 100% confident. There is another funny orange line on the side of the plate, next to one of the holes. I assumed this was some sort of reflection too, but I didn't think to try it without the B+ supply.
It didn't occur to me until this morning that the ST70 has the other pair of sockets rotated 180 degrees. I'll get a good view of it once I swap tube pairs.
Darryl called it. Here is a picture of the tube in question with only the heater going. The long exposure lets you really see the source: the bit of heater sticking out of the bottom of the cathode. It's reflecting off of the glass and lighting up a stripe across the plate. It hits that little spot near the welds and really lights it up.
Whew. I've had a string of bad luck with new production big power tubes lately. Glad this wasn't a continuation of the trend (so far)!
Whew. I've had a string of bad luck with new production big power tubes lately. Glad this wasn't a continuation of the trend (so far)!
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Good to hear that these don't seem to be problematic...although long term use is another story. I have been thinking about grabbing another set and giving the Golden Lions a rest for a while. I actually love the EH 6l6GCs I picked up a while back. So if these are anything like how those perform I should be pretty happy.
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