glowing red output tube, but don't think it's red-plating
Hello auidionaughts. Had an interesting circumstance last night; output from the right monoblock began to sound slow and of lower output. Line of sight to the tubes (both input and output) are largely blocked from the listening seating position by a ginormous power transformer and an equally sized output stage inductor, so neither me or mrs. x immediately noticed that the plates on tube #3 were glowing red...but the change in output got our attention. I jumped up to check it out and found tube #3 as described; of course I turned the amp off.
Inspecting the tube this morning, it shows two anomalies: silver plating on the inside of the glass opposite the ridges that hold the support rods and a very small amplitude dimple in the glass roughly centered within the two fields of the aforementioned silver plating. All else looks normal including the silver plating on the top of the tube which looks unchanged, even though I am guessing that was the source of the silver plating now on the sides of the tube. Could it have come from somewhere else?
During the incident, it didn't look like red-plating to me; i.e. not red at the right angle crease in the plate, rather, the whole plate was red. Oddly, every other tube failure with these amps was fast, taking out a bias resistor and fuse and sometimes breaking the glass. Neither of these happened with this failure and it was slow. Was able to replace the tube and slightly adjust the bias and away we go again, so now resistor or fuse damage. I did (stupidly) turn the bias pot down before installing the new tube ( I know doing so is good practice, but doing it removed a potentially informative data point), so can't know for sure, but I'm pretty sure that the bias was spot on where it is supposed to be (40 mA), or very close, when the failure happened. My understanding is that red-plating is due to incorrect bias.
Any ideas what caused this failure?
Looks like a pretty amp from your photo What is your system? Fortunately I have no experience with failing tubes, so I can't proffer and answer. But it does sound like you have a lot of tube failures. Crossed fingers. I run nearly forty tubes and have for many years... less numbers for over a decade and never had more than a whistle in one.
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I am not an electrical/tube guru, but I believe that if something failed within the tube (but not the amp) that caused that tube to draw way too much current, that would affect the bias on that tube and cause that tube to red plate. And I also think that you did the right thing by turning the bias all the way down before powering up with the replacement tube, even though you were interested in what your bias of your amp had been doing with the bad tube in it. I take it from your post that your amps have one bias pot for the whole shebang, as opposed to biasing each tube individually? Again, I am not a guru, so I may be wrong on this part, but if that happened and then the tube actually died, I think that the bias reading (which would have been reading all tubes on that pot) would then have been quite a bit low. If you can bias each tube individually, I guess you could put that tube back in the socket, back the bias for that tube socket all the way down, and then turn the amp on and slowly advance the bias and see what happens with the reading on the meter, but if things are running good right now, I wouldn’t--I’d just accept that the tube failed and move on. |
Thank you @immatthewj ; that is very helpful. The amps do have individual bias pots for each output tube. I certainly agree that the best course forward is to consider the bad actor tube as not fit for service and to be willing to move forward in ignorance of why it did what it did. I do have plenty of KT77s on hand and the effected amp is happily singing along. I was simply interested in becoming better informed about what electrical condition could cause the described failure. Full disclose, was hoping Ralph would chime in; he must be among the world's experts on tube systematics. I do still have the tube and will make it's photo observable if sufficient force is exerted upon me to do so. But I'm lazy; lotta effort would be needed. |
