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.
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?
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Are they ARCs? I know that some ARCs are biased by each individual tube. Not that it applies in this case, but I used to have a pair of ARC VTM 120s and they sounded absolutely fantastic, but when I turned them on I would cross my fingers and grit my teeth. But those amps each had one bias pot for all four 6550s. I don't know how much time @atmasphere spends on here these days, but he may respond if his screen name notification shows up.
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No, they are two of a kind monoblocks. Complicated history, but short version: customer asked Gary Dodd in about 2014 to build the best amps he can. He designs these and starts building them. He dies. A friend and colleague of Gary’s inherits these amps and builds them out. I buy them. They’re awesome. Just had to modify the individual bias circuits to stop those dramatic output tube blows. The amps do run at very high voltage, very low capacitance. All is good now though. Just curious about how a tube does what the tube last night did. I’m OK with remaining ignorant though; it’s my most familiar state 🙃 |
- 27 posts total