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?
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. |
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 🙃 |
Ha! Thanks Ralph. I really wish I had thought of leaving the bias pot as is with the new tube to see where bias was. Oops. Not sure what runaway means, but instinctively, that seems like an apt description of what happened. Happily, popping a new tube in and setting bias to 40 mA has worked just fine for two consecutive evening antics. No component issues. Tubes die. I get it. Thanks folks. |
@wharfy - I sure don't miss horking those 75 lb. monsters onto the workbench! They are far and away the best sounding amps I've ever heard. Charlie said the same thing and he's heard a LOT of amps! For anyone who may be interested in a bit more about them, I posted a bit over at AudioCircle shortly after receiving them: https://www.audiocircle.com/index.php?topic=171939.0 Looking forward to cranking them up tonight to play the Pink Floyd Tour 75 album that I recently scored: https://www.discogs.com/release/4915556-Pink-Floyd-Tour-75 |
@wharfy the amps are custom made; the only two on earth. The original designer and person who initiated their build was Gary Dodd. He died before finishing them. 6 years later in 2021, his friend and colleague Charlie Cocci completed them and sold them to me. These amps are optimized to run KT77s in 43% tap ultralinear mode; so much so that they literally cannot run any other tube type. So yes, all previous failures where of (Gold Lion) KT77s. From the get go, I was able to set the individual bias for each output tube to just barely 40 mA...usually with the bias pot turned all the way down. Sometimes even at fully turned down the tube would be at ~45 mA. After about half a dozen spectacular tube failures which always took out the sacrificial resistor on the bias circuit, the fuse and sucked in the glass envelope to the point of breaking, I contacted Charlie to ask what the heck was going on with these amps. Long story short ended with him incredulous that the bias circuit could be the problem but that he ultimately was even more incredulous that the three multimeters that measured >40 mA at full down turn of the bias pots could be wrong. So he advised me to buy two sets of resistors, I forget the values. Then one day, while he was on speaker phone I turned one of the amps on that had one of the sets of resistors installed...still got 40-ish mA at full down turn, so he told me to swap those resistors with the 2nd set and try it again. Perfect! At full turn down the output tubes are at 0 mA and 40 mA is at about 5 turns of the 10 turn Bourns bias pots. There hasn’t been any tube failure since then until a few nights ago when the I guess red-plating failure occurred. Much less spectacular and much less hassle as I didn’t have to unplug everything and lift an turn over the 75 lb. beast in order to replace the sacrificial resistor. Soooo, now that the bias circuits are correct, I’m finding great longevity, which I think can also be considered durability, from the Gold Lion KT77. I'm convinced that the tube just died of old age. |
@curiousjim - all is well now. The tube in question was just announcing it's demise. I intend to be much less dramatic with my own. A new tube, a quick bias routine and we're cooking with gas again. Really cranked it last night and both amps where singing beautifully. If I had any pride I'd be embarresed by my silly question, but happily I don't, so I'm not. I could be wrong, but I seem to recall that you're a former (current?) Dodd 120 amps owner; if I'm right about that and you read this, I'd appreciate knowing what you think (thought) of them and if you've replaced them, what you replaced them which, why you replaced them with what you did and what you think of what you replaced them which. |
