40 ma of idle current should be fine for any KT77, EL34, etc. Dynaco biased the Stereo 70 pair at 50 ma. Sometimes a red plated tube survives if the duration is short, but it's usually done. Look closely at the plate for signs of shading or discoloration....usually that's a tell tale sign the damage has been done. Some events like this can be caused by tube socket issues, cold solder joints, etc. Since these amps are unicorns it would be best to have them thoroughly checked out by an experienced tube amp tech.
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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@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. |
Red is usually a bad thing when it comes to tubes and I’m guessing that for what ever reason, that tube needs replacing. A simple test would be to replace it with whatever you have laying around and see if the output gets better. The beauty of having individual pots is that you can bias the replacement so it sounds pretty much like the others. All the best. |
@xenolith -Great to see you have your ride or die system. |
@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. |
I've lived through a red-plating event and the screening resistor blowing up in a spectacular fashion. Indoor fireworks isn't fun. From my 1-time experience and the successor investigation, repair(s), re-tubing, etc. here's my opinion: Red Plating often involves pulsating. This is not necessarily exclusive to bias, and can be a problem with the tubes that govern the output tubes. I'm in the camp that you have a bad tube. FWIW I learned the hard way that all new matched tubes is the preferred and safest method. long story, but in my case a painfully expensive set of matched ARC tubes turned out to be the end of all my amp's problems, and a surprisingly improved sound and volume capability. |
Thank you for the continuing education you are providing, @atmasphere .
I think I got from the OP that his issue was fixed by replacing the tube, and the system sounds good again. Meaning that it was a tube failure, which I would think would be preferable to issue with bias due to something happening with the gear. Earlier I referred to a pair of ARCs I used to own that sounded great, but were unreliable. Often enough that it was not a blue-moon even, on start up one or the other would blow a grid resistor and that tube (it used 6550s) would take on a strange glow. To be on the safe side, I always swapped out the pair (or sometimes the whole quad) of tubes. I am assuming that these failures were due to the component and not the tube, although the component malfeasance was bringing down a tube when it occurred?
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@immatthewj 'Red Plating' is simply the tube dissipating too much power- too many Watts. It can be caused by a variety of things. Usually in an amp that was functioning the main reason is a loss of bias, which is to say a loss of control Voltage on the control grid of the tube. A typical large pentode for example might have a bias Voltage on the grid that is -40 Volts or so with respect to the cathode of that tube. If the grid resistor opened up, or the contact of the tube or the tube socket failed, the control Voltage would be absent and so the tube would conduct as hard as it could- hence red-plating. IOW this is a loss of bias. If the bias Voltage is simply insufficient that could do it too. A shorted output transformer (let's hope that didn't happen), too much screen grid current, excessive plate Voltage can all do this too. The lucky bit is tubes are pretty forgiving about this. If you detect a tube is red plating and shut things down quickly enough, the tube may well survive undamaged, assuming it wasn't the problem in the first place. |
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. |
Ghat's what I was thinking; I enjoy finding out that I am not always wrong.
But I didn't know what it is exactly that would cause a tube to red plate; I am not the OP, but as always, thanks for the education, @atmasphere . |
@xenolith Pretty much the definition of red plating! First step: try replacing the tube. If the grid of the tube opened up (which can happen due to poor solder joints in the base of the tube, which is a lot more common these days with Russian and Chinese power tubes) the tube can go into runaway. The silver you described is normal for a tube that has been overheated. If that does not fix the problem you have a component failure, which can be an open resistor or damaged socket. |
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 🙃 |
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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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. |
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. |
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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