2. amps-preamps
  3. 195000-59357

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? 

xenolith

Showing 2 responses by atmasphere

atmasphere's avatar
atmasphere
12,133 posts
 

@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. 

atmasphere's avatar
atmasphere
12,133 posts
 

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. 

@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. 

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