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What is the nature of a gradual darkening on the inside of the glass tubes where the filament is not covered by the plate? I have seen this on the 6C19Pi triodes as well as on the TJ 300B/n so-called mesh-plates. The 6C19Pi shows up as a vertical band where the filament is open to the glass. On the 300B's it makes more of a soft mimicking of the mesh plate. None of my other power tubes show this as the plates cover the anode/grid assembly completely.
Just wondering what the nature of this is. Am assuming it is a sign of age as well as evidence of the cathode losing mass.
I've noticed some Chinese power tubes (and rectifier tubes) have a tendancy to do that, whereas American and Russian tubes do not.
What I've seen is sort of a translucent brown stain. I don't think it's anything to worry about.
"I've noticed some Chinese power tubes (and rectifier tubes) have a tendancy to do that, whereas American and Russian tubes do not.
What I've seen is sort of a translucent brown stain. I don't think it's anything to worry about."
Agreed with the non-worry aspect. The 6C19Pi tubes are Russian and The T-16 has 16 of them and they all have it. The TJ 300B/n are Chinese and have it as well. I did a check on the filament voltage per Chris O's post and they are still drawing 5.0 volts so that may not be the issue. I have only seen this where the filament can "see" the glass. The filaments on all the other other power tubes, such as the EL34, KT88 and EL84 tubes, are fairly well covered by the plate assembly.
My monkey curiosity at work...
I have some of the perf-plate TJ 300bs and they have less current draw on the filament than other tubes. When I plugged them in I measured the filament voltage in my amp at 5.8 volts. I had to up the dropping resistor by 0.7 ohms to get it back down to 5.0 volts.
Overvolting your filament will cause outgassing and can blacken the glass.
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