What's up?
I bought these tubes a few years ago as NOS. They were clear at the time. They were then used in an asymmetrical circuit (one side higher current than the other) and now there is a resulting flash spot on the glass, one larger than the other. Clearly this is a product of convection, but in a vacuum there is no such thing. Am I right to assume there isn't a very hard vacuum in these?
Is it heater or cathode material I wonder?
They work perfectly fine for all intents and purposes (not tested in a tester, just in the circuit where they work with nearly 600V B+). Just wondering who else has seen this before. Also, since most tubes have a top getter who would even see it?
I bought these tubes a few years ago as NOS. They were clear at the time. They were then used in an asymmetrical circuit (one side higher current than the other) and now there is a resulting flash spot on the glass, one larger than the other. Clearly this is a product of convection, but in a vacuum there is no such thing. Am I right to assume there isn't a very hard vacuum in these?
Is it heater or cathode material I wonder?
They work perfectly fine for all intents and purposes (not tested in a tester, just in the circuit where they work with nearly 600V B+). Just wondering who else has seen this before. Also, since most tubes have a top getter who would even see it?
That is cathode material on the glass. If they still work, they are fine. All valves, (with exception of gass filled devices), have a high vacuum.
Hopefully they never see 600volts between Anode and Cathode, if they do, they are not much longer for this world.
Hopefully they never see 600volts between Anode and Cathode, if they do, they are not much longer for this world.
The B+ is about 600V. There is about 300V across the tube at idle. I've been using 6N8S Reflektor for years, but they don't do this. They still have clear tops. I have GE that don't do this. Just four RCA 6SN7GTB from 1957.
So it's not the heater? I have seen that effect in old incandescent bulbs before.
I use them in the front end of my MA-1...
So it's not the heater? I have seen that effect in old incandescent bulbs before.
I use them in the front end of my MA-1...
Look at an old output valve that has had a hard life. You will see Barium Oxide condensed on the inside of the glass in patches, top, bottom and adjacent to holes/gaps in the anode.
Barium Oxide is the coating used on the cathodes to aid atom flow but vapourises at around 1800°C condensing on the much cooler glass.
I can't say that I have ever seen a coating like this inside an incandescent lamp but I have seen many black patches caused by the filament breaking and sparking as it fails.
Barium Oxide is the coating used on the cathodes to aid atom flow but vapourises at around 1800°C condensing on the much cooler glass.
I can't say that I have ever seen a coating like this inside an incandescent lamp but I have seen many black patches caused by the filament breaking and sparking as it fails.
I calculate the voltage over the 6SN7 to be about 320 V, at about 5 mA per section and at a bias of about -13.4 V. That is a bit over the maximum plate voltage of 300 V in the datasheets for the 6SN7GT (and the 6SN7WGT).
The two datasheets for the 6N8S that I looked at, state a maximum plate voltage of 330 V, so they probably functioned within their limits in your amplifier.
I noticed that the 6SN7GTA and 6SN7GTB have a maximum plate voltage of 450 V so those would probably be the best choice for your amplifier.
The two datasheets for the 6N8S that I looked at, state a maximum plate voltage of 330 V, so they probably functioned within their limits in your amplifier.
I noticed that the 6SN7GTA and 6SN7GTB have a maximum plate voltage of 450 V so those would probably be the best choice for your amplifier.
I don't know for sure what it is. I just assume that it is vapor deposition from the heater, cathode, or both. It's pretty common in some RCA built tubes, especially old 6V6GT's.
The 6SN7GT did find it's way into early B&W TV sets as the vertical sweep oscillator and output tube. The specs were revised to reflect that application in 1954 with the 6SN7GTA. This spec sheet for the GTA and GTB version shows an updated 450 volt DC maximum plate voltage with a peak voltage rating of 1500 volts. Some early 6SN7GT's may not be built to withstand 450 volts or more, but anything built after 1954 should be capable. A 6SN7GTB in an RC coupled amplifier on 600 volts should pose no risk to the tube.
The 6SN7GT did find it's way into early B&W TV sets as the vertical sweep oscillator and output tube. The specs were revised to reflect that application in 1954 with the 6SN7GTA. This spec sheet for the GTA and GTB version shows an updated 450 volt DC maximum plate voltage with a peak voltage rating of 1500 volts. Some early 6SN7GT's may not be built to withstand 450 volts or more, but anything built after 1954 should be capable. A 6SN7GTB in an RC coupled amplifier on 600 volts should pose no risk to the tube.
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I think they bounced around too much on the bmx ride home 😏...
But seriously, could those particles have been in the glass bottom that somehow got transferred to the top over time?
But seriously, could those particles have been in the glass bottom that somehow got transferred to the top over time?
No , that is material boiled off from the obvious hotest place , heater/cathode .
Usually you see this in TVs tubes or other equipements made to run continuously for many hours every day .
Usually you see this in TVs tubes or other equipements made to run continuously for many hours every day .