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Hi there , I am building a bass pre amp using 5670 tubes. After running them for a while they developed black burn spots right on the getter flashing above the heater filaments is that because the current is too strong for the filaments and its over heating them?
Follow Ups:
Jim's right (of course). I just pulled these out of their boxes, and most of them have the darker marks within the getter flashing.
Cory
Many tubes develop the black (or gray) marks above the cathodes and they are of no consequence. Most likely these occur due to impurities in the glass or within the tube itself.
These are the burn marks on the getter flashing
So these burn marks are normal and nothing to worry about?
These tubes often come with that sort of coloring right out of the box, brand new. It's nothing to worry about.
When I got them they had the silver black burn coating on the inside glass but there are black burn marks on the silver black coating now after using them for a bit
Keep in mind that those are not signs of burning. Really, your tubes look just fine to me, and some here say I know what I'm talking about!
Do remember that if the tubes sit for a long time there may be some outgassing of the internal parts, and the getter is there to absorb that. Once that task is done the rate of getter useage drops off.
Don't worry about them, just enjoy them. They are fine.
Thanx you so much I feel really relieved now I waited al long time fot them to arrive wouldn't want them burning up or something
You should check the heater Voltage, and check the plate current.
Black is usually not a bad color for a getter.
Brown is bad.
White is bad.
Invisible is bad.
Chunks flaking off is bad.
The voltage's are fine but the current on the heater transformer is rated at 1.25A might that overheat them? Because the tubes are rated at a max current of 340mA sorry for the stupid questions I'm not that experienced with electronics
Not stupid at all. The rating of the transformer has noting to do with the load. Meaning that you could use a transformer rated at 1,000 amps to deliver the 0.34 amps and it would be just fine. The voltage on the tube is just following Ohms law. At 6.3 volts and 0.34 amps, the heater is really just a 18.5 ohm resistor. (V/R = A)
Thank you so much for the advice. I would've been lost without this my heater circuit is running of a old 12v power supply for a internet modem would it ba safe to earth this with all the other earth's? or should I keep the circuit isolated because I'm having a bit of earth noise . One more question when I flick my standby on or off ,it clips and spikes heavily I was told to put a series resistor behind the switch but wouldn't that lower the voltage? And if I use the resistor what size should it be? And should it be on the negative or positive side? Sorry for all the questions in one post
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