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I just replaced the stock 12au7 tubes on my Eastern Electric MiniMax with NOS Tungsol tubes.When I power up the pre, these tubes flash bright and then dim. I have not experienced this with my other tube pres or my tube amp.
Is this normal or is there a potential problem here? Both tubes behave exactly the same.
Follow Ups:
Tube flash is caused by the cold filament having a point of higher resistance (usually near the pin at the base), which will take on most of the voltage potential initially applied across the filament, hence glow brightly. When the rest of the filament heats up, the voltage potential becomes distributed across the filament, and will glow normally.A filament that "flashes" takes longer to glow normally, because the "flashing" portion of the filament resists current flow through the entire filament. This could be an issue if used in a device with solid-state or direct-heated rectification, where the instant B+ is more-likely to be applied to an insufficiently-heated cathode, which can shorten tube life.
Thanks to all of you.I am now just relaxing and bathing in the glow of my tubes!!
I have seen the same phenomenon in a Siemens made ECC801-S and two Valvo Hamburg made 6201 tubes.
This is normal for some of the European tubes. The flash happens in the Siemens silverplate 12au7, CV491 German ITT tubes, some Amperex tubes. This is normal for these tubes.Relax and enjoy.
Bill,I just took another look and the tubes I amusing are in fact the German ITTs. Exactly as you described.
Sounds REALLY good and I will now forget about this matter.
Problem is poor thermal contact between the heater and the cathode.It doesn't take a rocket scientist to figure out that bringing the wire to over 1000 deg C temperature every time you turn the tube on is not going to be good for it. How this will affect the *real* tube life depends on many other things, and the fact that such tubes have lasted for long time in equipment is well known.
So yes, a defect, but don't lose any sleep over it.
Ds,Euro 9-pin minis are NOTED for heater flash on initial fire-up, especially.
The tube normally works just dandy and will aslt as long as any tube.
Check out the link to see a neat little blurb on tube flashing and more neat stuff about tubes.
Cheers,
Dman
GREAT GEAR DESTROYER!!!
Thanks very much for the quick and reassuring response.Now I can relax and enjoy.
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