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Tubes Asylum Questions about tubes and gear that glows. FAQ |
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Tubes Asylum Questions about tubes and gear that glows. FAQ |
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In Reply to: Tube flash at start up posted by rdgjr on March 24, 2001 at 08:54:28:
Tube "flash" at power-on occurs when a cold filament gets instantaneous power from a solid-state-rectified power supply. (Tube flash almost never occurs w/ tube-rectified power supplies, because the filament voltage is not instantaneous.) The exposed wire at the base of the tube glows relatively bright when the rest of the filament is cold. This is because that part of the filament can heat up much faster than the rest of the filament, and since a cold filament is less resistive (acts like a wire), that's where most of the filament voltage drop initially takes place. When the rest of the filament heats up, it becomes more resistive- The base part of the filament no longer is taking the bulk of the filament voltage drop. And ultimately, when the filament is glowing normal, the voltage drop is distributed evenly across the filament. (Note a tube that flashes at power-up puts *less* stress on the power supply than a tube that does not.)Tube flash usually occurs with certain brands/types of small-signal tubes. RFB and Siemens tubes to my recollections. Also note that tubes that flash at power up also do so with an intensity/brightness that varies from tube to tube. (A tube model which tends to flash at power-up may have samples which don't flash at all.)
Tubes that flash brighter do so more quickly at power up, and the filament does take *longer* to get to normal operating temperature. Tubes that flash dimmer do so more slowly after power up, and the filament gets to normal temperature more quickly. This is because a tube that flashes brightly will pass *less* current through the cold part of the filament, so it takes longer to heat up.
Usually, if the tube flash doesn't get to the brightness (color temperature) of a fully-powered incandescent light bulb, there is little to worry about in regard to tube life or tube failure. If the tube *does* flash as bright as a small fully-powered incandescent lamp, the tube (filament) could be a candidate for failure at power-up. (Note that this is *exactly* how incandescent lamps usually fail- at power-up. Except the time-frame is much faster with an incandescent lamp. I have seen small-signal tubes fail this way.) The only other time tube flash should be a real concern is if the flash gradually gets brighter each time you power up the device.
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Follow Ups
- Re: Tube flash at start up - Todd Krieger 15:24:18 03/24/01 (4)
- Re: Tube flash at start up - tubby 23:42:24 03/24/01 (0)
- Re: Tube flash at start up - Story 18:54:48 03/24/01 (1)
- I second that statement. - mb-de 08:47:29 03/26/01 (0)
- Re: Tube flash at start up - Psychopower 15:46:16 03/24/01 (0)
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