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Old 27th August 2005, 04:18 AM   #1
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Default Tube flashing

Can someone please enlighten me on the following.

I have a preamp that uses 2 X 12AT7 and 2 X 12AX7. I just purchased some used Telefunken 12AX7 and when I switch on the preamp, none of the the tube flashes. Previously, when I was having Mullards 12AX7 in place, one each of the 12AX7 and the 12AT7 flashes. Why does this happen and does the flashing indicate the condition or quality of a tube?

Thanks.
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Old 27th August 2005, 04:25 AM   #2
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I have several tubes that flash and other brands that don't.
I think the ones that don't flash were made that way. So a heater that flashes does so because of inrush current and localised heating. Once the coil heats up the resistance increases and the current falls and heating becomes uniform over the heater wire.

I would guess that heaters that flash will benefit from a soft start heater voltage , for longer life. The ones that don't flash probably are already tougher regarding this kind of faliure .
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Old 27th August 2005, 04:41 AM   #3
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> soft start heater voltage , for longer life.

In theory. But in practice, not counting series-string TV sets, I have only ever seen two broken heaters.

One I put on a Variac and turned to double rated voltage. It glowed real bright, but did not burn out. I slid it up and up, and it burned-out at four times rated voltage in a few seconds.

One was in lab gear that I knew had been physically abused (thrown into and off of a truck). It acted like a mechanical break or broken weld, not a burn-out: it worked a few seconds when cold, but would not stay hot.

In light bulbs, and in high-voltage tubes that can't use oxide emitters, you do have to run the heater close to burn-out. But common oxide cathodes can run 2/3rd, even 1/2 that temperature. Filament life falls roughly as the 13th power of temperature. So if a light bulb can live 1,000 hours, a filament at 2/3rd that temperature can run 1/(0.66^13) times longer, or 200,000 hours. Few tubes were factory rated for even 10,000 hours, so filament failure is not limiting tube rated life, even if you occasionally get a little hotter for a few seconds. I have tubes in low-stress duty coming up on 100,000 hours and working fine, and I don't pamper their heaters.

OTOH, a slow-start scheme that fails may put 10V or 15V across a "6V" heater. Even this is not instant disaster, but you don't want that to happen for long.

I say enjoy the light.
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Old 28th August 2005, 12:28 AM   #4
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Default Flash

Look carefully into the tube. On most 12A*7 type tubes there is a steel link that connects the filament of the two sections together. On some tubes the link is made out of filament material. It is this link that flashes on power up. I have also seen this on other dual tubes like 6U8's. I have also seen the link run across the top of the tube. There are some tubes where there is a considerable amount of filament material exposed outside the cathode. Since the cathode is cold on power up and acts like a small heat sink the exposed material heats up much faster and glows pretty bright during power up. This is a normal phenomenon for these tubes. If I remember correctly, I have only seen this flash in European tubes, Amperex I believe.
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Old 28th August 2005, 12:02 PM   #5
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Does this mean that it doesn't matter whether if flashes or not? I was told by some friends that when a tube flashes brightly, it is a sign of a high quality tube.
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Old 28th August 2005, 12:09 PM   #6
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Old 28th August 2005, 03:40 PM   #7
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Quote:
Filament life falls roughly as the 13th power of temperature. So if a light bulb can live 1,000 hours, a filament at 2/3rd that temperature can run 1/(0.66^13) times longer, or 200,000 hours.
As an aside, I remember taking a tour of Edison's winter laboratory in Ft. Myers, Florida, a few years ago. The lab was partially illuminated by dimly lit, orange-glowing antique light bulbs that, we were told, were from Edison’s day - still working all these decades later. I never heard the 13th power rule before, but that's a huge increase in life for a small decrease in temperature. That would explain Edison's bulbs.

Tubes in one location in my old ARC SP3A1 used to flash with terrifying brilliance, especially with some tubes. While I never suffered a failure due to this, it bothered me that the heater was undergoing such thermal shock and I worried about fatigue due to mechanical expansion-contraction cycles. It turns out that ARC combined heaters in a series-parallel arrangement, but lacking the needed even number of tubes, inserted resistors (whose values matched the fully hot heater resistance of a 12AX7). At turn-on, there was an imbalance in the series-parallel arrangement as the tubes started cold, but the resistor had a “hot” resistance. I ended up replacing the resistor with just the heater part of an older 12AX7, and tie-wrapped this dummy tube to a filter cap. No more flashing.

More recently, I have used voltage regulation with LM317s or equivalents, but I’ve implemented the simple slow turn-on circuit with an extra transistor, resistor, cap and diode as shown in the app notes. Seems to work very well. I too have never experienced a failed heater in audio amps, but I would prefer to be cautious.
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Old 28th August 2005, 08:31 PM   #8
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Default The "flasher", captured

The "flash" has no bearing on the quality of the tube. I looked through my tubes and found this "flasher". I took several pictures attempting to time the camera and the power supply simultaneously. This tube is a Zenith 6GH8A, Made in USA. I don't think that anyone would accuse this of being a "quality tube". The flashing is due to the filament construction. The fact that the link between the sections is made from filament material instead of steel is probably a cost reduction move, not a quality move.

I have seen other tubes that "flash" and some were from well known European manufacturers. I don't believe that they are any better or worse than a non "flasher" from the same manufacturer.
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Old 30th August 2005, 09:33 AM   #9
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SY, that's an option.

Ok, now I have a better understanding. Now I know why they flash when the pre is cold on turn on.
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Old 6th September 2005, 12:55 AM   #10
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I have a Mullard 12AX7 currently in use in a preamp that used to flash when it was first installed. The preamp provides it's heater power via a conventional silicon diode based rectifier (which of course applies it's power, pretty well stright away). After a few weeks of normal use -- it now doesn't flash anymore on startup. (Though still seems to work {i.e. sound} fine).

Now the interesting thing is that this tube was originally 'pulled' from tubed real-to-real tape recorder, which used tube rectification. Looking at it, it had appeared to have been well used. Yet still flashed after all these years when first installed in my preamp. But after just a few weeks of being fed from a non soft-start circuit, it looses its 'flash'.

Whether is has any sonicaly deleterous effects or ramifications for tube-life, I'm not sure. But whilst outright heater failures might be rare, there clearly seems from the above example to be *something* changing/wearing out here. Hence I now have some niggly concern (admitedly minor) about using non- soft-start cuircuits in any of my future designs.

Just thought I'd share to provoke any further thoughts.
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