Re: At power on tube filaments light up more than normal for a sec

"flipper" <flipper@xxxxxxxx> wrote in message
news:tv2qj41a6c2vdveo579pelmeq8290fptfm@xxxxxxx

Well, I found this explanation.

http://reviews.ebay.com/Why-Mullard-Amperex-12AX7-flash-when-1st-energized_W0QQugidZ10000000002972031

That jives with what my first guess was. That a small bit
of filament was 'bare' while the rest is sort of
heatsinked inside the cathode section so the 'bare' part
flashes as the rest warms up slower. That would make it
just an artifact of their particular construction
technique.

Seem questionable:

"The "flash" is perfectly normal and it's a trademark characteristic of
Mullard / Amperex tubes. There are a couple other brands that have a similar
flash but I'll discuss Mullard / Amperex tubes specifically since they are
my store specialty."

IME the flash was very common in the Japanese equipment that Lafayette used
to import, back in the day of. Now, the Japanese may have been building
tubes using Mullard/Amperex technology, but it was mostly the U.S. brand
tubes that *didn't* flash.

"The flash occurs on one side of the filament wires between the bottom plate
and the inside bottom of the tube and this phenomenon only occurs when
powering up from a cold start. The sudden influx of current on the cold
heater filament encounters very little resistance along this wire (hence the
sudden burst of light). So in effect one side of the tube always warms up
1st, the second takes a while to catch up but before long the heat is
evenly distributed (and your amp comes to life). "

Electricity flows at the speed of light, and the whole tube is in a vacuum.
The thermal intertia of the components the filament contacts would be a
possible variable. Most tubes that have separate and symmetrical sides are
are duals, and they seem to light up about the same. There might be far less
flash at the tops of tubes because the filament wires are just barely poking
out, while the ones at the bottom run all the way down to the pins.




.

Relevant Pages

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