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In Reply to: 12ax7a flashes when turned on posted by mes7800 on August 11, 2002 at 15:40:22:
Hi there,the question has been asked by others time and time again - an archive search in the forum would have yielded numerous replies.
On the other hand - you have asked a question and deserve a proper
answer:The described behaviour is normal for the valves you are using - albeit it does look scary, it does not damage the valves. It has been seen with Philips ECC81/82/84 = 12AT/U/X7 vor decades - 'Philips' is to be read 'Philips, Valvo, Mullard, Amperex Holland, and other Philips NV valve factories'.
I do admit it scared me, too, when I saw it first. It is just the inrush of the heater filaments.
So - do not worry - and enjoy listening to music or playing it.
Best regards,
Follow Ups:
Has anybody noticed that light bulbs flash and burn out usually when first turned on...?A cold tube is the same as a light bulb ...The filiment has a much less restance when cold,so when full power is applied to a cold filiment it is a SHOCK...
Broadcast Equiptment stays on all the time because of that reason or stand by modes run 1/2 filiment voltage till full turn-on is needed..Much longer tube life is realized..
Light bulbs on dimmers last longer for the same reason: *Slow turn on*... A 1 ohm resister in series with the filiment can help to provide a SLOW TRUN ON for most 12AX7 style tubes...
Has anybody noticed that light bulbs flash and burn out usually when first turned on...?Sure, but tube filaments are meant to create heat so
as to spur the emission of electrons and light bulb
filaments are supposed to create light.
So they're not the same thing.The usual failure mode for tubes is to either short out,
get noisy or just a wear out its cathode, which quits emitting
electrons.The German guy is right about filament failure
being unusual, and the folks at Telefunken were including, I'm sure,
series string filament tubes, where that would be more common.There are slow-warmup filaments for equipment that
runs the filaments in series-string arrangement,
but that's got nothing to with extending tube life except
for keeping the first or largest filament in the string
from blowing when the set is turned on..
Run with filaments in parallel, the slow warmup is moot point.
Broadcast Equiptment stays on all the time because of that reason or stand by modes run 1/2 filiment voltage till full turn-on is needed..Much longer tube life is realized..Yeah, but those are thoriated tungsten filament tubes that draw
amps and amps > of filament current (and often cost thousands
of dollars to boot), not heater-cathode 12AX7's drawing
300 milliamps!BTW, one can also extend the life of transmitting types through
judicious use of the filament voltage control, if the
transmitter has one.I read that in Radio World
a long time ago.
I have never, ever seen a nine pin tube with an open filament. I have seen exactly 3 power tubes with one--they were NIB. Soldering the pins fixed one. The health of the cathode in the lion's share of concern, and the primary problem, cathode stripping when the anode has DC present and the filament hasn't gotten the cathode hot enough, is handily solved w/ instant on filaments. Chris
... that less than 0,5 % of small signal valves failed with open filament, if operated within the heater voltage limits, i. e. less than 10 % overvoltage at startup.For Special Quality valves they even guaranteed several thousand cycles of cold turn on at 25% overvoltage.
Therefore this dog doesn't hunt.
In all the thousands of valves I have gone through since the 1970ies - since then I collect pullouts from all sorts of electronics gear, test and classify them, I had only maybe half a dozen valves with heater cathode(indirectly heated) and open filament - two of them were ECC81s. Most dead ones died of exhaustion of the cathode, cathode poisoning for too little current draw, vacuum loss or overheat (TV sets collect amazing amounts of dust, and so do PA amplifiers in bars).
The situation was different with direct heated valves for battery or mains operation - of those, many had broken filaments - by mechanical mishandling or overvoltage.
Slow turn on may be a good idea, but is apparently not strictly necessary for the small indirectly heated valves, and standby at normal or reduced heater voltage without current flow through the cathode is in many cases counterproductive. Philips wrote about this extensively in their data and application books published in the 1950ies and 1960ies - they recommend standby at full or reduced heater power for large transmitting and control valves with direct heated cathodes for up to 24 hours of operation interruption, for medium sized ones (Qa> 0,125 kW) for up to one hour, and for small or indirectly heated ones for no more than 10..20 minutes. Exception were special TV CRTs and camera tubes which were designed to be run at 2/3 heater power in standby, to reduce the startup time.
The only valves having no or less problems with cathode poisoning (resistive layer buildup when cathode is heated but no current flowing through it) are computer valves like the 7119/E182CC - their cathodes
are using different coatings than normal small signal valves, with the penalty of higher shot and other noise.Best regard,
Hi there,my apologies go to mb in Singapore - I have mistyped my moniker in
the previous post - should have been mb-de.Best regards,
Hi,Besides it isn't really the heater that brights up at start but rather the internal leads btw pin and heater :-)
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