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Old 12th April 2004, 11:58 AM   #21
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Hi,

Got it from an old Philips book about tube manufacturing, it’s from 1946 Think you are right and most more modern small signal tubes have just a folded filament wire. But that is also a “bifilar” construction if you can say so. Some years ago there was debate if we should bias the filament below or above cathode potential for best sound but that discussion faded away without consensus. Concerning the hum in pre amps, I’ll stick with DC heater power also for indirect heated tubes.

Also in that book found something along these lines: A lot of the efficiency of coated cathodes depends on small amounts of metallic barium in the coating. When this metallic barium oxidises the efficiency dramatically drops. When the temperature is high enough the tiny oxide layer evaporates freeing the metallic barium. Maybe this is something why we must not run coated tube cathodes too cold (when filament voltage is low) ??? Still curious about a more extensive explanation of cathode “poisoning”.

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Old 12th April 2004, 05:13 PM   #22
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Default cathode poisoning

this occurs when the space charge is low under low heating conditions, and the valve is passing current. Ions formed by the passage of electrons colliding with residual gasses ( There is always a few molecules of gas left) can then collide with the cathode surface and cause its eventual poisoning Apparently a poisoned cathode can be recovered by overheating by 20% untill emission returns, I havent tried with a poisoned cathode, just some dead of old age, but it does work!

the space charge is adequate at the +- 10% range of heater supply, and great gains in cathode life can be obtained at around 10% under heating on signal valves, where the current demands of the cathode are low. O/P valves should always be operated at as close to data sheet voltage as possible, or under extreame current use even around 5% higher than data sheet spec.
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Old 13th April 2004, 10:31 PM   #23
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Quote:
Originally posted by Pjotr
Hi,

Usually filament voltage is specified as ±10% for regular tubes. But I was also told long time ago that too low filament voltage will shorten tube life either. Did that Mullard engineer also told you why Guido? Curious about it.

Cheers
Hi Pjotr

Yes, too low is no good, he advised never to go below -5% so 6.0V

In addition:

When not using tubes (standby), take away anode voltage, and switch the heater to ac, half of specified voltage.

When using DC when in operation, the polarity should be switched every month or so.

I have a Dutch report available of a conversation I organised with this engineer, it can be bought in Dutch only at

www.audio.nl

best regards
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