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richard8976 16th October 2010 08:00 PM

Running on filament only
Does running a tube with only the filament connected, for hours or days, (all other electrodes disconnected), do any damage to the tube?

costis_n 16th October 2010 08:09 PM


Enzo 16th October 2010 08:20 PM

Let us hope not, because at this point there would be a few zillion guitar amps we could never switch to standby then.

No, no harm, that is what tubes are doing at idle, in standby, anyway. I personally don;t know of any in the hifi world, but in guitar amps, there are a few models with an unused half of a 12AX7 sitting there warm.

MrCurwen 16th October 2010 08:30 PM

Yes it does.

Hot cathode - Wikipedia, the free encyclopedia


The activated electrodes can be destroyed by contact with oxygen or other chemicals (e.g. aluminium, or silicates), either present as residual gases, entering the tube via leaks, or released by outgassing or migration from the construction elements. This results in diminished emissivity. This process is known as cathode poisoning. High-reliability tubes had to be developed for the early Whirlwind computer, with filaments free of traces of silicon.

Merlinb 16th October 2010 08:30 PM

In pre-1950s valves, leaving them in cutoff for hours on end would lead to the growth of 'interface resistance' in the cathode, which reduces the gm and useful working life.

In late-generation valves this was mostly cured by the use of high-purity nickel for the cathode sleeve. However, whether this still applies to modern valves is anyone's guess...

richard8976 16th October 2010 09:11 PM

Well, perhaps I need to be more specific:

My GEC KT88 of 1960 vintage, appears to draw 4uA ionic current.

So, there is some gassyness.

If I simply connected the filament, would this represent a situation where some damage would or could result to the tube?

(Not interested at this time whether running the filament only could significantly reduce the gassyness (probably not). Just interested in whether heating up the cathode by connecting the filament (no other electrodes connected to anything) could harm some element of the tube.)

P.S. Remember, no high velocity electrons if no voltage on plate.

Also, I found - but not read yet:

Rod Coleman 16th October 2010 09:28 PM

The hazard is, as Merlin says, 'interface resistance', 'sleeping sickness' or Zwischenschichtbildung. Any valve with a sleeve-type cathode base, as opposed to the complex [ie expensive] lock-seam cathodes - which were developed for high-reliability SQ (etc) valves - is at risk.

I would expect the KT88 to have ordinary tubular sleeve construction, since there's little point in building a power valve with high durability to long-term low-current duty.

And since a real GEC KT88 is a prized piece, it hardly seems worth the risk. Once the contaminated layer has formed, you cannot retrieve the valve.

richard8976 16th October 2010 09:42 PM

On that link I gave above I was reading the 7th post down by Ray C... . He mentions arranging for a "sniff of cathode current".

Perhaps I should do that. By perhaps putting a small positive voltage on g1.

P.S. I need to absorb what people have said on that link. Then I can have a better grasp of the matter.

DF96 16th October 2010 10:04 PM

If I had a genuine GEC KT88 I would certainly not want to risk sleeping sickness. However, don't panic - it usually takes months or years of running with just heater on to develop this. A few hours will not hurt. That is why guitar amp manufacturers can get away with adding 'standby' switches - they don't actually achieve anything apart from a reduction in electricity bills but at least they do little harm to the valves.

To avoid sleeping sickness you need different cathode materials, as used in computer valves and some high reliability versions.

bob91343 16th October 2010 11:59 PM

I think the cathode interface phenomenon is reversible. That is, if you start to draw plate current, while the emission will be down at first, it will eventually creep up to normal again.

That's just what I think. I can't back it up with any reference.

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