• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

Running on filament only

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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.
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.
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:

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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.
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.
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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.
The particular circumstances are a slightly gassy tube.

Ion bombardment

It seems, I think, the issue is not ion bombardment of the cathode, because ion creation calls for accelerated electrons which calls for positive voltages on the plate or some other electrode.

Interface resistance

I think this would be the issue of concern. But perhaps not if the tube is run for several hours or a day or two.

I'm not sure how interface resistance occurs, I think it's some interaction between the cathode and impurities in the tube.
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