Not zero, but small.
If you look at figure 4 in the Metson (1952) paper, the effect of soft x-ray is a minor deviation of the vacuum curve at extremely minuscule (pA) currents.
The contribution of these current is small, for sure. Especially with gassy NOS, or gassy new production, or even if you operate at normal heater voltage. 
There is one documented application where ambient light does corrupt valve operation, though - Electrometer triodes!
See the GEC data for the ET3: 'Operating Conditions'
'valve should be operated in darkness'.
That's if you want your 10fA grid current, anyway.
I'll accept the word of a GEC/OSRAM data sheet as authoritative on that much.
There is one documented application where ambient light does corrupt valve operation, though - Electrometer triodes!
See the GEC data for the ET3: 'Operating Conditions'
'valve should be operated in darkness'.
That's if you want your 10fA grid current, anyway.
I'll accept the word of a GEC/OSRAM data sheet as authoritative on that much.
Attachments
Actually, I did that experiment (though was really looking at the effect on cathode bias LEDs, but the tube was right there) a couple years ago and posted the results. As you might guess... no measurable effect.
I'm not saying you need worry about photons in a normal, practical amplifier.
The illustration is only there for those that don't accept the fact that the effect occurs at all - should anyone really be thinking so.
If you want to measure a photo flash event (in grid current terms), you must start with a grid current of, say, 10pA or less. Good luck finding some valves to fit that socket!
Now, let's continue checking your physics and calculate impact of ionization of gases. Then compare results of calculations to empirical data.
That's not checking my physics, that's just adding more variables. In a gassy valve, gas current can easily be expected to dominate.
That's my point, exactly: "in glass valves...", the rest is the same. I even don't mean energy of absolute vacuum that use modern physics; I mean a real vacuum in real tubes.
My diversion through electrometer triodes turned up something even more relevant, if we care to go back to the original question of the thread.
The attached extract is from the BRIMAR (Footscray, Kent) data for the 6BS7 pentode, a nice audio valve, that doubles as an electrometer tube, when selected samples were used.
See the precautions for electrometer use:
1. A Stabilised HT must be used and applied not less than 1 minute after the heater voltage.
The only reasonable conclusion we can draw from this warning being issued for electrometer applications is that grid current will not remain low if you ignore it.
The only explanation on the table for increased grid current in crash-started amplifiers is -
the van de Weijer effect.
A blob of emissive material splats onto your grid, for every crash-start.
The attached extract is from the BRIMAR (Footscray, Kent) data for the 6BS7 pentode, a nice audio valve, that doubles as an electrometer tube, when selected samples were used.
See the precautions for electrometer use:
1. A Stabilised HT must be used and applied not less than 1 minute after the heater voltage.
The only reasonable conclusion we can draw from this warning being issued for electrometer applications is that grid current will not remain low if you ignore it.
The only explanation on the table for increased grid current in crash-started amplifiers is -
the van de Weijer effect.
A blob of emissive material splats onto your grid, for every crash-start.
Attachments
There's one important one you're missing- anode composition. Some anodes are made with built-in gettering (e.g., 6528, 6336) which MUST be heated before application of voltage. Those are exceptional tubes and this requirement is spelled out on their datasheets.
For electrometer service, the pico-amp stuff that we can neglect for all other uses becomes much more important- for example, they also specify that the tube be operated in the dark and that it be conditioned for 200 hours.
SY, are you referring to the zirconium coating on the plate or the graphite itself acting as a getter? I always assumed the Zr was there to reduce secondary emission.
The 6528 data specifies a cathode conditioning period of thirty seconds due to the peculiarities of a series pass regulator circuit. See note 13 on page 3 of the Raytheon data sheet.
John
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...so you finally agree that we can apply HT at the same time as the heaters! Great, now we can post our conclusion and build happily here after
Haven't you had enough yet?
OK, Let's go back to your post #1 where you tell us that the book by Merlin Blencoe (would that be the Merlin here?) converted you to the Crash-Start school of design.
The author claims:
This claim is directly contradicted by almost every guitar amp maker.
Hartley Peavey even wrote a paper "Standby for the Truth" to demonstrate that Cathode Stripping was the reason his amps are fitted with a switch:
going on to say:
To be blunt, all of those articles are nonsense. They were either written by people who themselves were brought up with the same cathode-stripping misinformation as a world of guitarists, or they are written by marketing folk. They do not 'demonstrate' anything, they simply re-state the myth and help deluded customers feel better about themselves. Since soft start is unlikely to do any harm, who is going to challenge them? Only scientific pedants, and who cares about them, they're not bringing in the dollars. No company would ever write "the standby switch is to stop our underrated capacitors from blowing up" or "we don't know what the standby switch is for, we just copied it from some older amps". Remember, just saying something over and over again does not make it true, it only makes it received truth, or perhaps religion.
So far no one has presented a shred of evidence to suggest that the delayed application of the HT gives any extension of the working life, or that a shock start has any deleterious effects, in receiving valves.
Electrometer use is very different from normal use of course (5pA grid current is astonishingly low!) but I will do my best to hunt out the science behind the 6BS7 datasheet advice; it must exist, unless it was only a precautionary measure they added 'just in case'.
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Er, I carefully began with Hartley Peavey, because he founded Peavey Electronics in 1965, making amps HE DESIGNED. He is not a marketeer.
If you think that the Founder and original designer of Peavey amps is ignorantly making claims about the proper use of amps he designed, then the onus is on YOU to substantiate what you say.
This is conspicuous by its absence.
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