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Components and Circuits For discussions about component types, alternatives and availability, circuit configurations and modifications etc. Discussions here should be of a general nature and not about specific sets.

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Old 11th May 2019, 11:43 am   #21
GrimJosef
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Default Re: How much anode glow is acceptable in a KT88 ?

Quote:
Originally Posted by G6Tanuki View Post
... When the anode starts turning red, its efficiency as a heat-radiator actually _falls_ compared with when it's black and at the same temperature - so making it likely to get hotter, and redder, and so even less-effective as a radiator, so it gets hotter still. Thermal runaway, and a pain in the wallet ...
Thermal runaway certainly does happen and can do a great deal of damage, not just to the valves. But I think the anode's perceived colour doesn't explain it.

As I understand it a black body is defined as one where the interaction between the material and the light is dominated by radiation and absorption, not by reflection, and also where the radiative/absorptive properties of the material don't depend on the wavelength of the light. In general, I think, those radiative/absorptive properties rarely depend on temperature either. So if something is a black body at room temperature then it will still be a black body (from a thermal point of view) when it's red hot. Indeed, neglecting the spectral lines, which are characteristic of individual elements, the spectrum of the sun is quite accurately described by the black body equation and it looks very far from 'black'.

One of the things which confuses the situation is the limited spectral range of the human eye. When we look at a room-temperature black thing we don't see it radiating at all. But actually it's emitting black body radiation all the time. It's just that at room temperature the peak of the radiation spectrum is far into the infrared (around 10um wavelength) and our eyes don't respond to that.

Cheers,

GJ
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Old 11th May 2019, 12:40 pm   #22
Silicon
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Default Re: How much anode glow is acceptable in a KT88 ?

Another factor to consider is the transparency of the glass envelope at infra red frequencies/wavelengths.

We can see that visible light passes through the glass and the glass 'looks' clear.

However, from the point of view of the anode, at infra red frequencies the glass may 'look' like a mirror and 'feel' like a duvet.
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Old 11th May 2019, 1:12 pm   #23
trobbins
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Default Re: How much anode glow is acceptable in a KT88 ?

A quick google indicates that it can start to get a bit complex, as emissivity can be a function of temperature and wavelength.
https://www.irsm.cas.cz/materialy/cs..._2016_0023.pdf
It will obviously depend on the actual performance of materials being compared, although the strong T^4 relationship is likely to dominate any small change in emissivity, and not lead to a run-away scenario.

A not uncommon technique for setting PA amp bias using valves was to 'tune' the anode redness.
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