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Plate temperature measurement
Plate temperature measurement
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Old 4th September 2019, 05:25 AM   #31
PRR is offline PRR  United States
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> approximately 485C at the plate (and 212C at the envelope)

These are roughly as described in the J. P. Welch essay cited above. 500C in plate, and we would "like" <175C on glass for longevity, but a 6L6GC's work suggests compromise. I don't know the number for 6L6GC, but Tung-Sol (who knew hot glass seals) made 5881 to replace 6L6 in applications where 6L6 was not lasting; and made 6550 for bigger chores; 6550 is rated 250C on glass (and seems to be OK with that). I happened on a lesser tube rated 165C glass (a tight spec 6SN7 so probably conservative for low drift).

> 5W *screen* of course

In _6L6(GC)_ screen current is typically quite small, 5%-7% of plate current. Simplifying, we dart-toss 2W-3W, rarely 5W (except in severe overdrive). (Ah, but 6L6GC has a twin with a top-cap, for TV H-sweep duty, and there screen dissipation may be quite high.)

Last edited by PRR; 4th September 2019 at 05:29 AM.
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Old 4th September 2019, 10:15 AM   #32
pcbampmaker is offline pcbampmaker  New Zealand
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Quote:
Originally Posted by PRR View Post
In _6L6(GC)_ screen current is typically quite small, 5%-7% of plate current.
Yes, in my circuit there seems to be approx 2mA flowing through the screen, which at 450V approx would make just about 0.9W dissipation.
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Old 5th September 2019, 11:25 AM   #33
pcbampmaker is offline pcbampmaker  New Zealand
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I've read a few papers, and put together the plot attached:
- left axis is for Index of Refraction (complex: real part in red, imaginary in blue)
- right axis is for the rest
- green is the relative response of the FLIR i7
- purple, light blue and orange are the transmittance of the glass at thicknesses of 50, 250 and 750 um, where 750um is approximately our thickness.

The data is from this paper:
R. Kitamura, L. Pilon, and M. Jonasz, 2007. Optical Constants of Fused Quartz From Extreme Ultraviolet to Far Infrared at Near Room Temperatures. Applied Optics, Vol. 46, No. 33, pp. 8118-8133.

The absorption curves come from the imaginary part of the index of refraction: it's also called the extinction coefficient, and it's related to transmittance as

T = exp(-2\pi k x / \lambda_0)

where k is the extinction coefficient and \lambda_0 is the wavelength in vacuum and x is the distance traveled.

This Stack Exchange answer has a very nice derivation of the relation above from John Rennie.

Lastly I observe the very complex shape of the real part of the IOR, goin from 0.35 to 2.5 in one micron and then staying mostly above 2 in the region where the instrument has an sensitivity.

The next step I have in mind is to plot the emission resulting from this and its angular dependence, looks like it'll be quite shapey
Attached Images
File Type: png AbsorptionPlot-1.png (47.1 KB, 60 views)

Last edited by pcbampmaker; 5th September 2019 at 11:27 AM. Reason: missing factor in equation
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Old 5th September 2019, 11:35 AM   #34
trobbins is offline trobbins  Australia
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So the FLIR response shows that it pretty much won't 'see' the anode, cos the glass is effectively opaque at those wavelengths
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Old 8th September 2019, 10:45 AM   #35
pcbampmaker is offline pcbampmaker  New Zealand
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Isn't it interesting how the curve of the FLIR rises just where the transmittance drops, though...

Yes, the question is more what correction factor is needed to apply to the FLIR to have a guess at the temperature of the glass
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Old 8th September 2019, 11:59 AM   #36
mondogenerator is online now mondogenerator  England
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Or perhaps it shows that the FLIR CCD is ineffective at measuring the plate through the glass envelope, regardless of any sort of attempt to 'mis calibrate' the device to in an attempt to overcome the short coming of the device and read the temperature better.

I'm sure I read somewhere that the temperature can be approximated by the perceived colour, and if ball park is good enough, then +/- 25 degrees may be good enough, and feasible?

Other than cracking a tube opens and then putting the FLIR and tube electrodes into a new vacuum....

But then, that much effort, or this much effort? What does it actually achieve?

Say it's about 450 degrees at full barely visible red. I'm not sure what any more accuracy gets you.

Surely it's now more of a semantic study.

The anode is then either, hot enough, or too hot.
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Last edited by mondogenerator; 8th September 2019 at 12:02 PM.
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Old 8th September 2019, 03:28 PM   #37
gideon1990 is offline gideon1990  Netherlands
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Quote:
Originally Posted by mondogenerator View Post
Other than cracking a tube opens and then putting the FLIR and tube electrodes into a new vacuum....
at that point I would just spot-weld a thermocouple to whatever you want to analyze
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Old 8th September 2019, 04:23 PM   #38
mondogenerator is online now mondogenerator  England
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So true
Depends if you have isolating amplifiers at hand or not....
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Old 11th September 2019, 10:48 PM   #39
pcbampmaker is offline pcbampmaker  New Zealand
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Quote:
Originally Posted by mondogenerator View Post
Or perhaps it shows that the FLIR CCD is ineffective at measuring the plate through the glass envelope, regardless of any sort of attempt to 'mis calibrate' the device to in an attempt to overcome the short coming of the device and read the temperature better.
My point was more that the FLIR misestimates the temperature of the glass, because really it's the spectral distribution of the glass radiated field that violates the assumptions built in the thermal imager.

I've been a bit quiet here because I'm reading some papers from the 1950's about how this stuff is measured, this person named Gardon has published a fair bit on the subject. I'm intending to come back with further information once I get some closure, likely in a few days.
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