# Power tube biasing



## Guest (Mar 28, 2009)

I'm curious if anyone has ever used an infrared thermometer to measure tube bias. I've got one at work and I'm tempted to see if a small variance in bias current results in a change of temperature on the envelope after a few minutes.
Are the tubes running cold or hot, literally? I would guess they are and that measuring that temperature would be helpful. I could see feeding that information back to a biasing circuit to keep the tubes at a constant temp.


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## Wild Bill (May 3, 2006)

konasexone said:


> I'm curious if anyone has ever used an infrared thermometer to measure tube bias. I've got one at work and I'm tempted to see if a small variance in bias current results in a change of temperature on the envelope after a few minutes.
> Are the tubes running cold or hot, literally? I would guess they are and that measuring that temperature would be helpful. I could see feeding that information back to a biasing circuit to keep the tubes at a constant temp.


Doesn't matter, Mr. K!

A change in bias current will of course change the temperature, due to more or less current flow through the tube, which is controlled by the bias voltage.

The heat is wasted energy which is radiated out of the tube elements, through the vacuum inside the glass envelope and then radiated into the air by the surface of the glass. 

The tube doesn't care a whit what the actual temperature of the glass may be, as long as the heat is radiated away fast enough that the tube elements don't break down or melt.

There is also some inertia in temperature changes. If the tube current goes up it takes some time before the increased internal temperature makes its way to the glass and the outside air. This would make a feedback circuit too slow to respond.

Also, you're talking a bit of circuit complexity to do what you're suggesting. What would be the advantage? It's much easier to measure the plate voltage, calculate the optimum idle current and adjust the bias voltage. No special temperature sensing circuit with parts is required. The idle current will only change very slightly as the months of tube life go by anyway.

Did you consider that when the tube is really working when the volume is cranked up that the tube will also get hotter? A temperature feedback loop would mean that after a while it would notice the higher temperature and would crank down the idle current, leaving the tubes biased very cold. The amp would totally change its biasing depending on whether you were playing loud or soft and for how long a time you were doing it.

Just doesn't seem practical to me.

:food-smiley-004:


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## Guest (Mar 29, 2009)

This would not be instantaneous, at all. It would be looking for slow, long term change, otherwise you get into oscillations. It would give you the luxury of running a bit on the hot side or cold or wherever , whichever you prefer and keep you there over the life of the tubes in a pretty consistent manner. It would be microcontroller guided so you could implement a small PID routine to compensate for varying environments.


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## Wild Bill (May 3, 2006)

konasexone said:


> This would not be instantaneous, at all. It would be looking for slow, long term change, otherwise you get into oscillations. It would give you the luxury of running a bit on the hot side or cold or wherever , whichever you prefer and keep you there over the life of the tubes in a pretty consistent manner. It would be microcontroller guided so you could implement a small PID routine to compensate for varying environments.


Well, as I said, due to different internal structures between brands the distance from the plate element to the glass would vary, meaning you'd have to recalibrate after a tube change.

I still don't see the benefit. The difference in tone over even a 20-30% change in idling current is not detectable by the human ear! I've found by experience you have to change it 30-50% before the difference becomes noticeable.

To have microcontroller like a 68HC something or other hold the idle current to within a fraction of a percent would be like the old engineering adage: "Measure with a micrometer. Cut with an axe!"

Still, chaque a son gout!:smile:

:food-smiley-004:


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## Guest (Mar 29, 2009)

Yup, I concur. I first started thinking about a dirt simple method to bias the power tubes, something even a non-techy could do and then wondered if the life of a tube could be extended by keeping a closer eye on the bias, maybe only at startup during idle or maybe even on demand.


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## Balou (Apr 1, 2008)

Wild Bill said:


> Still, chaque a son gout!:smile:
> 
> :food-smiley-004:


Mr Wild Bill, your french is getting better day after day LOL kkjwpw


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## Wild Bill (May 3, 2006)

Balou said:


> Mr Wild Bill, your french is getting better day after day LOL kkjwpw


Merci, mon ami!:smile:

J'ai etudier la belle langue dupuis quelques ans, a les 70's, a Hurst et Kapuskasing, aux 'hotels' et avec le 'rock et roll' et du bier!

J'etait un 'roadie'!

After those times, I ran into my high school professeur de francais. He parlay'd at me and I parlay'd right back!

He cocked his ear and then told me that I had picked up a bad accent!

I told him, yeah, I guess so. I had picked up a CANADIEN accent!

I have never forgiven him for teaching me only Parisian french. Why don't they teach us Anglos Quebec french? When I was with my friends from Pointe Claire my accent was a dead giveaway, like someone from Ste Therese coming to Toronto and speaking English like Winston Churchill!

This is a screwy country! Merci au le bon Dieu que le bier sont bon! :smile:

:food-smiley-004:


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## Jim DaddyO (Mar 20, 2009)

ahh yes, being from Sudbury originally, I am quite familiar with "cereal box" french! lol


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## Wild Bill (May 3, 2006)

Jim DaddyO said:


> ahh yes, being from Sudbury originally, I am quite familiar with "cereal box" french! lol


What most amazed me was the french in Hearst! We had met a local girl (at the church beside the bar we played, honest!) and her patois was so far out there that I couldn't understand a word. Far more than just 'joual'! 

Her french was so far from the Parisian I was taught or any I had ever heard from a Quebecois friend that we were reduced to sign language. In fact, she herself couldn't understand the french on the TV in our room!

:food-smiley-004:


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## WCGill (Mar 27, 2009)

I doubt you'd notice much difference in temperature between 40 and 44 ma bias current in an idling tube. A controlled experiment seems to be in order here, peut-etre.


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