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Author Topic: 811A Tube lifespan  (Read 16829 times)
W8JI
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« Reply #30 on: September 12, 2011, 06:37:32 AM »

I wonder how many newbie hams, after reading this thread, began to wonder why they didn't take up Civil War reenactments instead of ham radio.

Allen

I agree Allen.

Generally when something gets so far off track it is best to let it go.

Unfortunately there is a chance someone might actually the misinformation about tube life and fan speed, or worse yet how heat transfer works. It's a somewhat common myth that more airflow will noticeably increase tube life, or worse yet will increase dissipation capacity, with a glass envelope internal anode tube.

I think it is important to paint the correct picture, and it's great Jim and W6RMK chimed in with accurate technical points.

73 Tom
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W6RMK
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« Reply #31 on: September 12, 2011, 07:21:19 AM »


On the black shirt/black paint, we cannot paint a tube black and a black chassis is not going to do better than positive air flow and may make amp run warmer as more heat would be absorbed my chassis rather than vented out.

I don't think you understand.

Take two identical pieces of shiny aluminum sheet - same thickness, same size. Say a foot square and 1/16 inch thick.

Paint one flat black, leave the other natural aluminum.

Place them both out in the sun on a sunny day, in such a way that they both get the same angle and amount of sunlight.

Leave them out in the sun for an hour and see which gets hotter.

Dollars to donuts it's the black one, because it absorbs more heat and the natural aluminum one reflects it.

Actually, no.. for the same reason metal door handles and seat belt tabs get so darn hot in the sun (hotter than the air temp inside the car).  There's both an absorption and an emissivity factor at play, and shiny metal has high alpha/epsilon ratio (it absorbs better than it re-radiates).  If you were to take a sphere and put it in orbit in the sun, the temperature from low to high would be white paint(.17/.92=.18), black paint(.94/.94=1), shiny metal(a/e =.09/.03= 3).

Rough metal surfaces are sort of in the middle, as are chemical conversion coatings (most of them wind up with a/e =1 or thereabouts)

 


Quote
The same thing happens inside an amplifier. An amplifier enclosure painted black on the inside will absorb heat radiated by glass tubes while a shiny aluminum one will reflect it back into them.

This isn't new stuff. Way back in July 1968 there was an article in QST about using sweep tubes in amplifiers, and forced-air cooling. They mentioned how to make tube  chimneys from tin cans - and how any such chimneys had to be painted flat black to avoid reflecting heat back into the tube.


That's an interesting case, because one might think that the cooling air for the chimney is cooling the tube, but really, what it's doing is cooling the chimney, which is getting radiantly heated by the tube.  (and the side benefit of cooling the pins/seals as well)  

The previous comment about multiple tube setups is also well taken, one tube will radiate to another.  I don't know that it's a huge effect though.  The visual angle of one tube is fairly small from the viewpoint of another tube, so there won't be  any radiative dissipation in that path, but the tube still "sees" a lot of case at roughly ambient air temp.  A clump of 4 tubes is going to be worse.    I suspect it's one of those things that is usually designed by empiricism, not analysis, particularly in the ham market.

It's not like folks are taking any efforts to make the inside of the chassis non-shiny for instance.
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W8JI
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« Reply #32 on: September 12, 2011, 11:37:37 AM »

That's an interesting case, because one might think that the cooling air for the chimney is cooling the tube, but really, what it's doing is cooling the chimney, which is getting radiantly heated by the tube.  (and the side benefit of cooling the pins/seals as well)  

The previous comment about multiple tube setups is also well taken, one tube will radiate to another.  I don't know that it's a huge effect though.  The visual angle of one tube is fairly small from the viewpoint of another tube, so there won't be  any radiative dissipation in that path, but the tube still "sees" a lot of case at roughly ambient air temp.  A clump of 4 tubes is going to be worse.    I suspect it's one of those things that is usually designed by empiricism, not analysis, particularly in the ham market.

That certainly is true for me. I do it all by measurements.  :-) I can measure air pressure, air flow, noise, and temperature in a variety of ways. I sort of have developed a feel for things, but adjust while making measurements.

Quote
It's not like folks are taking any efforts to make the inside of the chassis non-shiny for instance.

Actually some of us do.  :-) Usually with a specified matte finish, or for regular steel zinc dichromate, that does not cause electrical problems like anodizing or painting.

Thanks for your input on this.
« Last Edit: September 12, 2011, 11:40:54 AM by W8JI » Logged
N6AJR
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« Reply #33 on: September 12, 2011, 11:41:51 AM »

Well, back to the original question.  You want the tubes  to last, tune them up properly, and quickly.  First You need to use a dual cross needle  swr/power meter, as it shows both the forward and reflected power in real time.  You will notice when you "peak the load and dip the plate" you will find the spot for max dip and max peak will also be where the max power output is.  so after a while you can learn to tune up quickly, by

use a matrix  to preset the knobs to the approximate  setting from past usage on that frequency

start with a lower power input level when first starting to tune ( like if the amp will take 60 watts in for max out, start at 10 watts till you are close)

Go for  the peak/dip/ max power out at the low level first.

be sure the antenna is a good match.

once you are close  bring the input power up to near normal usage range. (like 50 watts in this example) and re-tune ( really just tweak it again.)

don't try to get every last erg of power from the amp. It will not make a difference on the other end, but will certainly shorten the life of the tubes.

remember that 811's and 572b's are relatively cheap and those amps  are good for causal operation and "learning" about amps.

The tubes in a Henry amp, or AL80 or such are more expensive  but will make more power.

The tubes in my alpha 87A or several other ones  using tubes like a 3cx800a7 or 8877's or 8874 can run 500 bucks or more for a tube, so care and experience is needed here.

Another point is you will see that there is a point with most amps where the output stops increasing even though you add more power.  this is what I call flat topping.  on the low end , every time you increase the power 10 watts  the output goes up a hundred watts.  But when you get to 60 or so watts , an increase of 10 watts input only gives you a small  (5 or 10 watt) increase, and it is no longer a linear progression.  back down from that spot a bit,  you will have  about the same output, but you will not be over driving the tube.

I hope this helps.

I leave all the stuff those folks are talking about on thermal dynamics and such to the experts, I just  play and enjoy.
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W9KDX
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« Reply #34 on: September 12, 2011, 04:16:59 PM »

Well, back to the original question.   .......You will notice when you "peak the load and dip the plate" you will find the spot for max dip and max peak will also be where the max power output is.  

Go for  the peak/dip/ max power out at the low level first.

I have heard of "dip" but none of the instructions I have read, nor any of the Ameritron videos mention it.  I would greatly appreciate a guide to how and when this step is done.  I have an idea but do I go past the peak and then stop on the dip?  Do I back off from the dip and go to the peak?  Do I do this every time I go from plate to load of it it just done on one of these?

Also, is the Yeasu FT-950 subject to ALC overshoot, as W8JI mentions on his site and is there anything I can do about it?

Thanks
« Last Edit: September 12, 2011, 04:29:08 PM by KD0PLD » Logged

Sam
W9KDX
N3JBH
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« Reply #35 on: September 12, 2011, 04:33:56 PM »

Hey sam here is a video that shows that Dip....
http://www.youtube.com/watch?v=QoxiL-HDYtI&feature=related
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K4RVN
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« Reply #36 on: September 12, 2011, 06:18:34 PM »

Reply number 31
What Jim  N2EY said in reply #26 is exactly right according to my view. The black would be hotter. Although the black metal would emit heat the shiny metal would reflect the solar insolation rather than absorb it as well as black. That's why flat plate air or water heating solar collectors are usually black. If what was said were fact, then solar collectors would be shiny metal. They do employ mirrors to concentrate the solar insolation on a black absorber. If you measured the entering air temperature at the fan intake on an amp, and the leaving temperature at the outlet, the heat removed would be 1.08 x
the td. x the cfm of the fan at the exit. It is that simple in btu/hr. Many of the propeller fans for amps are rated at 0 inches static pressure so the actual cfm at the outlet from the amp after pressure losses would have to be measured, not calculated. Painting the inside of the cabinet black would certainly contribute to a cooler amp interior for a fan ventilated amp. Black absorbs heat very well and also emits heat well when swept with ventilation air. Just my opinions expressed here nothing personal intended. Sam sure has created a monster for a guy who doesn't own an amp yet. It has been interesting reading Sam. I like what N6AJR wrote and also Allen.

Frank
« Last Edit: September 12, 2011, 07:24:31 PM by K4RVN » Logged
W8JI
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« Reply #37 on: September 12, 2011, 06:56:29 PM »

Frank,

What you said is correct in this application.

In the 811 amplifiers, internal metal finish is spec'd to be a matte finish. That's really as good as black in this application. The metal plate that primarily parallels the anodes has airflow on both sides.

I'm not sure what is with one person and fan speed, but here are actual measurements:

http://www.w8ji.com/al811h%20fan%20speed.htm

You can see even with heavy duty cycles and high power, the fan is far more than enough. Part of this is the way the air routes along the back and front of the matte finished heat dam and RF shield near the tubes. Adding that shield lowered interior temperature and increased exhaust temperature a noticeable amount.

I did try black, and the change was not worth it over matte. Polished or shiny increased internal temperature, and reduced exhaust temperature.

73 Tom
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N2EY
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« Reply #38 on: September 12, 2011, 07:47:07 PM »

If you were to take a sphere and put it in orbit in the sun, the temperature from low to high would be white paint(.17/.92=.18), black paint(.94/.94=1), shiny metal(a/e =.09/.03= 3).

I think you're right.

But the point I was trying to make still holds if you painted one plate white and the other black - the black plate would get much hotter. Correct?
 
That's an interesting case, because one might think that the cooling air for the chimney is cooling the tube, but really, what it's doing is cooling the chimney, which is getting radiantly heated by the tube.  (and the side benefit of cooling the pins/seals as well)  

The previous comment about multiple tube setups is also well taken, one tube will radiate to another.  I don't know that it's a huge effect though.  The visual angle of one tube is fairly small from the viewpoint of another tube, so there won't be  any radiative dissipation in that path, but the tube still "sees" a lot of case at roughly ambient air temp.  A clump of 4 tubes is going to be worse.    I suspect it's one of those things that is usually designed by empiricism, not analysis, particularly in the ham market.

It's not like folks are taking any efforts to make the inside of the chassis non-shiny for instance.

The 1968 article was specifically about multiple-tube amps using sweep tubes near each other. Reradiating heat into each other could be a real proble, As you say, if the tubes were in black painted chimneys the heat would radiate to the chimneys and the air would cool the chimneys.

Of course nowadays it would make almost no sense to build a multi-sweep-tube amplifier. Besides poorer IMD, the rarity and cost of suitable sweep tubes makes it uneconomical.

73 de Jim, N2EY
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W9KDX
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« Reply #39 on: September 13, 2011, 04:19:48 AM »

Hey sam here is a video that shows that Dip....
http://www.youtube.com/watch?v=QoxiL-HDYtI&feature=related

Thanks,  I watched that one and he is not very good at explaining what he is doing when he gets the effect on the meters.  I'll try a few more times and see if he starts to make sense.

Any more help on dip would be great.
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Sam
W9KDX
W8JI
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« Reply #40 on: September 13, 2011, 07:27:49 AM »

Hey sam here is a video that shows that Dip....
http://www.youtube.com/watch?v=QoxiL-HDYtI&feature=related

Thanks,  I watched that one and he is not very good at explaining what he is doing when he gets the effect on the meters.  I'll try a few more times and see if he starts to make sense.

Any more help on dip would be great.

Do not focus on the dip. I can give a long accurate but boring technical reason why you should not focus on the dip, but the shortest way to say it is the dip is very often not the most reliable indicator of proper tuning in a grounded grid amplifier.

I'm sure that argument will come up though.  :-)

Focus on grid current peak, and on power output peak.  The ONLY attention you should pay to plate current is not grossly exceeding maximum limits.

If this was a neutralized grid driven amp, or was a very low intercept grid triode with very high isolation from input to output inside the tubes, THEN you could watch the plate dip more. With a grounded grid amp, and in particular tubes with high cathode impedances, the plate current dip is just a rough starting indicator. Never use it as a tuning indicator. If you read the manual, it is pretty clear the plate current dip is SECONDARY to watching power output and grid current.

Any advice to the contrary is wrong.

73 Tom
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W9KDX
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« Reply #41 on: September 13, 2011, 09:15:20 AM »

Thanks Tom,

Things specific to my amp are very helpful.  The manual never mentions dip, neither does the video.  If it is not important, I will be glad to skip it.

Sam
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Sam
W9KDX
N3JBH
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« Reply #42 on: September 13, 2011, 12:05:51 PM »

"Do not focus on the dip. I'm sure that argument will come up though.  :-)"

WHAT !!! How can you have any power of you don't dip your meter?   Ok sorry weird take on a Pink Floyd song  Roll Eyes
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WB9YRI
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« Reply #43 on: September 13, 2011, 05:07:08 PM »

Has anyone ever tried to submerge an 811 in transformer oil or PAO (oriented properly) and see how much power one of these tubes can take with "ideal" cooling of all external parts (glass envelope, pins, and cap).

The oil bath should tell us if it is possible to do something to the outside of the tube to improve its power handling capability, or reliability.  If we saw a substantial improvement, then it might be possible to experiment with the individual external components to see where the biggest improvement is.

It sounds like fun.

Thanks
Joe

 
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W8JI
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« Reply #44 on: September 13, 2011, 07:15:02 PM »

Thanks Tom,

Things specific to my amp are very helpful.  The manual never mentions dip, neither does the video.  If it is not important, I will be glad to skip it.

Sam

The manual does mention the dip Sam. It says to tune for maximum output power and peak the grid current with the plate control, and that plate current will generally dip at that point or something to that effect. :-)

But you are correct. It does not say tune for a dip....just that it happens when you tune for maximum output. :-)

The dip thing comes from when we had grid driven tubes and no power meters. I used to load my 807 transmitter and 6L6 transmitters by watching for the dip.   I didn't have a power meter for years because we were not required to know power into the antenna. That stuff all started in 1982 or 83.  :-)
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