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Author Topic: • Favorite Old Wives' Tales •  (Read 32452 times)
W7SMJ
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Posts: 126




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« Reply #165 on: November 19, 2011, 07:26:48 PM »

  by reading between the lines. 

What does that mean???

I don't have access to the latest and greatest Care and Feeding publication so I can't comment on that.  But I was able to find this on CPI's website: http://www.cpii.com/docs/related/23/Tube%20Maintenance.pdf

I took note of three things:

1)"The operator, by adjusting the filament voltage, can control the operating temperature.  Each tube is unique; while one tube may make full operating power at a filament voltage of 7.3 V, a replacement device may require 7.4 V to attain the same power."  This tells me that you can't make wholesale generalizations about reducing filament voltage.

2) It is very clear that as you lower filament voltage you must monitor the tube to ensure it is meeting all specifications.  You can't just say "reduce your filament voltage by 0.25V for greater tube life".

3) I consider this to be the most damning to your argument: "A power tube operated in this manner will generally yield life 50 percent greater than a tube run continuously at rated filament voltage."  So if you lower filament voltage in the way CPI dictates and sanctions you can only expect tube life to be 50% greater.  This doesn't fit your formula either...


73,
Scott
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G3TXQ
Member

Posts: 1530




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« Reply #166 on: November 20, 2011, 12:50:34 AM »

RE:  Rich's antenna tuner, the 1500-ohm antenna, and the location of the balun.  With  1100W from my pair of 3-400Zs, there is 234v across the balun on the input.  If I move the balun from the input of the tuner to the output there will be 1284v across the balun.  Would it be a good idea to move the balun to where there would be more voltage across it?
What was the degree of imbalance in the load? The relative CM voltages at input and output change, depending on the degree of imbalance!

Steve G3TXQ
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G3TXQ
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Posts: 1530




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« Reply #167 on: November 20, 2011, 01:14:32 AM »

His formulas are only good for an L, T or Pi.

If by that we mean unbalanced L, T or Pi, I agree.

The bottom line is, the only time you can move the balun and use a poor balun on the input, is when you really don't need any balun at all. That makes sense when we think about it. Even a poor balun system works fine when it is not needed.

You follow what I am saying about the balanced network? The series reactance in the return line has the same current as the reactance in the output line, and drops the same voltage with the same phase shift. This pretty well balances the network even if the lower leg is just tied to ground. If you run a Monte Carlo analysis with tolerances in components, and in the load balance  limits, you will see you might just as well not use the balun at all. :-)

I guess running the numbers on one particular load and one specific balanced network doesn't qualify as a Monte Carlo analysis Smiley

However, when I dig into that specific case it's clear than any reduction in the required Zcm of the balun arising from placing it at the input progressively disappears as the load becomes unbalanced. So if you were designing a tuner for worst-case load conditions there would be no performance advantage by placing it at the input, but a significant cost penalty.

In retrospect it would have been helpful - and saved me a few hours work - if Roy's note had made clear that it specifically applied to unbalanced tuners. Then again, I learned a lot working through the numbers Smiley

73,
Steve G3TXQ


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AG6K
Member

Posts: 1




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« Reply #168 on: November 20, 2011, 05:04:14 AM »

RE:  Rich's antenna tuner, the 1500-ohm antenna, and the location of the balun.  With  1100W from my pair of 3-400Zs, there is 234v across the balun on the input.  If I move the balun from the input of the tuner to the output there will be 1284v across the balun.  Would it be a good idea to move the balun to where there would be more voltage across it?
What was the degree of imbalance in the load? The relative CM voltages at input and output change, depending on the degree of imbalance!

Steve G3TXQ

  The load is a Hertz antenna (I.E., an end-fed halfwave) so the problem was the RFV-drop in the 1.4m lead from the L-network to the earth connection.  Thus, as long as there was a device that would confine nearly all of the current to the earth lead there would be no RF getting back to the TS-850S radio.  In order to do this I put a 1:1 choke balun on the input to the antenna tuner and parked the tuner on a wood shelf to insulate it. I could have put the balun on the output but there would have been 5.5x as much RFV across the balun. Since experience has taught me that baluns are not high-potential friendly, I put the balun on the input and floated the tuner. 
Rich, ag6k
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AG6K
Member

Posts: 1




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« Reply #169 on: November 20, 2011, 05:48:56 AM »

  by reading between the lines. 

What does that mean???

  It means that there is nothing written there that slam-dunk guarantees max emissive life when the tube is operated at the upper limit of its filament V. range . 

Quote
I don't have access to the latest and greatest Care and Feeding publication so I can't comment on that.  But I was able to find this on CPI's website: http://www.cpii.com/docs/related/23/Tube%20Maintenance.pdf

I took note of three things:

1)"The operator, by adjusting the filament voltage, can control the operating temperature.  Each tube is unique; while one tube may make full operating power at a filament voltage of 7.3 V, a replacement device may require 7.4 V to attain the same power."  This tells me that you can't make wholesale generalizations about reducing filament voltage.

  One can quite safely say that operating the filament potential 2% higher than the PEP decrease point is a practice that will not damage the Th or unnecessarily decrease the life of the ditungsten-carbide emissive layer.

Quote
2) It is very clear that as you lower filament voltage you must monitor the tube to ensure it is meeting all specifications.  You can't just say "reduce your filament voltage by 0.25V for greater tube life".

  But we can say:  operate a Th-W filament 2% above the PEP decrease point and you will not be flushing your electron-tube $$ down the crapper.  However, if the tube is slightly gassy and one inadvertently operates the filament below the minimum rated V, the resulting Th degradation by O2 and N2 can be reversed by operating the tube at the specified filament-V. -- so it's not like spilled milk. 

Quote
3) I consider this to be the most damning to your argument: "A power tube operated in this manner will generally yield life 50 percent greater than a tube run continuously at rated filament voltage."  So if you lower filament voltage in the way CPI dictates and sanctions you can only expect tube life to be 50% greater.  This doesn't fit your formula either...

  My Ham-friend in San Diego, who gets 22k-hours out of 4CX15.000As in the FM broadcast transmitter he maintains, pretty much confirms that Eimac's supposedly only theoretical statement that each excess 3% filament-V = 50% decrease in emissive life is not just  "nonsense"  Scott.
•••  Rich, ag6k

Quote
73,
Scott
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VK4TUX
Member

Posts: 37




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« Reply #170 on: November 30, 2011, 04:49:12 AM »

 
AG6K has moved to QRZ.COM Forum: Amateur Radio Amplifiers


________________________________________
Richard asked to inform those that may wish to pursue technical comment.
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