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bdcst
User Rank
Platinum
Power tetrodes and screen supplies
bdcst   7/25/2011 10:47:46 AM
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Just a little correction to one of the posters.  A power tetrode amplifier will put out at least 10% power even with the screen grid at zero volts as long as the plate circuit is active.  The screen will not cut-off the tube but it will reduce its gain substantially if not energized.  Thus in the case of the missing 13,000 watts, there was also no plate voltage/current on the tube regardless of screen grid or control grid excitation.

OLD_CURMUDGEON
User Rank
Platinum
Metyering Plate current...
OLD_CURMUDGEON   7/25/2011 8:09:39 AM
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In the 1960s, fresh from college, I was assigned the projects of designing HF power amps in the 2-30 MHz range, with outputs of 400w to 3KW PEP AM, CW & SSB).  In EVERY design, I used EIMAC transmitting tubes, and in EVERY design, I used a 50ohm, 50watt WARD LEONARD resistor in the C.T. to ground of the Plate supply transformer.  In this leg was where the PLATE CURRENT meter was circuited.  In thousands of installations (both land-based & maritime) across this entire globe, it was NEVER a cause for concern or problem.  Some of the tubes were EIMAC 3CX1500B (8877), 4CX1500B (8660) , 4CX3000B,3-400Z, etc.

William K.
User Rank
Platinum
Re: this wasn't a metering problem!
William K.   7/22/2011 4:07:13 PM
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The screen voltage had been set to "almost zero" because that was the only way to have the screen current within it's limits. IT would then seem that the plate current was much less than normal, but indicated normal, because of the missing shunt resistor. Grounding the screen grid is a fairly common way of limiting power input to a tube. My concern would have been about the control grid drive, since that would perhaps still be present. I am not familiar with the control grid ratings of a 4CX10000A tube.

bdcst
User Rank
Platinum
Re: this wasn't a metering problem!
bdcst   7/22/2011 4:02:16 PM
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It was both a design and a metering problem.  Metering in that the open shunt would not stop the meter from displaying a reading even if false.  Design issue if you assume the metering shunt was not sized correctly for the continuous load current.  Of course if you add some EMP from lightning, quite common at a tower site, all bets are off.

And as I said earlier, newer designs moved the ammeter to the low side of the HV supply for both safety and lower stress issues.

William K.
User Rank
Platinum
Re: this wasn't a metering problem!
William K.   7/22/2011 3:57:31 PM
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Putting a meter in series to read the plate current is entirely reasonable and safe if it is done correctly. The way to do it is to make certain that the meter is physically isolated from everything else, which it sounds like that is what was done, with the meter placed inside the enclosure, behind a window, and probably supported by the carrying strap. So that allows the meter to float at the circuit voltage with no problems. Ofr course, the range had to be set with the power off, and no range changing with the power on. But it does work. No, it would not satisfy a safety-fixated person who can only go by one script, but it would certainly allow reading the plate current safely.

bdcst
User Rank
Platinum
Re: this wasn't a metering problem!
bdcst   7/22/2011 3:57:09 PM
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Yes, and that's a gotya too if you don't understand how these transmitter meters were set up.  The  plate voltage meter was reading a sample from the HV power supply before the ammeter, not the tube side.  So it indicated plate voltage was available even though the opened shunt kept it from the tube itself.

I've run into a lot of situations where you really have to know the machine, read the print carefully before jumping to conclusions.  I've seen the opposite where a voltmeter read nothing yet the HV was there ready to kill on contact. I suspect that's why the voltmeter in this model transmitter is tied directly to the output of the power supply as an indication that dangerous potentials are indeed available inside the cabinet.

jljarvis
User Rank
Gold
Re: this wasn't a metering problem!
jljarvis   7/22/2011 3:46:58 PM
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I can't get at the original article to verify this, but nowhere did I read, "the plate voltage indicated zero".  What I saw was "the screen voltage measured zero".  

 

It might have been my mistake.   Still, I wouldn't call an off-air trouble call "a metering problem".   If that shunt evaporated somehow, it might better be considered a DESIGN problem!  Or a component selection problem.  

 

 

bdcst
User Rank
Platinum
Re: this wasn't a metering problem!
bdcst   7/22/2011 2:27:16 PM
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It most certainly was a "metering" problem.  Glad the commenter is no longer working on transmitters!  The loss of the current shunt on the PA tube high side ammeter means there was only a few milliamps of current to a tube expecting at least 1 ampere.  Thus the input power was near zero.  It had nothing to do with lack of screen current.  In fact, had the screen voltage been up around its normal 400-800 volts it would have resulted in a blown fuse, burned up screen supply or maybe even a burned up screen grid in the power tube.

Without plate voltage, a power tetrode tube will attempt to draw operating current through the screen circuit thus overloading it.

With an open shunt, the sensitivity of the ammeter would have increased drastically so a few milliamps would have driven it way upscale.  Obviously it gave the false impression that there was real input power when in reality there was just about none.

As for foolishly using a Simpson 260 as an HV side ammeter, it was indeed foolish.  More modern tube rigs nowadays eliminate high side metering and instead are in the return path, grounded end of the power supply.  Furthermore, the shunt resistors even on this low voltage end are physically large and external to the meter.  Finally, a hefty zener diode is often across the shunt circuit limiting the voltage should the shunt open, yet another safety feature.

jljarvis
User Rank
Gold
this wasn't a metering problem!
jljarvis   7/22/2011 2:03:39 PM
NO RATINGS
Remember the early part of the story?   RF Power out was zero, and listeners couldn't hear the station?  

The problem wasn't the screen metering.  The problem was that the current shunt was in series with the screen supply, and the 4cx10,000 had no screen voltage, ergo no output.

One wonders whether that meter shunt vaporized, or there were pieces within

the transmitter.   In addition, that Simpson meter isn't rated for 13kv withstand, so there was a potential life safety issue here.   Most likely, however, the screen voltage was in the 600-800V range, and the meter wasn't exposed to the Vp.

 

Color me:  recovered broadcast engineer.   Paid for college working on AM/FM's in the metro NYC area.  ♠

Ivan Kirkpatrick
User Rank
Platinum
Test Instrument calibrations
Ivan Kirkpatrick   7/20/2011 1:12:41 PM
NO RATINGS
It is a good idea to make sure your instruments and testing tools are calibrated and verified as working correctly.

In my US Navy career as an engineer we frequently, as in every 6 months I recall, had to test and calibrate every pressure gauge and thermometer.  We kept records of this in the maintenance logs for historical purposes.  

It was hard to tell that this actually made a big differnece in the overall efficiency of the engineering plant but it was good to know that we were operating the equipment with good data and presumably at the best conditions.

This 1200 PSI steam plant was really an interesting machine.  It made a big difference if we were in cold sea water or warmer water.  the main condenser vacuum could vary by a few inches of mercury, 29 for very cold and about 26 or so in warmer water like the Indian Ocean.

I learned a lot about good engineering operations adn studied the design of that power plant extensively.  I guess it was kind of a 1950's classic. 



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