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Author Topic: How to get 4KW out of a pair of 4cx250s  (Read 1354 times)
KL7AJ
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« on: March 25, 2010, 08:45:30 AM »

This is not directly related to ham radio, but perhaps of interest to those wondering how people do things in the commercial/scientific world.

At Hipas Observatory, we had an ionosonde built by Stanford Research Systems.  The final amplifier in this unit put out 4 kilowatts of R.F. between 2 and 20 megahertz in 50 microsecond pulses.  The amplifier used a pair of 4CX250 tubes with special transverse flow cooling fins....internally nothing different than your garden variety '250s.

  How did we get this kind of power.  (This thing ran 24/7 for YEARS with no tube failures).

   It's all about duty cycle.  These tubes ran in what's called "Upside Down Class C"... a common radar trick.  The grids are pulsed with a high positive voltage which SATURATES the tube for about 90% of the R.F. cycle.  Unlike class C, where the tube is OFF for 90% of the cycle, or so, these tubes were FULLY ON except for the negative peaks of the R.F.

What this did is create a very LOW source impedance for the amplifier....just a few ohms.  This is what you want for a very BROADBANDED amplifier.  There was no tank circuit on this whatsoever, just a lowpass filter.

Since the PULSE duty cycle of this transmitter was only about .1%, the average dissipation was almost nothing.  The tubes ran cold to the touch immediately after shutting down!

The designt pretty high voltage on the plates of these...around 3500 volts, which is more than the max rating for the tubes, but less than the ABSOLUTE max rating.

Again, this is definitely a "Don't try this at home" concept...but interesting nonetheless.


Eric
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AD4U
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« Reply #1 on: March 25, 2010, 11:29:34 AM »

This reminds me of something an EE professor, who was also a HAM, said when I was in college back in the dark ages.  We were discussing the weird things that happen when (if) you approach absolute zero.  As a point of discussion this EE professor said, "You can get 1000 watts of RF (the legal limit back then) out of a 2N2222 transistor, if you can keep it cool.

I guess similar things apply in the application you described.

Dick  AD4U
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KF6QEX
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« Reply #2 on: March 25, 2010, 11:56:42 AM »

Quote
if you can keep it cool.
...so where can you get liquid nitrogen from in "small quantities" ? Smiley
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KD4LLA
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« Reply #3 on: March 25, 2010, 01:53:55 PM »

This question not directly related to ham radio either.  I read that the HIPAS Observatory, located near Fairbanks, Alaska was closed in December 2009?  As recent as two days ago there was an article in the Fairbanks Daily News-miner saying there will be an auction of the equipment there.  Maybe you are going to purchase some of the items sold?

Mike
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KH6AQ
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« Reply #4 on: March 25, 2010, 02:05:58 PM »

Interesting. The 4CX250 will run 1 amp of cathode current with 0 volts on the control grid and 350 volts on the screen.

Driving the grid positive must have made for some high grid current. This tube does not have - as far as I know - a low grid current interception cathode-grid alignment.

Was the heater run at 6.3 volts or was this cranked up for higher emission?
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KL7AJ
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« Reply #5 on: March 25, 2010, 02:22:59 PM »

Yep...I plan on making the little jaunt out there to see what I can pick up.

There's a great 432 antenna array there...4x4 array of 17 element KLM yagis.   I built it back in '94....maybe I can pick it up. Smiley

eric
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KL7AJ
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« Reply #6 on: March 25, 2010, 02:25:01 PM »

XG:

   Indeed...it had high grid current...but the duty cycle was VERY low. Smiley  So the average grid current was well within specs.

Eric
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W8JI
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« Reply #7 on: March 25, 2010, 04:43:38 PM »

Interesting. The 4CX250 will run 1 amp of cathode current with 0 volts on the control grid and 350 volts on the screen.

Driving the grid positive must have made for some high grid current. This tube does not have - as far as I know - a low grid current interception cathode-grid alignment.

Was the heater run at 6.3 volts or was this cranked up for higher emission?

I'm confused too. :-)

Quote
   It's all about duty cycle.  These tubes ran in what's called "Upside Down Class C"... a common radar trick.  The grids are pulsed with a high positive voltage which SATURATES the tube for about 90% of the R.F. cycle.  Unlike class C, where the tube is OFF for 90% of the cycle, or so, these tubes were FULLY ON except for the negative peaks of the R.F.

What this did is create a very LOW source impedance for the amplifier....just a few ohms.  This is what you want for a very BROADBANDED amplifier.  There was no tank circuit on this whatsoever, just a lowpass filter.

I can't imagine running a tube that way. The transmitters I did for pulse ran very high negative grid, and the grid was slammed with very high voltage positive RF pulses to turn the tubes on. The RF pulses were clamped on the negative swing of RF to load the exciter, and as the sine wave a swung positive the clamp tube turned off and the exciter was unloaded to drive the very low grid impedance. The clamp kept the grid from going very far negative when the grid impedance went high during negative cycles withou wasting power by swamping the grid. It was like the driver load shifted from the grid to the clamp tube and back.
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KU7I
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« Reply #8 on: March 25, 2010, 10:30:54 PM »

Wickedly cool post man!!!

Lane
Ku7i
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