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Interstage RF Impedance Matching




 
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Author Topic: Interstage RF Impedance Matching  (Read 291 times)
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N1BCG
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« on: November 25, 2019, 12:42:27 PM »

The attached circuit from a Collins 300G shows how a dual 807 driver is connected to a dual 810 final. The plate circuit of the 807s is connected to a tap on a parallel tank circuit connected to the grids of the 810s.

What I'm not seeing is how this arrangement can step-down the 3100Z 807 plate impedance (412V / 136mA) to the 1900Z 810 grid impedance (210V / 110mA).



* 300G_IPA.jpg (173.4 KB, 775x380 - viewed 73 times.)
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N1BCG
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« Reply #1 on: November 25, 2019, 02:38:28 PM »

You forgot the "K" factor:

Estimated RL (based on E, I and K) = E / (K * I) where K is the class of operation of the tube (class A, AB1, AB2, B or C)

Values of K:
Class A: 1.3
Class AB: 1.5-1.7  
Class B: 1.57-1.8  
Class C: 2.0

You're actually matching 1548Z up to 1900Z not 3100Z down to 1900Z.

K cars, Kmart, and K factor. All life-changers.
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W1RKW
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« Reply #2 on: November 25, 2019, 05:19:27 PM »

you forgot K-Pax
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Bob
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Home of GORT. A buddy of mine named the 813 rig GORT.
His fear was when I turned it on for the first time life on earth would come to a stand still.
W1ITT
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« Reply #3 on: November 25, 2019, 05:24:40 PM »

Neither of the examples cited represents much of a mismatch, less than 1.5 to one in either case.  Back in the 1940s, when the Collins 300 series was designed, people were not as OCD about SWR, particularly if it did not involve transferring power through a long transmission line.  The object here was to generate some RF voltage and tickle the grid of the next stage fairly efficiently and get on with life.
Now that every Tom, Dick and Hiram has an SWR bridge or, even better, a 50 buck NanoVNA, we are at risk of spending too much time tweaking ourselves down to inconsequential increments.  In the case of the Collins 300 series, there is no shortage of drive power so Arthur's boys just had to get it pretty close.
Having said that, I will admit to being as bad as the rest of them.  When I was building FM transmitting arrays, I tuned to 40 db return loss, just because I had the equipment to measure it accurately, and just because I could.  When I was tuning HF curtains for the BBC, everything had to be under  1.35 across a whole octave, even outside of the SW broadcast bands.  The Marconi transmitters didn't give a hoot either way.  We worry too much about this stuff.
de Norm W1ITT
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DMOD
AC0OB - A Place where Thermionic Emitters Rule!
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« Reply #4 on: November 25, 2019, 10:23:33 PM »

Look at the positions of the respective taps on the coil.


What does feedback do to the input impedance of an amplifier?


Phil - AC0OB
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"What kind of Koolaid do they make you drink in the Physics Department?" Charlie Epps to Dr. Larry Fleinhardt, NUMB3RS   Smiley
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