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Author Topic: Where does the drive power go?  (Read 4834 times)
K0BT
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Posts: 29




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« Reply #15 on: February 19, 2018, 01:54:37 PM »

I have heard other's say the same thing. The parts are on my list! I have a couple small hamfest coming in a couple weeks. I would also like to get down to New Hampshire for the big one this spring. IN the mean time I will still be building my own if I come up with some cores. Carl

Lou at King Conversions removes the SB-220 input stages for his 6M conversions. He was very reasonable when I wanted to add 10M to a SB-221.
« Last Edit: February 19, 2018, 01:57:06 PM by K0BT » Logged
AB1ZI
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Posts: 9




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« Reply #16 on: February 19, 2018, 03:38:39 PM »

I sent him an email. Thanks for the tip, Carl
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W1BR
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Posts: 4196




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« Reply #17 on: February 19, 2018, 03:46:10 PM »

Coil inductances will be different for the SB-220,  your 572  tubes will have twice the input impedance of a pair of 3-500z.  Try for the SB-200 coil set.
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AB1ZI
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« Reply #18 on: February 19, 2018, 04:38:09 PM »

Good to know. I told Lou what I was using,  thanks. Carl
« Last Edit: February 19, 2018, 04:40:42 PM by AB1ZI » Logged
N3QE
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Posts: 5598




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« Reply #19 on: February 19, 2018, 06:56:21 PM »

I have never seen the answer or heard anyone ask this. If I drive an amp with 100 watts does some of that drive power combine with the output or is it lost? Carl AB1ZI

Back when the FCC power limit was DC Input Power, the answer for how to calculate power for grounded-grid amps, was written into the FCC rules and was also on the license exam.

Possibly this was a mid-60's update in response to those who abused the previous rule that did not require combining the drive power with DC power?
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AC2RY
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Posts: 761




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« Reply #20 on: February 19, 2018, 08:06:07 PM »


And how is that different from a solid state exciter???  The amp stills needs a tuned input.

It does not. FET is DIRECTLY connected to tube - being in series with it for both DC and RF providing current drive. And to drive FET from exciter, you do not need tuned circuit at all.

You should do some digging to find circuit like that. They a not really common, as they became feasible only a couple decades ago. Here are few examples:

http://www.cqham.ru/pa_ur5zd.htm
http://dirallian.narod.ru/sxema/LAMP/rd1325.htm
http://dirallian.narod.ru/sxema/LAMP/910511.htm
http://www.cqham.ru/forum/attachment.php?attachmentid=192470&d=1421737892





I am actually surprised why similar circuits are almost unknown among DIY crowd in the Western world.




« Last Edit: February 19, 2018, 08:11:46 PM by AC2RY » Logged
HAMHOCK75
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Posts: 640




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« Reply #21 on: February 19, 2018, 10:27:47 PM »

Those who have designed with dual gate FET's will recognize this immediately. The configuration is called a cascode amplifier which is what dual gate FET's are. One characteristic of the cascode amplifier is high isolation between input/output. Presumably that high isolation prevents the harmonics of the class AB amplifier from getting back to the input.

It is those harmonics that create the problem with solid state exciters. Solid state exciters have wideband SWR bridges but those bridges cannot distinguish between reflected power and harmonic power created by the amplifier which both go back towards the exciter so harmonics can trigger the solid state exciter to shut down just like reflected power. The purpose of input tuned circuits usually in the form of pi networks is similar. Pi networks are low pass filters and passively block harmonics from going back to the exciter.

Below is an image of a solid state exciter driving a grounded grid amplifier with no input tuned circuits. The yellow trace is the solid state exciter output, the blue trace is at the cathodes of the amplifier. Those ripples are the harmonics. As you can see some of the harmonics are starting to appear at the exciter. If they grow in amplitude, those harmonics will trigger the exciter to shut down.



It is a neat idea. AC2RY thanks for sharing.
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W1BR
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Posts: 4196




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« Reply #22 on: February 20, 2018, 01:35:17 PM »



I am actually surprised why similar circuits are almost unknown among DIY crowd in the Western world.

 


Interesting.  I wish those pages were able to be translated.  Has anyone posted measured IMD for one of those amps?
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