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Author Topic: MLA-2500 swamping resistor  (Read 2918 times)
WA3RSL
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Posts: 40




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« on: January 11, 2013, 08:31:24 AM »

Hello all,

I'm building up a mini-gun station. My radio is a Elecraft KX-3 with a max output of 10 watts.

I plan to drive my MLA-2500 with it. I see in my MLA-2500 manual that the swamping resistor is 100 ohms at 25 watts.

Will I have any problems if I reduce the size of the resistor to get more output from my amp driving with a max of 10 watts?

Thanks

73

Frank

WA3RSL
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W8GP
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Posts: 215




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« Reply #1 on: January 11, 2013, 09:05:39 AM »

The swamping resistor was just a cheap and dirty way of smoothing out the input SWR without the expense and complexity of a "real" tuned input.You could add tuned input and possibly get a little more output, but I wouldn't expect more than a couple hundred watts. How much output are you getting now?
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WA3RSL
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Posts: 40




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« Reply #2 on: January 11, 2013, 09:13:51 AM »

Hello,

I don't have it hooked up yet. And I'd be happy with a couple of hundred watts out. You made me think of something else. With no tuned input, will I get a benefit of using the tuner on my KX3 while connected to my MLA-2500?

73

Frank
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W8GP
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Posts: 215




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« Reply #3 on: January 11, 2013, 11:35:56 AM »

The 8875s actually require very little drive because they have a very sensitive grid. In fact, that is the weak link of the tubes because the grid is easily damaged by excessive drive.But with only 10W, you can use the easy-drive characteristics to your advantage.The swamping resistor will potentially be dissipating up to 1/2 of your drive power so eliminating it and using a proper tuned input will definetely maximumize drive to the amp. In your case, using a tuner between rig and amp may help, especially if you eliminate the swamping resistor.But again, you'll just have to try it and see.Let me know the results of your testing.
                      Good luck, Greg
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WA3RSL
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Posts: 40




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« Reply #4 on: January 11, 2013, 05:28:56 PM »

Thanks Greg,

I'll let you know how it works when I get it going.

73

Frank
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KU7I
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Posts: 122




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« Reply #5 on: January 14, 2013, 01:17:13 AM »

I would like to hear about your test results also. Thanks. Lane Ku7i/JH1JCM US Naval Hospital Yokosuka Japan
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K9MHZ
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Posts: 424




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« Reply #6 on: May 03, 2014, 06:32:58 PM »

The swamping resistor was just a cheap and dirty way of smoothing out the input SWR without the expense and complexity of a "real" tuned input.

Yes it was....lame doesn't begin to describe it.  Man, you won't believe the spurs you'll get out of that, too.

To the OP....your Elecraft is a much, much classier piece of gear than that old POS Dentron linear.   Go a different route and get rid of that linear.  I had one years ago, and rejoiced the day I got rid of it.

Best,

Brad, K9MHZ

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W9WQA
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Posts: 136




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« Reply #7 on: May 04, 2014, 06:03:15 AM »

i use the internal tuner in my yaesu 450d to tune into my untuned input hb amp. it works fine and they say it sounds good.
on some bands/freqs its necessary to get swr ok.
some have said it should not be done but no one said why not!
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G3RZP
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« Reply #8 on: May 04, 2014, 11:49:01 PM »

In order to get the best IMD performance from a GG stage, the impedance from cathode to ground at the harmonics (especially the second) needs to be low. Thus a low Q pi network is the usual way to go. Tuners of the T network variety won't do this as well as a pi network right at the tube socket, and unless the tube input impedance is greater than 50 ohms, neither will an L network. The capacitance of the cable may be such that it suffices when the tuner provides a suitable load for the transmitter, depending on cable length and amplifier input impedance.

Whether or not it 'sounds good' is not only highly subjective, but also requires a good signal to noise ratio (30dB +) and a low in-channel IMD selective receiver at the far end to mean very much - and the adjacent channel needs checking. That has to have noise and QRM at least 35dB down for it to mean very much.
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W9WQA
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Posts: 136




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« Reply #9 on: May 05, 2014, 07:01:26 AM »

Quote
Whether or not it 'sounds good' is not only highly subjective, but also requires a good signal to noise ratio (30dB +) and a low in-channel IMD selective receiver at the far end to mean very much - and the adjacent channel needs checking. That has to have noise and QRM at least 35dB down for it to mean very much.
Posted on: Yesterday at 06:03:15 AM
Posted by: W9WQA

what you say is valid , interesting,but how does the typical ham, even with technical knowledge check it?
most of us depend on,"signal reports" and when it's, "you're 30 over here", its rare to here anyone say they are cutting power.

based on reading in these forums i have decided to workup input circuits ,one band at a time with some nice coil forms ive had around. just cant tie a switch in with my existing band sw.
might do it external. how critical can the distance be? close to the socket?

me,i run barefoot mostly.!
« Last Edit: May 05, 2014, 07:07:45 AM by W9WQA » Logged
K1ZJH
Member

Posts: 1074




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« Reply #10 on: May 05, 2014, 07:07:47 AM »

Sometimes grid load resistors are added to act like attenuator pads, to allow higher power exciters to be use without worrying about overdriving the amp.

Sometimes they are added for the same reason, but may also mask amplifier instabilities that may occur if the grid load resistors
are removed Smiley

I once owned a Hallicrafters HT-33 amp that ran a pair of 4CX300 tubes, AB1, grid driven. I removed the grid load resistor, and
I could easily drive it to over 600 watts CW output with the grid resistor removed.  I ran it with a home brew 80 - 10 meter
transceiver for several years. The five watt PEP output from the rig easily drove it to full power.

Pete
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