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Author Topic: SB-200 How Low will it go?  (Read 25524 times)
W1BR
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Posts: 4179




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« Reply #30 on: October 31, 2014, 08:20:34 AM »

One possible problem with driving an amp to only 100 watts is if it`s loaded at that level output. For example if your rig has a power "spike"issue. Driving it with 10 watts gives you 100 watts for example. If the rig spikes to say 100 watts,that could cause the amp to arc if it`s loaded for 100 watts output only. Of course only some rigs have that problem. But an arc could cause damage in some cases. We still don`t know why the op wants to do this. Using a qrp rig maybe.

Load the amp to the full output power of the exciter, and then it should be fine when run with the exciter RF drive reduced for QRP operation.

Pete
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K6IGB
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« Reply #31 on: October 31, 2014, 09:20:11 PM »

To a point that would be OK, but won't the tubes' output impedance change when you go that much lower in power output. From my understanding the current draw will be different. Also, won't linearity suffer? I know it will when you overdrive it, but because it isn't a class A amplifier, doesn't it rely on a certain amount of excitation to keep things forward conducting? Or is that only with transistors, I can't remember. But back to my point..I am pretty sure if you tune for resonance, then lower the power output more than 60% or so things will change.
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W1BR
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« Reply #32 on: November 01, 2014, 06:27:00 AM »

If the devices are properly biased, then there is no problem with low drive. I'd expect that would be more of a problem with a CB amplifier or incorrectly biased tube amp. None are perfectly linear, and the gain may be a bit higher at lower drive and output levels.
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W3RSW
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« Reply #33 on: November 01, 2014, 08:02:35 AM »

Oh Jeeze, the demon haunted world keeps sneaking back in.

If a properly designed and built LINEAR amplifier is tuned and loaded properly for the maximum peak envelop power it will be driven to, then ALL waveforms driving it UP TO but not Over, that peak power will be faithfully reproduced in their waveform be it sine wave or complex.

That's why such amplifiers are called LINEAR.

Do not confuse the max. Rated pep of an amplifier for the max. pep accepted in derated service.
Both conditions if properly loaded and tuned and not overdriven for that maximum expected pep will linearally amplify all lesser driving waveforms up to the maximum pep expected.


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Rick, W3RSW
W1BR
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Posts: 4179




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« Reply #34 on: November 01, 2014, 04:49:44 PM »

per W8JI website:

Power Gain of AL811H Amplifier

The older AL811H (prior to drive resistor modification) with good tubes and stable power line, when properly tuned and operating in the linear region, has about 11 dB gain.
Note: If you tune the amplifier at reduced drive power, gain increases.
 
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K6IGB
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« Reply #35 on: November 01, 2014, 05:06:53 PM »

HuhHuhHuhHuh?Oh Jeeze, the demon haunted world keeps sneaking back in?HuhHuhHuhHuhHuh?

Not sure how to reply to that except to maybe say it was Halloween, so I deemed it appropriate?? I was just asking. and didn't mean to cause anyone to get upset. Just wanted to learn.
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W3RSW
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« Reply #36 on: November 01, 2014, 06:35:14 PM »

c.f. Carl Sagan
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Rick, W3RSW
W9OY
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Posts: 1843


WWW

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« Reply #37 on: November 01, 2014, 08:36:36 PM »

so then tell us what you learned because much good stuff was taught

73  W9OY
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W3RSW
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Posts: 606




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« Reply #38 on: November 02, 2014, 06:40:44 AM »

Beware the obfuscation of the message.
The facts are in the data, not to be re-confused over and over again. Grin
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Rick, W3RSW
W1BR
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Posts: 4179




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« Reply #39 on: November 03, 2014, 05:11:15 AM »

http://www.mirageamp.com/Product.php?productid=B-1018-G

Theoretically, the output follows the input in a truly linear straight line ... but in the real world very few amps are even close to being perfectly linear.

Here are some worst case examples (which I would not want to hear on the air!)

http://www.mirageamp.com/Product.php?productid=B-1018-G  Note the gain starts falling off well before 1/2 rated power output.

Pete
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G3RZP
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Posts: 1225




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« Reply #40 on: November 03, 2014, 08:47:39 AM »

Pete,

That one is in reality about a 60 or 70 watt maximum PEP output amp for AM/SSB, but 160 watts on CW/FM.....The mistake is when people think of it as a 150 watt PEP SSB amplifier, which it certainly isn't.

Tube amps are generally a bit better in having gain compression starting nearer to full output power and it is obvious that the gain of a tube amplifier will increase if the amplifier is tuned for maximum output at low power - and that linearity at higher powers will suffer.

One would not (should not?) expect something with devices that have a logarithmic transfer characteristic to show as good an IMD or gain compression/full output power ratio as something with 3/2 characteristic.
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WB6BYU
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« Reply #41 on: November 03, 2014, 09:57:16 AM »

The manual for the Mirage B-1018-G specifically states in several places that maximum output
for linear modes such as SSB is 110W PEP due to increasing distortion at higher power levels.

That's not a problem for CW or FM modes, however, where linearity isn't required.
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G3RZP
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« Reply #42 on: November 03, 2014, 10:20:51 AM »

I would have said that 110 watts was pushing it rather more than is desirable.....
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WB6BYU
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« Reply #43 on: November 03, 2014, 02:14:01 PM »

It may well be the case that even the 110W number comes from Marketing rather than Engineering, but at
least their manual doesn't pretend it can do 160W PEP on SSB.
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G3RZP
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Posts: 1225




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« Reply #44 on: November 03, 2014, 02:18:09 PM »

Ah! Yes! Marketing - numbers from 'vapourware!'

Been there - suffered that, got the ulcers......
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