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Author Topic: Lowering output power from SB200  (Read 14495 times)
KE7VOB
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Posts: 22




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« on: February 23, 2009, 12:46:19 PM »

Hello group,  

I just put up a dipole for 40 today and would like to try my amplifier with it. I don't want to run the full 600 watts to it right away.

The problem is that my FT-757 doesn't have any provision for lowering output power in SSB without taking the cover off and messing around.

I did see a mod online that allows you to use the drive knob for output power in SSB but don't really want to cut anything inside my radio.

Is there a way that is safe to load the amplifier at lower power?
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AA4PB
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Posts: 15067




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« Reply #1 on: February 23, 2009, 01:12:23 PM »

On SSB, lowering the mike gain will lower the power output.
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Bob  AA4PB
Garrisonville, VA
WB2WIK
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Posts: 21837




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« Reply #2 on: February 23, 2009, 01:46:18 PM »

You can't "load the amplifier" by tuning up on SSB anyway.

You need to use a carrier, like CW with the key closed.

The Yaesu can turn its CW output power all the way down to 5W or less.  That should be more than enough power control to let you load up at reduced power if you wish.

However, to operate SSB, you should *always* tune the amplifier up at maximum power; to tune up at reduced power and then switch to SSB is never a good idea.

WB2WIK/6
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KE7VOB
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Posts: 22




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« Reply #3 on: February 23, 2009, 02:14:45 PM »

Ok  I follow you guys and I do load the amplifier in CW mode.

What I want to do is after loading in CW, run in SSB without the full amplifier output.

My radio as I stated does not have variable SSB power output. It is 100 watts period unless you do a mod that allows you to use the drive knob for SSB power output.

I was just wondering if there was another way. As I understand it, mic gain doesn't change output power. Please correct me if I am wrong.
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K5MBV
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Posts: 265




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« Reply #4 on: February 23, 2009, 04:07:37 PM »

The manual says not to load up at full power, so you
are trying to do the right thing. All you have to do
is detune the SB200 input to the level you need so
it's not overdriven until completely tuned up. You can
use AM or CW to get the carrier for tuning.
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WB2WIK
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Posts: 21837




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« Reply #5 on: February 23, 2009, 06:54:43 PM »

>RE: Lowering output power from SB200       Reply
by KE7VOB on February 23, 2009    Mail this to a friend!
Ok I follow you guys and I do load the amplifier in CW mode.

What I want to do is after loading in CW, run in SSB without the full amplifier output.

My radio as I stated does not have variable SSB power output. It is 100 watts period unless you do a mod that allows you to use the drive knob for SSB power output.

I was just wondering if there was another way. As I understand it, mic gain doesn't change output power. Please correct me if I am wrong.<

::You are 100% correct.  Mike gain does NOT change output power with the Yaesu.  If you turn down the mike gain, your peak power remains the same but overall "punch," talk power, readability and clarity will suffer greatly.  Don't do that.

First, why do you need to run less power?  The SB200 only runs 600W PEP output anyway.  If you need "less power," shut it off and run 100W.  The difference between 600W and 100W is exactly 7.78 dB.  Not huge.  It seems prudent, practical, and perfectly normal to run "one or the other" (either 100W or 600W, no steps in between) because the actual difference is only about an "S" unit, anyway.

Second, if for some reason you really NEED to run a power level between 100W and 600W, make the modification to the Yaesu!  Otherwise, you could build an attenuator (like 3 dB, 50 Ohms, 50W) which will reduce drive power to 50W and reduce the amp's output power to about 300W PEP.  If you want some advice about building the attenuator, just ask and I'll provide you with the component values.

WB2WIK/6
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K5MBV
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Posts: 265




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« Reply #6 on: February 23, 2009, 07:47:09 PM »

I can't imagine how lowering mic gain doesn't reduce the
power. That would mean that with no audio there
would still be full power. Hunh Unh. Doesn't work that
way if it's SSB. You're getting some strange answers.

Just reduce the mic gain to reduce SSB power.
.... Good Grief!
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KE7VOB
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Posts: 22




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« Reply #7 on: February 24, 2009, 07:48:15 AM »

Okay sounds like the mod to the radio is easy enough to do. I agree that there isn't a massive difference between 100 and 600 watts. I just wanted the flexibility.

STEVE,  I heard you on 40 last night and tried to call you right after your QSO but you were gone, or just couldn't hear me.

Thank you for the replies

Dave
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WB2WIK
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Posts: 21837




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« Reply #8 on: February 24, 2009, 08:39:53 AM »

>RE: Lowering output power from SB200       Reply
by K5MBV on February 23, 2009    Mail this to a friend!
I can't imagine how lowering mic gain doesn't reduce the
power. That would mean that with no audio there
would still be full power. Hunh Unh. Doesn't work that
way if it's SSB. You're getting some strange answers.

Just reduce the mic gain to reduce SSB power.
.... Good Grief!<

::You're wrong about this.  If you turn the mike gain all the way down, you'll have no modulation power left, only the suppressed carrier.  But crack it open a little tiny bit to the point where you have modulation, and use a scope or true PEP wattmeter to indicate PEP output (not a regular average reading wattmeter, like 99% are) and you'll see that the moment you have ANY power output, your PEP will be just about the same as if you turn the mike gain up all the way.  The only difference is the average power will be MUCH lower (maybe close to nothing) and you won't sound good at all on the air.

Once you reduce mike gain to the point where your ALC is no longer working, the rig has lost control over everything it's supposed to control.  With ALC operating normally (normal mike gain, full modulation), your peak-to-average ratio is probably between 2:1 and 4:1, as it should be.  With ALC not operating at all (mike gain too low), your peak-to-average ratio can be 100:1 (100W PEP, 1W average).  As such, you won't sound good, and won't have enough "punch" to get through interference or noise, but the PEP will still be normal.

That's the way MOST SSB rigs work, regardless of model.

Turning down the mike gain to "reduce power" is a bad idea, because it defeats the rig's ALC.  The ALC is what makes the rig sound full and well modulation, and easy to understand.  You don't want to defeat it.

Much better to use a 3dB or 6dB resistive attenuator between the rig and the amp (bypassed for receiving, so as not to sacrifice received signals, if you wish) and leave the mike gain set normally.

WB2WIK/6
 
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WB2WIK
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Posts: 21837




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« Reply #9 on: February 24, 2009, 08:43:25 AM »

>RE: Lowering output power from SB200       Reply
by KE7VOB on February 24, 2009    Mail this to a friend!
Okay sounds like the mod to the radio is easy enough to do. I agree that there isn't a massive difference between 100 and 600 watts. I just wanted the flexibility.

STEVE, I heard you on 40 last night and tried to call you right after your QSO but you were gone, or just couldn't hear me.

Thank you for the replies

Dave<

::Call me next time.  I had to leave the station to make an important phone call at one point, that's probably when you called!  (I turned the volume all the way down and removed the headphones, sorry.)

The "flexibility" isn't very much in your particular case.  I'd either use the amp, or not.  Anything in between is really splitting hairs. Although my rigs all have the capability of reducing output power on SSB without sacrificing ALC or modulation quality at all, I never do that to reduce power with an amplifier.  I figure if I can reduce power, I might as well put the amp in STBY or OFF, stop making the electric company rich, and enjoy the quiet.  :-)

WB2WIK/6
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K5MBV
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Posts: 265




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« Reply #10 on: February 24, 2009, 08:58:46 AM »

I have to tell you that you are mistaken about this
simple question. The answer to reducing power with SSB
is just turning the mic gain down. You must be
confusing some parts of AM theory with your answer
which sounds very technical, but is dead wrong.

Forget the carrier. It's not there for all purposes.
All the useable power is in the single sideband which
is controlled by the mic gain.

ALC is another matter entirely and isn't what is being discussed.

Ken
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KB9CRY
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« Reply #11 on: February 25, 2009, 05:35:51 AM »

unless you do a mod that


Do the mod


Actually I've never seen a radio that doesn't have a Mic Gain control.  Are you a CBer?
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KB9CRY
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« Reply #12 on: February 25, 2009, 08:29:32 AM »

Belay my previous comment.  Posted too early in the morning without coffee.  Sri.
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WB4JZY
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Posts: 76




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« Reply #13 on: February 25, 2009, 11:17:43 AM »

Wow....this is a strange thread.  I have built a few SSB exciters from scratch, and with all of them I could vary the output power with the mic gain.  Maybe I was doing something wrong.
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WB2WIK
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Posts: 21837




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« Reply #14 on: February 25, 2009, 03:14:34 PM »

>RE: Lowering output power from SB200       Reply
by WB4JZY on February 25, 2009    Mail this to a friend!
Wow....this is a strange thread. I have built a few SSB exciters from scratch, and with all of them I could vary the output power with the mic gain. Maybe I was doing something wrong.<

::Yes, actually, you were, but you're sure not alone.  This would make good fodder for a new eHam.net article, "Why you should never turn down your mike gain to reduce power on SSB."  And it's true, you really never should.  This is NOT the way to reduce power and still maintain any effective communications.  

The whole reason that most rigs made in the past 20 years do have a separate PWR LEVEL control, which reduces power output on SSB without impacting modulation quality, is because this is the right way to do it.  It allows the ALC to still work properly and provide full modulation with a high average-to-peak ratio when running reduced power, which can never be achieved by "turning down the mike gain."

Article forthcoming!  With detailed explanation and signal analysis showing clearly why turning down the mike gain is not a good idea.

73

Steve WB2WIK/6
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