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Author Topic: Getting started with new amp  (Read 4292 times)
KJ6TSX
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Posts: 116




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« on: August 28, 2012, 09:24:13 PM »

I just bought a Dentron 1000b amp and want to connect it to my FT-950. What is the best way to do this in regard to the relay cable?

Now for the dumb questions  Grin since I just broke the bank buying the amp (very small bank) I would like to keep my output below 300 watts in till I can upgrade my tuner. what is the best way to do this? lower the radio output and tune the amp normally or drive the amp with 100 watts and lower the current and then peak the drive to that lower current?

Thanks in advance for the help
George
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KB4QAA
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« Reply #1 on: August 28, 2012, 10:30:12 PM »

Look at the operator manuals for each item.
-What are the voltages for each?

Tune up the amp at the highest input power you plan to use.  You can always lower the input power by adjusting the drive.  If you tune up at a lower power then go above it, you will have less protection from splatter, etc.
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KM3F
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« Reply #2 on: August 28, 2012, 11:17:22 PM »

I think you can run safely at 'somewhat' above 300 watts but...but you must have the system tuned for low SWR first to prevent high RF voltages developing in the tuner from an out of match condition going above the tuner's component ratings.
If you do, it's at some risk if something changes.
Good luck.
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KC4MOP
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« Reply #3 on: August 29, 2012, 04:20:20 AM »

A very nice amp BTW. Gives that boost you need without a lot of dollars. I do not want to add to your bank account disappearing, but the next need is a dummy load that can handle legal limit. That way if you ever want to go "all the way", the dummy load will always be there.
The tuner should be replaced with one that can deal with the new amp. A 300watt tuner can flash over even if you are only close to its ratings if there is the slightest of SWR. Certain bands reacting with your antenna system and tuner can cause flash over and damage.
Some tuners include a dummy load.
Tuning of a linear amplifier must first be done at its full rated power... This model amplifier does not have an ALC. This is an adjustment that will throttle back your transceiver to regulate how much RF the amplifer will output to the tuner/antenna.
So, you will have to adjust the ALC in your FT950 to limit the the output of the transceiver so you do not go above 300 watts. It's getting complicated here. I would suggest getting the dummy and tuner that is rated for the amplifier or legal limit rating, then you will not have to go through all of the chatter I typed about ALC's
I also see that this amp uses 4 (old days of TV) TV sweep tubes. So tuning it properly is important.
Here is a source for the manual, if you do not have it.

http://bama.edebris.com/manuals/dentron/gla1000b/

There doesn't seem to be any links to free downloads.
From what I am reading from googling Dentron 1000B which is a GLA 1000B, that tuning is a little hard to understand and it must be done quickly!!!
And PLEASE use a dummy load  Shocked  PLEASE........to keep interference to others using the Ham bands to a minimum.

Fred
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K8AXW
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« Reply #4 on: August 29, 2012, 09:28:40 AM »

George:  First, you should do a bit of research here on eHam.com, Amplifier Forum.  This question has been asked several times and the resultant information is invaluable.  Please don't read this statement the wrong way.  I'm just pointing out that there is a LOT of information here on this particular subject that you should take advantage of.

Now specifically, you have a major problem.  The correct way to tune a linear is to tune it for FULL output and then if you want to operate at a reduced power you back off on the exciter drive.  But, the amp must first be loaded to full output.

The problem comes in with the tuner.  You can load it into a dummy (if you had one) and then switch to the antenna but since the tuner can't handle the full power you have the problem I mentioned.

If you have a resonant antenna (that doesn't need a tuner to give you a suitable SWR) then you can bypass the tuner and you're good to go with tuning full bore and then backing off the drive.  Your manual should give you he correct way to tune your amp.  Good luck.

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W8JX
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« Reply #5 on: August 29, 2012, 09:45:11 AM »

I think you can run safely at 'somewhat' above 300 watts but...but you must have the system tuned for low SWR first to prevent high RF voltages developing in the tuner from an out of match condition going above the tuner's component ratings.
If you do, it's at some risk if something changes.
Good luck.

I see no problems up to 400 watts out or so. The key here is to limit key down time during tune to about 4 to 5 seconds at high power then pause 5 sec or so and do it again till tuned. In SSB operation the sweep tubes in that amp can easily handle 400 watts out PEP.  RTTY, SSTV or digital is different and I would say 250 watts or so max. As mentioned further up thread, adjust drive for desired output level. When tubes are good you should be able to make 400 watts out with 40watts or less drive. If it takes more the tubes are getting soft. Also amp uses indirectly heated cathodes so wait 20 sec or so from turn on before applying drive to amp.
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N6AJR
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« Reply #6 on: August 29, 2012, 12:12:44 PM »

with all the stuff about peaking the load and maxing the dip, if you just look at your output on a meter and tune for max output then look at the grid current and turn down the radio output to get below the max grid current. the peak and the dip both occour at the point of max output from the amp.
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KJ6TSX
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« Reply #7 on: August 29, 2012, 07:19:19 PM »

Thanks Everyone for your help

Sounds like what I should do is start lookinn for a antenna tuner, any suggestions on a good tuner, I have heard good and bad about MFJ but I really don't know. Figure I should get something at the legal limit would be smart
Thanks
George
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VE3FMC
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« Reply #8 on: August 29, 2012, 07:28:24 PM »

I do not know the keying voltage of that amp. However I would suspect it is above the ratings that the FT-950 can handle.

Therefore you may need a keying relay in line so you do not damage the keying relay in the rig.

The experts will weigh in on this subject. But I would be sure to find out before I hook the amp up to the radio.
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W8JX
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« Reply #9 on: August 29, 2012, 07:29:59 PM »

I do not know the keying voltage of that amp. However I would suspect it is above the ratings that the FT-950 can handle.

Therefore you may need a keying relay in line so you do not damage the keying relay in the rig.

The experts will weigh in on this subject. But I would be sure to find out before I hook the amp up to the radio.

It is not a problem with this amp.
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KJ6TSX
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Posts: 116




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« Reply #10 on: August 29, 2012, 08:12:00 PM »

I figure a keying relay would be a good investment, I have to buy the cable anyway so the relay is pretty cheap investment this way I can run any amp. I checked out the SWR without the tuner it's 1.3:1 or less on 40, 20 and 10 meters the rest are about 3:1 So I think I would be safe on the resonant bands running without a tuner. Am I forgetting something??
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W8JX
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« Reply #11 on: August 30, 2012, 05:06:00 AM »

You really do not relay. As far as tuner, as long as SWR in under 2 to 1 or so you will be fine. I have used a amp for many years without a tuner.
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K2DC
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« Reply #12 on: August 30, 2012, 05:15:15 AM »

You should absolutely look into a keying relay buffer.  The GLA-1000B uses a 12V keying relay (good news) but there's no information in the manual as to the current requirements for the relay (not so good news).  The FT-950 uses a transistor switching circuit for the amplifier, and the manual suggests that a 12V relay requiring 25-75 mA is okay but with no info on the amp relay current requirements, all bets are off.  Look into the Ameritron ARB-704 or similar relay buffer, or build one yourself using a 12V relay with a high resistance coil.

GL & 73,

Don, K2DC
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W8JX
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« Reply #13 on: August 30, 2012, 06:30:23 AM »

You really only need a external relay with older amps than have higher keying voltages like a SB200, SB220, L4B etc that have not been modified.
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N4CR
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« Reply #14 on: August 30, 2012, 07:31:44 AM »

Key the amp with a short circuit with an milliamp meter in series (start on a higher scale). Measure the current the relay in the amp draws. Check the current limits of keying in your radio. Make decision.
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73 de N4CR, Phil

We are Coulomb of Borg. Resistance is futile. Voltage, on the other hand, has potential.
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