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eHam Forums => Amplifiers => Topic started by: KJ6TSX on August 28, 2012, 09:24:13 PM



Title: Getting started with new amp
Post by: KJ6TSX 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  ;D 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


Title: RE: Getting started with new amp
Post by: KB4QAA 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.


Title: RE: Getting started with new amp
Post by: KM3F 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.


Title: RE: Getting started with new amp
Post by: KC4MOP 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  :o  PLEASE........to keep interference to others using the Ham bands to a minimum.

Fred


Title: RE: Getting started with new amp
Post by: K8AXW 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.



Title: RE: Getting started with new amp
Post by: W8JX 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.


Title: RE: Getting started with new amp
Post by: N6AJR 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.


Title: RE: Getting started with new amp
Post by: KJ6TSX 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


Title: RE: Getting started with new amp
Post by: VE3FMC 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.


Title: RE: Getting started with new amp
Post by: W8JX 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.


Title: RE: Getting started with new amp
Post by: KJ6TSX 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??


Title: RE: Getting started with new amp
Post by: W8JX 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.


Title: RE: Getting started with new amp
Post by: K2DC 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


Title: RE: Getting started with new amp
Post by: W8JX 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.


Title: RE: Getting started with new amp
Post by: N4CR 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.


Title: RE: Getting started with new amp
Post by: AD6KA on August 30, 2012, 07:42:32 AM
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

I would get the dummy load first.
After all, even if you plan to run less
than full power, you have to first tune
the amp to full power......then back off the
input drive and leave the settings alone.

Good legal limit tuner?
I LOVE my Dentron MT-3000A,

But it does take up a lot of real estate
on the desk...and they are becoming
harder to find.
73, Ken  AD6KA


Title: RE: Getting started with new amp
Post by: KC4MOP on August 30, 2012, 02:14:39 PM


Good legal limit tuner?
I LOVE my Dentron MT-3000A,

But it does take up a lot of real estate
on the desk...and they are becoming
harder to find.
73, Ken  AD6KA
I missed several times on ebay to get on of those MT-3000A's. Still command a high resale. I did get a nice deal from eHam ads on a Dentron 3KA tuner.


Title: RE: Getting started with new amp
Post by: WB4M on August 30, 2012, 09:31:03 PM
K8AXW


Title: RE: Getting started with new amp
Post by: W8JI on August 31, 2012, 03:37:29 AM
Quote
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.

That isn't always true.

Older amplifiers generally do not have back-pulse cancelling diodes. When the relay line opens, voltage can spike to 50-100 volts or more.  While this Dentron is a 12 V relay, Dentron never looked at voltages closely. The 12V  supply comes from a 12V winding that is a capacitor input, and actual voltage is around 18 volts. Current is around 175-250 mA, depending on the TX light used. About 125-150 mA is the relay coil, which is rated at 100 mA but is operated over the rated voltage. The rest is amp current.

While the steady relay voltage and steady current is safe for his particular rig, the back-pulse from the Dentron when the relay opens exceeds 50 volts. Oddly, the backpulse voltage from Dentron's also varies with the particular lamp used in the transmit indicator! With no bulb, the peak voltage can be 80-100 volts. With a high current bulb, it is much less.

This probably will not hurt the Yaesu transistor, but it is only rated at 60 volts. The Dentron has no diode across the relay coil, and one should be added. $.10 spent in the Dentron can save wear and tear on the keying transistor.

We should all remember many radios use small reed relays or small transistors to key amplifiers, and just because an amplifier says it has 12 volts doesn't mean it is always + 12 volts with no nasty spikes. :-)


Title: RE: Getting started with new amp
Post by: K8AXW on August 31, 2012, 09:25:38 AM
WB4M:  Yes??

K8AXW