Correct way of tuning an SB-200 Linear
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| Feb 5th 2012, 17:38 | |
KD4SBYTotal Topics: 0 Total Posts: 0 |
I recently gotten a SB-200 Linear, and never have owned a linear, am wondering, what is the best way to tune it? In my case I have a IC-718, driving through a SWR bridge the SB-200 , which in turn is connected through a Heathkit SA-2040 Antenna Tuner to the antenna. I have been told that the best way to tune the linear is to first use the IC-718 at lower power levels and use the antenna tuner to obtain the lowest SWR with the Linear not operating. Then the next step is to switch in the Linear and provide just enough power to it to get the Linear to work so that its meter can be read for plate current and SWR. Here is where I get vague. Do I go back to the tuner and try to get a better match, or do I first try to do it at the linear, and then tweek up the Tuner for the best SWR on the linear's meter? Then, when whatever the correct way is done, can I confidently apply full power to the Linear without fear of burning anything up and be assured that the originally determined SWR is constant? Or do I have to readjust anything? It is assumed that I keep the plate current within its limits while doing all this. Any comments? |
| Feb 5th 2012, 19:35 | |
KB0HAETotal Topics: 0 Total Posts: 0 |
"I have been told that the best way to tune the linear is to first use the IC-718 at lower power levels and use the antenna tuner to obtain the lowest SWR with the Linear not operating." I would say that this is correct. You should make a chart to record the tuner settings for frequencies of interest in the bands you will operate on. This will help minimize on-air tuning. You should alwayso tune the amp into an appropriate dummy load, never directly into an antenna. SWR between thr radio and the tuner, and at the tuner output are both important. Never transmit for more than a few seconds into an SWR higher than 2:1. Doing so could damage the radio or the amp. The radio will have curcuits to help protect it from high SWR. The amp (being much older) does not have protection circuits. |
| Feb 6th 2012, 04:30 | |
WA0CBWTotal Topics: 0 Total Posts: 0 |
Tuning up the amp into a dummy load is good advice. I have basically the same setup, radio-->SB200-->Tuner. There are a couple of ways to do this. You can bypass the amp and use the tuner to tune the antenna. Then connect the amp to a dummy load and apply between 10 to 25 watts. The amp is about a 1 to 10 ratio so 10 watts in will get you about 100 watts out. Tune up the amp into the dummy load. Then connect the amp to the tuner and make small corrections with the tuner. Then bring up the power on the rig slowly checking the radio and the antenna for low SWR. Slight adjustments can be made to the tuner and the amp for maximum signal output. The important thing is to make sure the amp is tuned up into a dummy load (a good 50 ohm match) and that the tuner is tuned to provide a good 50 ohm match to the antenna. Small adjustments may need to be made to the tuner (and maybe the amp) for maximum power output. Then bring up the power on the rig. Be sure and not over drive the amp especially if you aren't using the ALC voltage from the amp. I use a monitor oscilloscope to be sure I'm not over driving the amplifier. 50 to 75 watts of drive should push the amp to 500 to 600 watts assuming the tube are in good condition. I have found that the input circuits in the amp may need to be adjusted for best match to the radio. There are several articles on the WEB detailing how to adjust the input network for best results. The SB200 is a great little amp to give you just a little extra punch. And I love the glow of the tubes in the dark. Good Luck! Bill - WA0CBW ARRL Technical Coordinator - Kansas Section |
| Feb 6th 2012, 16:34 | |
KD4SBYTotal Topics: 0 Total Posts: 0 |
Thanks fellows.
This true in general, but at a certain point you have to connect to an antenna, which unlike the dummy load, might not be 50 ohms. My question was directed in general. In my case, it is assumed that I have used the dummy load to approximate the settings of the Tuner and Linear for the frequency used. From what I read from WA0CBW there is no set way to finalize the tuning procedure after you have made sure that there is a good match to the antenna with lower power. Just keep an eye on the input power What would be a good estimate of the outpout power using the SB meter? If I read a plate current of approx 470-480mA and a HV of 2200V, what is my RF output? |
| Feb 6th 2012, 18:42 | |
WA0CBWTotal Topics: 0 Total Posts: 0 |
The formula for input power would be the plate voltage times the plate current (power = plate voltage x plate current) which would be around 1034 watts input power. Figuring about 60/75% efficiency would give you 600-700 watts out. The tuner that I use has a built in watt meter so I can monitor the power during the tune-up process. You could also put a Bird 43 watt meter or equivalent in line before the tuner. Be sure you use an element for the watt meter that exceeds the anticipated output power. It is possible to hook the amp up to the antenna. The output circuit of the amp is basically a pi network so depending on the impedance of the antenna it may be possible to tune up directly into the antenna and obtain a match. Start off again with only a few watts and see if the tune and load controls can make a match to your antenna. It is does then apply a larger input power until the amp is loaded correctly. I didn't mention it but you should be doing this in the CW mode. Another way of determining the correct amount of drive is to increase the drive until the relative output meter on the amp stops increasing. This is kind of hard to see with the meter on the amp. When you think you are there you might back the drive off 5 to 10 watts to be sure you aren't over driving the amp causing some splatter. A real or relative type of output power measuring device makes it easier to tune it up. Bill - WA0CBW ARRL Technical Coordinator - Kansas Section |
| Feb 10th 2012, 15:31 | |
KD4SBYTotal Topics: 0 Total Posts: 0 |
WA0CBW - Thanks Bill for your comments. I was asking about the RF output calculation because I always assumed that I would have a 50% efficiency. Turns out I may be a little bit too conservative there. Good! Then my amplifier is working OK. Yes, I do everything in the CW mode. I prefer to use the Tuner, for I can get an excellent match that way. I adjusted my antenna with a MFJ Antenna Analyser to get less than 1:3 on all bands I use (measured at the antenna site), and the tuner brings it down to less than 1.5 as read with the meter of the Linear at full power. I never tried to hook the linear directly to the antenna, I have to try that sometime. I do not now what the impedance of the antenna is, the Analyser I used did not have the capability of measuring that. I am on the lookout for a high power SWR bridge, so that I can keep an eye on the Plate current, the RF output and SWR at the same time as I tune up. (Looking at the finals to see if they start to run cherry red is definitely not the way to do it!) Bernie - KD4SBY |
| Feb 10th 2012, 22:27 | |
W0BTUTotal Topics: 0 Total Posts: 0 |
So far, nobody has mentioned a thing about grid current, which is about as important as anything else. Keep the grid and plate current in the white range (below half scale) and follow the excellent advice at http://www.w8ji.com/loading_amplifier.htm. 73, Mike www.w0btu.com |
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