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Controlling output power from TS-530SP

Discussion in 'Amateur Radio Amplifiers' started by KB1YFD, Jun 1, 2013.

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  1. KB1YFD

    KB1YFD Ham Member QRZ Page

    Hello Amp experts,
    My LK500-ZC does not have an ALC output for my rigs. Flex-3000, no problem, I just turn down the output power. Same for my FT-100D. My third radio is a Kenwood TS-530SP. When properly tuned, it puts out 140 Watts. If I drive my amp that hard, my tuner will arc. I installed a 0 to -10 Volt power supply and control it with a pot. I tested in CW and the control works just fine. It seems to work in SSB, except I still get voice peaks of 140 Watts! Is there a good way to control the output power of my TS-530SP???

    John kb1yfd
  2. WB2WIK

    WB2WIK Premium Subscriber QRZ Page

    Not really, but there's something wrong if the 530SP puts out 140W. Either your wattmeter is inaccurate, or something weird is going on. It uses a pair of 6146Bs and can only put out about 100W. I've owned several of them, that's all any of them did.

    If this is really the case, try a 3 dB pad between the rig and the amp. That would do it!
  3. KB1YFD

    KB1YFD Ham Member QRZ Page

    140 Watts according to my LP-100A.

    Someone on air suggested I de-tune the tank circuit by turning the load control full ccw and adjusting the plate setting for the desired power out. I've done that and it does work. However, is this OK for the 6146's?
  4. K9STH

    K9STH Ham Member QRZ Page

    You need to rotate the antenna loading control fully counter-clockwise. Then, use the plate loading capacitor to "dip" the final. Increase the loading control clockwise a little bit at a time and then "dip" the plate loading control again. Do this until the power output reaches the desired level. This makes sure the pi-network is actually matching the output to whatever the load actually is.

    Next, note the settings of the antenna loading control and the plate loading control. In the future, you can preset the controls to near these values and then "dip" the plate loading control to make sure that they are "spot on". That way, you can tune the transmitter in just a few seconds.

    Glen, K9STH
  5. WB2WIK

    WB2WIK Premium Subscriber QRZ Page

    I disagree.

    The "problem" with "underloading" the rig as suggested is the IMD products become terrible.

    PAs operate "best" when fully loaded, or even "over" loaded. That can reduce IMD by 10 dB or more.

    Doing what is suggested may reduce output power, but it's not a very "clean" way to operate. I like "clean," and would always use a fully loaded PA with an attenuator to achieve that, rather than underloading a PA. A 3dB 50W pad is cheap...can be built for $20 and often purchased commercially (surplus) for about that also.
  6. KB1YFD

    KB1YFD Ham Member QRZ Page

    Hi Guys,
    Thanks for all the input. I agree a properly loaded tank is the best for the tubes and the signal quality. I did some more experimenting and found that turning down the mike gain works really well. I tap the mike while checking the peak power and found I can control it quite well with just the mike gain. Seems kinda cheesey, but testing proves it works.

    John kb1yfd
  7. K9STH

    K9STH Ham Member QRZ Page


    The old "rule of thumb", which works pretty well, is to tune the transmitter "normally" and then increase the antenna loading 5-percent to 10-percent without "re-dipping" the final.

    Just saying "fully loaded" could indicate to the unknowing that the antenna loading control is to be turned fully clockwise which, in turn, could definitely cause serious problems because the pi-network would not be resonant!

    Glen, K9STH
  8. WB2WIK

    WB2WIK Premium Subscriber QRZ Page

    Glen, I agree that's usually about right.

    John (YFD) turning down the mike gain isn't a good idea.

    Two reasons why:

    1. Although your average power output will definitely decrease, occasional peaks can be exactly the same power as with the mike gain up much higher. This is because...

    2. The ALC loop doesn't operate normally when you reduce the mike gain. If you reduce the mike gain so the ALC indication on the meter is very low or "none," the ALC is now not functioning and can't do what it's intended to do, which is keep the average power up close to the peak power. So, you end up with an "undermodulated" signal that might sound good and strong on occasional voice peaks, and sound really difficult to hear with normal conversation; and the occasional peak will still be whatever it was before you turned the gain if that could be damaging to your amplifier or tuner, you haven't really solved that problem.
  9. KB1YFD

    KB1YFD Ham Member QRZ Page

    That was my understanding also until I just tried it. The radio acts as if the mike gain controls the absolute peak audio fed to the transmitter. My test was to tap the mike screen with my finger nail. This tapping always produces the biggest audio peak. To my surprise, this worked perfectly to reduce my SSB output power. Audio reports from my regular rag chew nets were good. I used the TS-530 with my amp for several hours last night without any problems.

    John kb1yfd
  10. KH2G

    KH2G Ham Member QRZ Page

    Never a good plan to control output power by de-tuning the amp for a good number of reasons. Backing off on the load isn't sooo bad but full peak out is best and as was suggested, you can damp the output with a pad which is the preferred method. (adjusting for full output will give the most linear operation as the plate load of the tubes will be matched making everything happy)
    Dick KH2G
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