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KB1YFD
06-01-2013, 12:43 AM
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

WB2WIK
06-01-2013, 12:51 AM
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!

KB1YFD
06-01-2013, 01:01 AM
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?

K9STH
06-01-2013, 02:07 AM
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

WB2WIK
06-01-2013, 02:28 AM
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.

KB1YFD
06-01-2013, 12:41 PM
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

K9STH
06-01-2013, 02:49 PM
WIK:

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

WB2WIK
06-01-2013, 05:28 PM
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 down...so if that could be damaging to your amplifier or tuner, you haven't really solved that problem.

KB1YFD
06-01-2013, 05:43 PM
WIK,
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

KH2G
06-01-2013, 06:27 PM
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)
Regards,
Dick KH2G

WA7PRC
06-02-2013, 10:50 PM
I've never used a "rule of thumb" when adjusting an amplifier. Going back to my broadcast engineering days, I just use an oscilloscope to ensure sufficient loading, and an audio limiter between the microphone and rig (no ALC). Once set, despite what Steve and a few others will say, there's NEVER any doubt about proper amplifier loading and drive level. The only thing better is a spectrum analyzer, which is considerably more expensive.

A suitable scope needs to have a vertical amplifier with at least twice the bandwidth of the highest radio frequency (a 60 MHz 'scope is sufficient for up to 30 MHz). You can find lab scopes for as little as about $1/MHz. I use a Tektronix 2236 that works GREAT. My audio box is a Symetrix 528E that brings a lot to the table. There are simpler & less expensive units that will also limit/compress.

WB2WIK
06-02-2013, 10:58 PM
I like ALC, though. Properly designed and implemented, it does a great job.

Hard to beat a "closed loop" control system, which is what it is.

Can't see IMD on a scope, no matter how good the scope is.:p You can see it on a spectrum analyzer, but it has to be pretty good, carefully adjusted, and used with two steady-state signals and not a voice.

There's a relationship between linearity and IMD, but it's not a very tight relationship. My rigs here (Ten Tec, Kenwood, Drake) all exhibit improved IMD when the ALC loop is working, compared to when it's not.

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