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Author Topic: Palstar LA-1K vs. Acom 1200S  (Read 1485 times)
G3RZP
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« Reply #15 on: May 10, 2019, 08:46:45 AM »

Back in the 1960s when I was an apprentice working on big - 10 and 20kW TWT and klystron ground station transmitters, there was a continuous monitor of cooling water resistivity that shut things down if the resistivity got too low.
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SM5GLC
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« Reply #16 on: May 10, 2019, 09:03:01 AM »

After loosing a BLF188XR (out of two) in my SSPA for unknown reasons, everything had been OK for a year. I decided SSPA with LDMOS is not ready for prime time. There are several things that need to be solved...

Fragile Gate. As the transistor is made up from many tinygates, and if only one breaks, the transistor goes QRT. Overdrive, bias supply transient is just two easy ways of killing the device.

Cooling Surface of the transistor and the amount of heat means the die will become very hot, fast. Using CW (or any non continous modulation) the temperature cycling will stress the material.

SWR Even if the drain can withstand high SWR for a short period and have the protection circuits to kick in, it may cause too much voltage on the gate (due to feedback), instatly killing the transistor.

And for those trying to reduce output power to save the FET's remember you need to lower the drain voltage, as the wide band output transformer is optimized for a certain load, and if you reduce power, keeping the drain voltage, dissipation will not be reduced to what you expect....

I have pretty much given up, and stays with tubes. My SSPA will be more of an experimental toy, until I feel everything is under control.
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N3FAA
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« Reply #17 on: May 10, 2019, 11:35:59 AM »

This posting has a bit of deja vu to it.

As you can see, just like in the other threads you linked, this has devolved into 2 pages of completely off-topic posts that have absolutely nothing to do with the original question.  I'll be asking a moderator to close it.
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K7JQ
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« Reply #18 on: May 10, 2019, 12:18:22 PM »

Personally, I'd go with the Acom, mostly due to their long history in designing quality, reliable amps. I believe the LA-1K is Palstar's first amp on the market.

But be careful...SS amps are more radio and antenna dependent than their tube counterparts. They can be quirky if the radio has a power overshoot problem...frequent overdrive faults and could damage the finals over time. Also, if your antenna isn't resonant to less than 2:1 SWR over the entire band, you WILL need a high power tuner. Otherwise, fault city and power foldback.   
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W9IQ
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« Reply #19 on: May 10, 2019, 01:09:08 PM »

Lasse,

Most of what you describe is not a fault of the final but of the design of the amp. Proper engineering and good supervisory control can make it bullet proof.

- Glenn W9IQ
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- Glenn W9IQ

I never make a mistake. I thought I did once but I was wrong.
W9IQ
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Posts: 3529




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« Reply #20 on: May 10, 2019, 01:13:42 PM »

Also, if your antenna isn't resonant to less than 2:1 SWR over the entire band, you WILL need a high power tuner. 

Equating resonance to less than 2:1 SWR is not correct. Simply saying "less than 2:1 SWR" would be sufficient as it doesn't matter then if the load has reactance or not.

- Glenn W9IQ
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- Glenn W9IQ

I never make a mistake. I thought I did once but I was wrong.
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