Trying the LK722's - an expensive experiment:
After first trying the SK702, I ordered a pair of LK722's to try out. In speaking with an engineer at PolyFET, it was determined that, for maximum power in a broadband amplifier circuit, this would be a better choice than the LK721 as it was designed for 12.5 volt operation and appeared to be more rugged. They were $104 each and with a $200 minimum invoice, I obtained two of them.
During the initial testing, it seemed to be fine in that I could obtain about 45 watts of saturated output power on 2 meters and about 30 watts on 70cm. Like the SK702, it, too, seemed to operate with best efficiency and power output without the output loading capacitors (C3553 etc.)
Everything seemed to be going fine: There appeared to be no obvious sign of instability and the devices seemed to be operating with good efficiency and were not getting unduly warm - and suddenly, the output power dropped by about 6 dB.
Investigation first revealed a loss of the gate bias supply, dropping the devices into Class-C operation and lowering their gain. Further investigation revealed that they had developed a low-resistance Gate-Source resistance - about 15-20 ohms - which swamped out the gate bias. All this time, I had been observing the output of the amplifier on the analyzer and I saw nothing that had caused concern, and output power versus drive seemed to be stable - a reasonable indicator of device stability.
Carefully re-checking the output circuitry for any obvious failure (and finding none) I put the 2nd device in: It, too, operated flawlessly for a few minutes before the same thing happened.
Hmmm... (Actually, I said other words - none of which I will repeat here!)
So, I put the original SK702 back in and it worked exactly as before. Being somewhat worried about ruggedness, I did several things to try to kill it such as unterminated/short the output at full output - but nothing seemed to faze the device, so I put everything back together again.
So what happened with the LK722's? I don't know, exactly. The most likely cause was some sort of spurious oscillation, but I saw no evidence of such in all of the testing. I will leave it up to someone braver than I to conduct further experiments along this line...
Additional Comment on the use of the LK722 or similar devices:
I have seen claims that an LK722 (or other devices) will simply "drop in" as a replacement and give full power output, but these sources, when asked for any specific details about the replacement procedure and about the performance, have not responded.
If you have successfully used the LK722 (or any other device) as a replacement for the SRF7043 (even if it is at lower power) please let me know, along with some details and your experiences so that they can be shared here!
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This page last updated 20100727Since 12/2010: