[Elecraft] KPA-500 high gain modification?

[Eric Swartz - WA6HHQ, Elecraft]

Removing this part of the circuit will definitely void the warranty. It 
is part of both the input overdrive protection for the amp and also 
stabilizes the input SWR as seen by the driving radio.  It's also 
important for amplifier stability.

We also check for excessive gain in the amplifier's protection firmware. 
Removing this circuit will likely cause an operation fault error, 
putting the amplifier in stand-by.

73,

Eric
---
www.elecraft.com


On 3/16/2012 8:14 AM, Joe Subich, W4TV wrote:
> On 3/16/2012 9:59 AM, mark roz wrote:
>> Interesting. I have build 600W amp with four Motorola MRF150
>> transistors and I can easily get 600W output with 7W input.
> Well, after a little more research (finding the KPA-500 schematics on
> the FCC web site) ... there is a fixed attenuator between the input
> wattmeter and the input matching network (L-net on 6 and an attenuator
> on HF).  The constants don't match those of classic 50 Ohm in/50 Ohm
> Out 3dB pad but are close.
>
> Of course, It is entirely possible that the KPA firmware includes gain
> measurement will cause a shutdown if the gain is too high
>
> 73,
>
>      ... Joe, W4TV
>
>
> On 3/16/2012 9:59 AM, mark roz wrote:
>> Interesting. I have build 600W amp with four Motorola MRF150 transistors and I can easily
>> get 600W output with 7W input. This was tested with input attenuator removed.
>> Are those Mosfets in KPA500 that much different?
>> I also would like to purchase the KPA500 but only if I could get 15 dB gain.
>> It would be great to drive KPA500 with KX3 10W and get around 300W output power.
>> I’m pretty sure that KPA500 has some form of attenuator at the input.
>> Mark WQ7X
>>
>> But I would buy a second KPA500 in a heartbeat if I knew I could
>> modify it for 10 watts drive for full output!
>   >
>   >  I think you're being optimistic about full output with 10W drive.
>> The VRF-2933 is rated for 23 dB gain (typical).  With perhaps 1 dB loss
>> in the low pass filter, a dB or so in the output transformer, roughly 3
>> dB in feedback to improve linearity one dB in the input transformer,
>> bias decoupling, gate balancing, and two dB in the input matching comes
>> very close to the roughly 15 dB gain limit in the US regulations.
>>
>> Some of those values are based on application notes from Motorola,
>> MicroSemi and APT for similar MOSFET designs while some values are
>> from Elecraft designers.  In any case, the inescapable conclusion is
>> that there is no "magic modification" to get the additional 2 - 3 dB
>> of gain necessary to get 600W with 10W of driver from the KPA-500.
>>
>> 73,
>>
>>       ... Joe, W4TV
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