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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