On Mon, 29 Nov 2004 16:05:00 -0500 "Tom Cathey" <k1jj@comcast.net>
writes:
> Hi All,
>
> I need to come up with a CLEAN 50W-70W IPA to drive my newly built 6
> meter 8877.
>
> I have 20W available from a DownEast 50-28 transverter.
> An older N4NTA 6M kit using a single MRF-492 (10W to 100W brick) looks
> interesting as an IPA, but I understand the 3rd order IMD is only
> around -25db? The 8877 is supposedly around -41db, so this would be
> a poor match up.
Hi Tom,
You really need something with at least 10dB better IMD numbers, if
you wish take full advantage of the 8877's specs.
>
> The problem is that the 6M noise floor is very low and when using
directive
> arrays, so splatter can be an issue for locals... :-( I don't know
what
> the 50-28 transverter IMD output is, but I have a call into them with
the
> request.
Either that or just take a look on a spectrum analyzer...
>
> I have even looked at building a small tube amplifier as an IPA, but
the IMD
> on 6146's, 811A's, etc does not seem better than -25 to 30db either.
Even
> my 6M FT-650 is speced at -31db. It appears hard to get a better
driver
> combo to match the 8877 specs.
How about a 4CX350FJ? Great IMD numbers, plenty of gain and it would
loaf at your required output.
>
> I've also thought of tapping into the 50-28 transverter at a low level
where
> it is cleanest and amplify that up through several tube stages using
> negative RF feedback. But, I do not have a design that would suit the
> purpose.
RF feedback could be applied from the 8877 output to the 4CX350 input
to good effect... reducing the IMD contribution from both tubes.
>
> Thought I'd ask here before searching further.
>
> Any ideas? Thanks!
Any... ideas!?.... Here's an odd one: Stick a FET, an MRF151 for
instance, in the cathode circuit of the 8877 and create a transistor-tube
cascode! The gain will be adequate for your purpose, not sure about the
resulting IMD. :-)
> Tom, K1JJ
>
73 & Good morning,
Marv WC6W
*
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