On Tue, 30 Nov 2004 14:06:37 -0500 "Tom Cathey" <k1jj@comcast.net>
writes:
> Hi Marv,
>
Hi Tom,
> Your idea about using a 4CX350J IPA is interesting. I suppose it would
have
> to be VERY physically close to the 8877 or the RF negative feedback
will
> have phase shift problems. ie, It will not work in a separate cabinet
with a
> short coaxial lead supplying feedback, I would imagine.
Yep... 20° per foot at 6M.
> The "J" [linear version] might be an expensive tube too.
Any of those parts are expensive new. I've seen a number of pulls go
for $20-30. They do not seem to be very popular as they have 26V
heaters and therefore will not drop in for 4CX250B's, et al.
>
> Also, I like the idea about putting an MRF-151 in the cathode of the
8877
> as a "gain booster", but also wonder if it would degrade the IMD of
the 8877
> significantly. Anyone have ideas about this?
On second thought, there are a couple other considerations in this
scheme. The transistor only needs to handle a couple amps so a MRF-151
is a bit on the largish side. And the cathode of the 8877 will exhibit a
considerable voltage swing.
Perhaps it would be better, conceptually, to use a real transistor
cascode to drive the cathode. A small FET like an MRF136 driving a high
voltage FET (something rated around 5A Id & 200V with low input
capacitance) placed in cascode with about 15 volts on the gate referred
to the '136 source. The drain of this second FET would then connect to
the 8877 cathode.
>
> Another interesting idea was about using a ~200W triode and letting it
idle
> in class A1, giving maybe 60W output without worrying about neg
feedback.
> Maybe that would approach -40 db, or perhaps the device itself has
> limitations - like using a triode connected 813, etc, dunno.
>
> The bad news: I just got the 3rd order IMD figures back from DownEast
on
> their 50-28 transverter. At 14 watts output it is NO WORSE than -26db.
> Typically -30db. She said at a lower power it would be better.
[sometimes
> true, sometime not]. So it appears that I will have to either look
into
> idling those devices higher or modifying somehow, or tapping off the
low
> level area which should be cleaner and amplifying from 100MW? up to 60W
with
> something cleaner - as suggested above... [sigh]
>
> Anyone have ideas for cleaning up the 50-28 final amplifier stages? I
wonder
> what the rest of the world has been doing about this or just living
with -30
> db IMD on 6M?
Just build a complete clean exciter from scratch? :-)
>
> 73,
> Tom, K1JJ
73 & Good morning,
Marv WC6W
*
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