Tom Cathey wrote:
> Hi Marv,
>
> 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. The "J" [linear version] might be an expensive tube
> too.
>
> 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?
>
> 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?
Probably easier to bypass it, or run at 100s mW max output so it's in class
A. A modern FET will give you 20+dB gain at 50MHz, so 60W out will mean 0.5W
or less input.
For comparison, we ran a 2m contest transmitter for many years and it was
regularly noted as one of the cleanest signals on the band. That was an 80W
fet driving 8877. The transverter output stage was a 20W bipolar running in
class A (28V/0.8A) at 2-3W out so the IMD was hard to measure.
Steve
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