[Amps] How to drive 6M 8877 with DownEast 50-28 ... IPA?

[Tom Cathey]

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?

73,
Tom, K1JJ


>
>   How about a 4CX350FJ?   Great IMD numbers, plenty of gain and it would
> loaf at your required output.
>
>
>   RF feedback could be applied from the 8877 output to the 4CX350 input
> to good effect... reducing the IMD contribution from both tubes.
>
>   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.  :-)
>
>
> 73 & Good morning,
>    Marv  WC6W
>