[AMPS] SB220 & 4-400s

[Alek Petkovic]

Spot on Marv, That is exactly how it is with the 4-400's. My best mate 
VK6JX got around 900W to 1KW out of his. He now has the 3-500's in but 
still has the tetrodes as spares.

73 de Alek
VK6APK


At 11:22 AM 04-04-01 -0700, you wrote:


>On Wed, 4 Apr 2001 16:45:40 +0100 "John Tait" <bravo@iol.ie> writes:
> >
> >All Hail...!!  Oh Enlightened ones...!!
> >
> >    A friend of mine has an old SB220... The 3-500s have departed for
> >Valhalla, but we have a number of 4-400As...(allegedly) good pulls.
> >  If we use the 4-400s instead of 3-500s (both grids grounded), what, if
> >any, circuit changes should be made to cope with the new scenario???
> >  Any suggestions gratefully received..
> >         73 to all
> >              de
> >             John EI7BA    http://www.iol.ie/~bravo/
> >
>
>Hi John,
>    This is from long ago memory...
>
>    First, I recall you have to wire the grid pins together (2,3 & 4) on
>the bottom of the sockets as only one of them has a DC return in a stock
>SB-220.
>
>    And then, add a jumper across the 5 volt zener in the cathode circuit
>to run the 4-400A's in true zero bias.
>
>    The plate impedance of the 4-400A's will be a skosh higher than the
>3-500's but, there should be sufficient latitude in the output stage to
>accommodate this.
>
>    The input impedance of the 4-400A's will also be a bit higher and the
>input network(s) should be re-adjusted for best match.
>
>    The converted unit will exhibit a bit less gain, i.e. require more
>drive for a given output, but should run just fine otherwise.
>
>73,
>   Marv  WC6W
>
>
>
>
>
>
>
>*
>
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