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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