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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