[AMPS] SB220 & 4-400s

[Lane Zeitler]

If the sockets are recessed like in the Henry 2KD-5 and the like then the 
tubes will not stay seated, not enough pin to grab the tube. You may have to 
mount the sockets on top of the chassis or make the hole a bit bigger so the 
tubes sit flush. IF you mount the socket on top of the chassis then you may 
have to go with shorter plate caps. I played with this with an SB-220 about 
15 years ago so my memory is rusty.

Lane
Ku7i


>From: Radio WC6W <wc6w@juno.com>
>To: bravo@iol.ie, amps@contesting.com
>Subject: Re: [AMPS] SB220 & 4-400s
>Date: Wed, 4 Apr 2001 11:22:48 -0700
>
>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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