Modified SB-220

[John C Adkins]

I have received several responses to last night's email asking how
extensively my SB-220 has been modified.  Here goes:

Note that I have no schematics for any of this.  I should say that the
modifications seem to have been done well, with nice cabling, wire routing,
etc.

1.  A second HV transformer has been added (outside the enclosure, using
cables/connectors).  I believe that this is a duplicate to the original.  I
haven't had a chance to figure out the wiring, but my guess is that it is
simply wired parallel to the original.  At this time, if I turn the unit on
with this xformer connected, the circuit breakers flip, so I don't know if
the xformer actually works.

2.  A filament step-start relay circuit has been added.  Seems to work.

3.  The filament/bias circuits to the final tubes has been completely
shielded, using an enclosure, feed-thru capacitors, etc.

4.  The antenna relay has been shielded similarly.  Interestingly, the
relay has 4 switches instead of the original 3.


I don't actually know whether this unit ever worked as modified.  The
3-500z's look new, and there are no filament/grid/plate shorts.  (The tubes
were reported as new to me by the former owner.)  When I tune it up, the
load capacitors reach full capacitance, and the rel pwr meter never peaks.

So I have decided to remove the modifications one at a time until the
amplifier works.  I have another, minimally modified SB-220, so eyeballing
it makes this easier.

In restoring the antenna relay circuit to the original, I find that I need
the "silver plated copper strip," which connects the variable capacitor to
the relay.  Any ideas where I can get one?

Thanks

John
w4kv

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