I have an old SB-221 donated from the estate of the original builder. I
think it was assembled in 1981. With 100 Watts of drive and the HV
switch in the SSB position (stiff 240V service), I am getting anywhere
between 700 and 1100 watts output depending on the band. Forty meters is
the best band (1100 watts). On eighty and twenty I am getting about 1000
watts output. On 10 meters, output is only about 700 Watts. IIRC, the
amp is equipped with Eimac brand 3-500Zs (they might be the original
tubes). I making measurements with a Bird 43 (2500 watt slug), so the
output power numbers should be reasonably accurate (FWIW, the Bird 43
readings are in good agreement with my Diawa CN-720 cross needle meter).
Does these numbers sound about right for an unmodified SB-221 with good
tubes? I have someone interested in purchasing the amplifier, but they
are kind of hung up on wanting a little more output power capability
(their exciter has a maximum output power of 100 watts). IIRC, changing
to 3-500ZG graphite anode tubes and directly grounding the grids
improves gain and output power slightly. Does anyone have any data on
how much this improves the amplifier's performance? If there is an
upgrade path that will deliver a little extra output power, I wanted to
let the potential buyer know what their options are.
Thanks & 73,
Mike W4EF.....................
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