Most likely the tuned input.
Does it have 10 meters in it? Did the builder or adder of 10 meters do
it correctly?
The best place to increase the efficiency of an SB220/221/HL2200 is the
tubed input.
Also, use a 6 foot piece of 66% velocity (RG8/RG58) coax between the
amplifier and the exciter.
Thanks
73
Jim W7RY
On 8/1/2020 1:01 AM, Michael Tope wrote:
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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