[Amps] SB-221 Power Gain

[Michael Tope]

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.....................