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Re: [Amps] SB-221 Power Gain

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Subject: Re: [Amps] SB-221 Power Gain
From: Michael Tope <>
Date: Sun, 2 Aug 2020 19:51:39 -0700
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Thanks to everyone who responded to my questions. The consensus of the responses is that the numbers I am getting are typical for an SB-221 with 100 watts drive. I agree with those who suggested that the power supply and cooling scheme in the amp are weak points so it doesn't make much sense push the tubes towards higher output power levels.

73, Mike W4EF............

On 7/31/2020 11:01 PM, 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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