[Amps] SB-221 Power Gain

[Artek Manuals]

Mike

Numbers are in the ball park

You might get up to 1200-1250 with a new pair of tubes . A pair of new 
3-500ZG's is going to run you in the neighborhood of $450. The bigger 
issue on power out is you are power supply limited, no easy fix for that.

Surprisingly you might find that improvements on 10M can be had by 
tightening every screw in the RF deck . And I mean EVERY screw. tank 
circuit variable caps, grid grounds , even the ones that hold the RF 
cage to the chassis, etc.  The issue is conductivity for circulating 
currents. 40 year old aluminum like the rest of us is not as good as it 
once was at doing the heavy lifting and the "joints" complain.

Dave
NR1DX

On 8/1/2020 2:01 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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-- 
Dave
Manuals at ArtekManuals.com
www.ArtekManuals.com


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