I also built an attenuator from 2 watt carbon composition resistors. My
application was a Kenwood TS-440 driving a SGC SG-500 amplifier. The ALC
circuitry in the TS-440 made the keying envelope funny at anything much
below 90 watts output. The SG-500 only wanted 50 or 60 watts drive, and
anything higher would kick in the built in input attenuator, which would
no doubt also make for a weird CW envelope. The attenuator I made would
also only handle about 25 watts continuous, so I built it in a one quart
paint can and filled it with mineral oil. A sort of "cantenuator".
There are web sites with attenuator calculators, wherein you enter
impedance, dB loss and whether you want a pi or tee configuration. I
have generally found tees easier to get the right resistor values. That
may not be the case for all attenuation values though.
Bob, where do you get your two watt carbon composition resistors?
> The 10 dB attenuator is a T configuration. The input series R is 25 ohms
> made of 4 x 100 ohm 2 W carbon resistors in parallel. The shunt R is 33
> ohms made of 3 x 100 ohm 2 W carbon resistors in parallel and the series
> output R is a 2 watt 27 ohm resistor. Close enough to 10 dB attenuation and
> 50 ohms in and 50 ohms out.
> If you need less loss then here values for 6 dB. Input series R is 17 ohms
> made of 4 x 68 ohm 2 W carbon resistors in parallel. The shunt R is 67 ohms
> made of 4 x 270 ohm 2 W carbon resistors in parallel. the series output R is
> a 2 watt 18 ohm resistor. Again, some 6 dB attenuation with about 50 ohms
> in and out.
> These values will handle 25 to 35 watts of SSB with no problems. Watch CW
> key down or tuning times as the dissipation of the R's may be exceeded.
> If you need other values of loss or input or output Z, let me know. I
> figure the values for you.
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