On Mon, 2007-01-29 at 19:26 -1000, Ken Brown wrote:
> I used to run a Kenwood TS-440, driving a SGC SG-500 amplifier. The
> SG-500 requires about 55 to 60 watts of drive for full output. It has
> protection circuitry, of various kinds. There are probably about a half
> dozen conditions that will shut the amplifier down to prevent damage,
> and there is an input attenuator, about 3 dB, that automatically
> switches in if you overdrive the amplifier. You don't want to overdrive
> the amplifier and you don't want that attenuator automatically switching
> in on every leading CW element of a transmission. Turning the RF output
> down on the TS-440 results in a funny sort of slow rise envelope. I
> investigated the possibility of adjusting the ALC (or mis-adjusting it)
> so that it would have a proper keying envelope at 55 watts output, and
> probably could have done that. Then I would not be able to get the full
> 100 watts out of the TS-440 when I wanted it, running without the amplifier.
>
> So, I made a power attenuator out of a bunch of 2 watt carbon
> composition resistors. I don't recall the exact values. It is a T
> network with about 2.5 to 3 dB attenuation, and four or five, two watt
> resistors in parallel for each of the three elements of the T. It could
> probably dissipate 30 watts continuously in air, and at CW duty cycle
> would be fine dissipating 50 watts. Just to be sure, I built it in a one
> quart paint can filled with mineral oil. When using the SG-500 and
> transmitting and receiving on a single antenna the attenuator was in
> line for both transmit and receive. My keying envelope was good, and the
> attenuation was not even noticeable in receive.
I have grown to like the power tin oxide film, sometimes called flame
proof. I've tested them operating glowing red without permanent damage
to the resistors element, only to the coating and their surroundings. At
Collins we used a bunch of them with high velocity oil cooling to handle
250 KW AM when developing the 821A-1. Such as the 3 watt PR03 series
from Vishay sold by Mouser. Catalog number 594-5093NW. Many years ago I
created a BASIC program (on my CP/M system) that would create a series
of cascaded T attenuator elements using two resistors in parallel for
the shunt element of each section and one resistor for each of the
series sections of a T and would cascade sections to make the total
desired attenuation. It could also add a third shunt resistor if needed
when standard values didn't have a close enough selection. It used the
same power rating for each resistor except that third shunt and it
computed the power dissipation in each resistor of each stage. It could
design a 100 watt input attenuator using 1/2 watt resistors. I used the
two shunt resistors as a better approximation of a disk resistor than a
single resistor. I should root that program out some day, it was handy
occasionally.
>
> Perhaps a better setup would be to have the power attenuator in the
> input circuitry of the amplifier, after the input/bypass switching
> relay. If you could get the right carborundum, or other non-inductive
> power resistors, to make the right sized (both in attenuation and power
> rating) pad, it might fit inside the amplifier.
>
> If you do a google search on " t pad attenuator calculator " you'll get
> plenty of results. The first one on the list worked for me.
>
> DE N6KB
--
73, Jerry, K0CQ,
All content copyright Dr. Gerald N. Johnson, electrical engineer
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