ai) Using an HW-101 as a 100-watt PA
Hot Water for the K2 By K1RFD
Note: I have included this article as others may find it useful. While the modification is
written for the HW-101, it should apply equally well to the SB-100 and SB-101. The SB-
102 already has the link on the driver output coil brought out to the rear panel jack
labeled “Driver Output” and implements the switching needed to switch the link on L801
between the antenna connector (receive mode) and the Driver Output jack (transmit
mode). It appears that the SB-102 final could be used as an amplifier for a QRP rig by
feeding the QRP rig output into the Driver Output jack on the SB-102 and providing a
way to key the SB-102 into transmit mode as described in this article.
When I was a young ham in 1977, I saved money to buy and build a Heathkit HW-101.
The rig has served me well over the years, going in and out of storage as my interests
and addresses changed, but it seemed destined to return to storage forever after I
finished building the K2.
However, it occurred to me that the HW-101 has a perfectly good final amplifier stage,
featuring a pair of 6146s, which might work well as a pair of "shoes" for the K2. The rig
easily produces >100 watts output on 80 through 10 meters, excluding the WARC
bands.
I got out the HW-101 schematic and considered various ways to drive either the finals or
the driver from an external source. I decided the project would not be worth doing if it
required extensive modification of the HW-101, since mine was (fortunately) in good
working order.
The modification turned out to be easier than I expected. It can be done without
rendering the HW-101 inoperative for its original purpose and it is easily reversible.
The key is that the HW-101's receiver front end is actually coupled to the plate circuit of
the transmitter's 6CL6 driver. This allows the driver's L-C circuit to double as a receiver
preselector. The signal from the antenna relay is coupled into this via an extra "link"
winding around one of the driver plate circuit inductors (L801, in the diagram below).
Hot Water for the K2 Page 183
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