[TenTec] ARRL Reviews 746PRO

[Dr. Gerald N. Johnson, electrical engineer]

The WWII Command Set VFO was and still IS as good as a Collins PTO, far
better than any other receivers contemporary to the 10B. It couldn't be
made today, though with good temperature control and solid state to keep
the heat completely away from the tuned circuits, a Command Set VFO can
stay within .03 Hz of WWV all day (e.g. 20 seconds per swish). The coils
were wound with heated wire so they shrunk to the form. The capacitors
were made of Invar, a zero temperature coefficient metal.

The sideband suppression of the 10B was suspect, and while it had a
sideband select switch it could only be aligned for good suppression one
sideband. That's because the audio phasing network was designed poorly
and gave something other than 90 degrees phase shift. About 30 years ago
I tested a network analysis program by running the 50s era phase shift
networks. Some were 90 degrees instead of 90. None were very good.

Using tubes like 807 and sweep tubes as linear tubes did not make for
linear amplifiers. Sweep tubes were the worst, 6146 as used by a few
makers including Collins S-line were among the best. Collins used RF
feedback to achieve imd results that are hard to improve on. At least
one commercial radio used the HI-FI 6550 to get very good linearity.
With the advent of solid state finals, the IMD response of transmitters
has gone down hill ever since, with the added "feature" that higher
order IMD products from a solid state amplifier tend to be far more
obnoxious than from tubes, even over driven sweep tubes.

73, Jerry, K0CQ
-- 
Entire content copyright Dr. Gerald N. Johnson. Reproduction by
permission only.