[R-390] RE: 6DC6

[Roy Morgan]

At 02:40 PM 9/6/02 -0700, Richard.McClung@Dielectric.spx.com wrote:
>GOTO: <http://www.r-390a.net/faq-collins-cost.pdf>
>
>Scroll to document page 9.   Report  PAGE 5  PARAGRAPH 2.3.15  PART 15 -
>RF UNIT


Which says (for the record):


2.3.15 Part 15 - RF Unit

Tests were made to determine how much the 2nd RF stage with its coils was
contributing to the performance of the receiver. Its advantages were that it
afforded additional AGC control ahead of the mixers, and better image
rejection above 8 mc. But because of the considerable saving resulting from
the removal of this stage with its associated coils, SCEL representatives
agreed it was worthwhile. In an attempt to regain some of the lost AGC
control, the mixers were put on the AGC line. Attempts were made to restore
some of the lost image rejection by improving coil Q and tracking, and by
using better switch materials. Better antenna trimmer action on the 16-32 mc
band was accomplished by mounting the trimmer closer to the high band antenna
coil. This reduces stray C allowing the trimmer to be coupled tighter to the
coil.

Field tests by SCEL engineers on current R-390 receivers revealed that cross
modulation is a problem in the presence of strong interferring signals.
Extensive tests were made on all available miniature RF pentodes to find a
tube which best combined the characteristics of low noise and low cross
modulation. Late in the investigation tube types 6DC6 and 6BZ6 became
available. These had the most desirable characteristics of any tube tested.
Because the 6DC6 affords slightly better AGC control, it was the one selected
for use as the RF amplifier. Cross modulation tests were also run on various
mixers, and the 6C4 proved to be very good in this respect, actually improving
slightly in performance as the bias was increased up to about 14 volts. On
this basis, it was deemed safe to use AGC on the mixers. Late tests show that
cross modulation is largely confined to the first IF tube at medium high
levels (.05-.2 volts input) and the RF amplifier at levels above this.
Investigation carried out by E. Read on the R-392 receiver showed a 
substantial improvement in cross modulation at signal levels
exceeding the normal operating bias of the 1st RF tube when a short-time
constant circuit was placed in the 1st RF grid. Subsequent experimentation on
the "A" model R-391 showed a small improvement particularly at antenna levels
exceeding 2 volts. The results are not nearly as spectacular as on the R-392,
however.


Roy

- Roy Morgan, K1LKY since 1959 - Keep 'em Glowing!
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