As others have pointed out, only a single I.F. amplifier chip is gain
controlled in the K2. The RF gain, the gain of post mixer I.F. amplifier
transistor, and the gain of the active product detector are not controlled
through AGC, yet they all contribute to the receiver's total gain.
Since the I.F. amplifier chip has a gain range of about 50 db (min to
max) (if memory serves correctly), that's about all the AGC range you'll
have for the entire receiver as a whole. The RF attenuator/bypass/RF
amplifier provide about 24 dB (-10 db to +14 db) of manual gain control.
The volume control provides the rest. :-)
About 20 years ago I converted an old Hammarlund HQ-140-X receiver
to a solid state design. The Hammarlund's AGC controlled the gain
of the RF amplifier and three I.F. amplifiers (all 6BA6 remote
cutoff pentodes). I kept the same philosophy after converting
each of these stages to 40673 MOSFETs.
The input of each 455 kHz I.F. stage was (is) through a tap on each
of the I.F. interstage transformers. This "gain deprives" each I.F.
amplifier a bit, and is possibly done for maintaining stability, rather
than for reasons of impedance matching. However, since there's still
more gain than is needed for the receiver, and since ALL of the RF
and IF stages are AGCed, a tremendous amount of AGC range is
available. (Significantly more than if only 2 I.F. stages were
used without the gain-depriving transformer taps.)
The result of having nearly all of the receiver's total gain under AGC
control is that the difference in volume when tuning between a strong
signal (such as an AM broadcast) and an empty band (10-meter band noise
at night) is almost negligible.
With a large amount of receiver gain under AGC control, the volume
control essentially becomes a set-and-forget adjustment.
Happy Holidays, everyone.
73, de John, KD2BD
=====
Visit John on the Web at:
http://www.qsl.net/kd2bd/
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