Morrow MB-6 Receiver

The Morrow Radio Manufacturing Company of Salem, Oregan, produced three mobile amateur band receivers in its brief four-year history, and the MB-6, shown below with its matching speaker/AC power supply, was the premier receiver in its line. This seldom-seen radio can be distinguished from the more common MBR-5 by its rotating drum dial mechanism. The MB-6 was made between 1958-59 and sold for $239.

Features: The MB-6 tuned the five amateur bands from 80-10 meters. Unlike the Multi-Elmac PMR-7 and PMR-8, which were its less-expensive competitors, the MB-6 had a calibrated S-meter, crystal calibrator, and sophisticated squelch circuit. The radio could also be used as a field strength meter, with a separate antenna input for this function. A tunable BFO was provided for CW and SSB reception, but there was no product detector.

 Circuit Description: Aside from an unusual squelch circuit, the MB-6 utilized a conventional double-conversion design. A 6BA6 RF Amplifier stage (the MBR-5 used a 6BZ6) was followed by a 12AT7 mixer with an IF of 1690 kHz. This signal was mixed in a 6BE6 second mixer with a 1470 kHz crystal oscillator to 220 kHz. Following one stage of first-IF (6JB6) and one stage of second-IF (6BA6) amplification, the signal was detected (6T8) and amplified by 12AX7 and 6C4 preamplifiers. The 2.5 W audio output stage utilized the ubiquitous 6AQ5. In all, the receiver used 13 tubes, four of which were dual-purpose.

Under the Hood: The mechanical layout of the MB-6 is a very good example of careful engineering. In order to cram all the circuitry into a small package only 11-3/4" long x 7" deep x 4" high, the designers had to mount three of the tubes sideways. There is essentially zero clearance between the top of the 6AQ5 audio output tube and the cabinet! The chassis is made of sheet metal, with numerous partitions and brackets giving it good rigidity. The tuning capacitor is coupled to the tuning knob via two anti-backlash gears with a 22:1 reduction ratio. A pulley/dial cord arrangement gangs the knob to the pointer.

In order to fit all the circuitry into the compact MB-6, several tubes had to be mounted sideways.

Many underchassis components are mounted on a large phenolic terminal board.

Surprisingly, the underchassis view of the MB-6 appears uncrowded, with many components mounted on a large phenolic terminal board. The wiring is very carefully done, with wires squared-off and components neatly oriented at right angles to on another. The wiring has a mil-spec appearance more reminiscent of an R390 module than a piece of ham gear. All screw threads and nuts were treated with dabs of lock-tite, as befits a receiver intended for mobile use.

Performance and Operating Impressions: What first caught my attention when I powered up this radio was its outstanding stability. After a five-minute warmup, I was able to monitor a 75 meter SSB net for over an hour with no further tuning. Banging on the cabinet produced only the slightest warble. A checkout with a signal generator on my test bench showed it to be quite sensitive, with 0.1uV signals easily detectable. The S-meter (S9+40db full scale) worked well, but was perhaps a bit too generous. One pleasant surprise was the AM audio quality, which was outstanding. I attribute this quality in part to the twin speakers in the AC power supply console.

The selectivity was adequate, but not outstanding, as might be expected for three tuned 220 kHz IF transformers. The gear-driven tuning was very smooth, with adequate bandspread, and while there was no backlash, I thought the tuning seemed a bit spongy. A bit of inspection revealed the tension of the dial cord/pulley mechanism to be the culprit. CW notes were crisp and clear, and the squelch worked flawlessly, with no thumping or poppng. All-in-all the MB-6 is a fine little receiver that outperforms many others in its price class.