Navy 6000-BAC morale receiver
The Navy 6000-BAC morale receiver was made by Industrial Tool and Die Works of Minneapolis, Minnesota under Navy contract N140 S 70560A. The receiver is designed for 3 way power. It can be operated from 115 volts AC or DC or from an external 6 volt storage battery. Its dual band coverage includes the standard AM broadcast band as well as shortwaves from about 5.8 MHz to 16 MHz. The seven tubes include 4 loctals, a 7B7 as RF amp, another 7B7 as IF amp, 7A8 as converter, 7C6 as detector/ AVC/ and audio preamp. The other tubes are octal with a 6G6 as audio output and two rectifiers, a 0Z4 and a 35Z5. An eighth socket is for a vibrator. A vibrator transformer feeds the 0Z4 as in a typical car radio of the era. The 35Z5 supplies B+ when 115 volt power is selected.
First impressions
I bought this radio at a swap meet in part because it was the first I had ever seen. When I saw the insides, I assumed that the radio was obviously built by a low bidder. The chassis was made of thin steel and the capacitors, the terminals strips and even the ground connection on the volume control had been soldered directly to the chassis metal. The only rivets were for the tube and vibrator sockets. The set was "hot chassis". When operated on 115 AC or DC, one side of the power line is directly connected to the chassis.
Second impressions
Looking at it more closely, however, I realized they had not skimped on the quality of the components or the actual design. Most of the caps were rated at 600 volts and were pretested to 1800 volts according to logos on the caps themselves. The set's B+ power resistors are definitely oversized. The set has separate RF and antenna alignment adjustments, a padder for the broadcast band, and obviously a tuned RF stage.
Mine was missing the vibrator transformer and the 0Z4. I did not have the schematic until after repairing the radio. In the absence of a schematic, I traced the power switch wiring. The set could indeed be switched from 6 volts for parallel filaments to a full series string fed by 115 volts. I originally thought the ST-shaped 6G6 output tube was an oddball and a left over from some other project. Wrong! The 6G6 was designed with a filament that takes only 0.15 amp at 6 volts just like the other non-rectifier tubes and could therefore be used directly in either a series string or 6 volts parallel.
Repairs and the low power draw
I did early testing and repairs with an external Heathkit power supply feeding B+ to the set as well as 6 volts AC to the filaments in parallel. The B+ draw was only 25 mA at about 110 volts. The filament draw for the 5 non-rectifier tubes in parallel was only 0.75 amp. Not only did the set take very little power, but I was surprised by the sensitivity of the set after alignment. The set was happy with just 80 volts of B+. Increasing B+ beyond about 95 volts made the set a bit louder but not more sensitive. Total wattage requirement at a B+ of 110 volts and the 6 volt filaments was only 7.25 watts, quite low. That would also have meant a minimal draw on a 6 volt storage battery since the 0Z4, as a gas rectifier, does not have a filament.
My modifications
With the missing vibrator transformer, and the possibility of directly feeding 6 volts to the filaments, I decided I did not want to put up with a hot chassis set or the additional 13 watts of heat that would be generated by the 35Z5 and the filament resistor in a metal case with no ventilation. In the space left by the missing vibrator transformer, I mounted a small power transformer to provide high voltage for B+ and 6 volts for the filaments. The missing 0Z4 tube was replaced with a tube base containing a pair of 1N4007 diodes. The electrolytics had been replaced earlier in the set's life. I replaced them once again. The relatively tiny new transformer barely gets warm after playing the radio for an hour or two.
With the transformer installed, the power cord was replaced with a 3 wire safety grounded version. If I had planned on using the 115 volt side, I would have changed the power cord to a polarized version so that the chassis would always see neutral only. The power switch is already properly connected to just switch the line side.
Performance
The performance of the little set impressed me. It is quite sensitive with its tuned RF stage and quality components, a definite cut above similar morale receivers such as the Minerva Tropic Master which has an RF stage that is resistance-coupled but not tuned.
After alignment, my favorite weak-signal oldies broadcast station came in quite well with just a short clip lead for antenna and phenomenally well with a proper antenna. I was beginning to truly admire the little set. I even began to appreciate the hot-chassis design as reasonable for the Navy considering that many ships of the era used 115 volts DC for power. Power from a 6 volt storage battery would also be easy to find. Every Jeep and most other vehicles had a usable storage battery.
After repairs and alignment, I still needed the schematic and the station log that are normally included as a card in the lid. Rich W5CSZ came through with excellent scans of both the front log sheet and the schematic on the back. Send me an e-mail if you need copies of these scans. I found from the schematic that the IF is listed as 465 KHz. Because my IF transformers had 455 KHz stamped on them, I had assumed 455 to be correct. They had obviously been aligned to 455 before I tweaked them. With the markings on the IF cans, it is possible that either may be correct depending on the individual set.
Some history
N140 has been identified as the Naval Supply Research and Development Facility in Brooklyn, New York and the contract has been dated to May 1945. One of the questions raised by the contract date is when the set was actually used if the earliest manufacturing date was May 1945. Victory in Europe occurred in May 1945, but I doubt that the military imagined that the war in the Pacific could be over in only 3 months. I suspect that most of these morale radios were intended for the war in the Pacific theater, but most were used during the occupation of Germany and Japan.
An instruction manual for the 6000-BAC radio is part of the personal papers of Joseph Trainor, who under the Supreme Commander for the Allied Powers was the Deputy Chief of the Education Division assigned to oversee programs of education for democratizing Japan during the allied occupation of 1945-1952. Trainor's papers are in the Hoover Institution at Stanford University. I can picture him and other military personnel using this little radio to listen to the Armed Forces radio network and news from home on the shortwaves.
It is also likely that some of these radios were used during the Korean conflict. If you used one of these during the war or occupation, tell us your story. And thanks for your service.
A bit more history
Industrial Tool and Die Works of Minneapolis, Minnesota also produced a radio for the Army, the 6000B that only ran on 6 volts. The company also built tooling for the famous Bailey bridges extensively used in World War II. Their interest in electronics is clear from two grants which they funded for the University of Minnesota in 1943-44, one for high voltage research and another for research on the electrical characteristics of powdered iron products.
After the war, three individuals associated with the company formed another company, Mound Metalcraft of Mound, Minnesota. They started producing metal household objects but quickly found their true niche, very tough stamped metal toys. They changed the company name to Tonka. The metal stamping may help explain why Industrial Tool and Die built the 6000-BAC radio in the first place. They had the tooling for metal work for the construction of the case and front panel. That along with those soldered-on-chassis components made for a Tonka-toy style of radio tough enough to survive at the bottom of a duffle bag.
Date 7-5-11,
Tube-era capacitor analyzers Part 2
Knight, Eico, Lafayette, Heathkit were the previous items on the bench.