posted 12-13-2004 08:05 PM                    

quote:

---

Some pics from my shop, where I am currently building sound racks for our new 12-plex:

---

Sort of looks like our shop too. A big mess and junk all over the place. We go from one job to another and clean up about every 6th job or so !

Mark

posted 12-13-2004 11:05 PM                    

quote: Paul Mayer

---

Am I reading those numbers correctly? 35V filaments?

---

Yep. all the heaters are connected in series across the 117V power line, eliminating the need for a large and costly power transformer:

35W4 (1/2-wave rectifier): 35V
2x35C5 (power outputs): 70V
12AX7 (preamp/driver): 12.6V

Total heater draw: 117.6V @ 150ma

The only drawback with such circuits is that one side of the power line is connected to the chassis, thus you have to be VERY careful how you plug the thing in...

-Aaron

posted 12-13-2004 11:10 PM                 

quote: Aaron Sisemore

---

...such circuits is that one side of the power line is connected to the chassis...

---

posted 12-14-2004 11:33 AM                       

quote:

---

Possibly the longest lived consumer electronic product design was the five tube "AC/DC" AM radio. Virtually every household had at least a few over the years. These radios were low cost, and one expensive item designed out was the power transformer. Thus the series heater string, and using the powerline directly rectified for B+ power. No power transformer also made it possible for smaller and lighter sets to be made.

---

quote:

---

Ed Engelken mentioned the potential risks associated with AC/DC receiver chassis in the October 2004 “Wireless Connection” column featuring his restoration of a National SW-54 SWL receiver (Photo A). Even more recently, a post in an Internet antique radio chat forum related the tragic electrocution of a young boy when he touched an outdoor water pump. Several years have past since we’ve discussed AC/DC receivers, so perhaps now’s a good time to review what you need to know to safely service and restore these sets. Line voltages can be deadly, so always treat any radio with respect, on or off the bench.

AC/DC Radios, What Are They?

AC/DC sets were designed to work on both AC and DC line voltages, and until fairly recently you could still find areas with DC power in larger cities. These sets used a tube lineup that was particularly suited for this service. For example, the typical 1950s AA5 tube lineup was a 12BE6 converter, 12BA6 IF amplifier, 12AV6 detector/first audio, 50C5 audio power amplifier, and 35W4 rectifier. These tubes were ideal sets using a series filament string that totaled near 110 volts, permitting them to directly operate from the AC or DC line voltage. Did you know that the “A” suffix on tubes meant they were specifically designed for series string operation? The filaments in A-suffix tubes were designed to withstand and share the initial startup surge current at turn-on. Other variations of the AA5 added a sixth tube for an RF or second IF stage.

AA5: The All-American Five

These simple AC/DC radio were nicknamed the “All American Five,” or AA5 for short, in honor of the seven-pin miniature 150-mA tube lineups consisting of the aforementioned 6BE6, 6BA6, 6AV6, 50C5, and 35W4 vacuum tube lineup. Earlier AA5 radios incorporated an octal-based tube lineup, generally using a 12SA7 converter, 12SK7 IF, 12SQ7 det/first audio, 50L6 audio, and a 35Z5 rectifier. Purists argue—and probably correctly so—that only those sets using the earlier octal tube lineup are true examples of AA5 radios.

---